Nicholas Cox

3.4 Molecular Concepts of Water Splitting: Nature’s Approach

Publisher: DE GRUYTER

Date: 12-12-2012

DOI: 10.1515/9783110266320.185

The First State in the Catalytic Cycle of the Water-Oxidizing Enzyme: Identification of a Water-Derived μ-Hydroxo Bridge

Publisher: American Chemical Society (ACS)

Date: 03-10-2017

DOI: 10.1021/JACS.7B05263

Abstract: Nature's water-splitting catalyst, an oxygen-bridged tetramanganese calcium (Mn

Theoretical Evaluation of Structural Models of the S₂ State in the Oxygen Evolving Complex of Photosystem II: Protonation States and Magnetic Interactions

Publisher: American Chemical Society (ACS)

Date: 17-11-2011

DOI: 10.1021/JA2041805

Abstract: Protonation states of water ligands and oxo bridges are intimately involved in tuning the electronic structures and oxidation potentials of the oxygen evolving complex (OEC) in Photosystem II, steering the mechanistic pathway, which involves at least five redox state intermediates S(n) (n = 0-4) resulting in the oxidation of water to molecular oxygen. Although protons are practically invisible in protein crystallography, their effects on the electronic structure and magnetic properties of metal active sites can be probed using spectroscopy. With the twin purpose of aiding the interpretation of the complex electron paramagnetic resonance (EPR) spectroscopic data of the OEC and of improving the view of the cluster at the atomic level, a complete set of protonation configurations for the S(2) state of the OEC were investigated, and their distinctive effects on magnetic properties of the cluster were evaluated. The most recent X-ray structure of Photosystem II at 1.9 Å resolution was used and refined to obtain the optimum structure for the Mn(4)O(5)Ca core within the protein pocket. Employing this model, a set of 26 structures was constructed that tested various protonation scenarios of the water ligands and oxo bridges. Our results suggest that one of the two water molecules that are proposed to coordinate the outer Mn ion (Mn(A)) of the cluster is deprotonated in the S(2) state, as this leads to optimal experimental agreement, reproducing the correct ground state spin multiplicity (S = 1/2), spin expectation values, and EXAFS-derived metal-metal distances. Deprotonation of Ca(2+)-bound water molecules is strongly disfavored in the S(2) state, but dissociation of one of the two water ligands appears to be facile. The computed isotropic hyperfine couplings presented here allow distinctions between models to be made and call into question the assumption that the largest coupling is always attributable to Mn(III). The present results impose limits for the total charge and the proton configuration of the OEC in the S(2) state, with implications for the cascade of events in the Kok cycle and for the water splitting mechanism.

Biological Water Oxidation

Publisher: American Chemical Society (ACS)

Date: 18-03-2013

DOI: 10.1021/AR3003249

Abstract: Photosystem II (PSII), a multisubunit pigment-protein supercomplex found in cyanobacteria, algae, and plants, catalyzes a unique reaction in nature: the light-driven oxidation of water. Remarkable recent advances in the structural analysis of PSII now give a detailed picture of the static supercomplex on the molecular level. These data provide a solid foundation for future functional studies, in particular the mechanism of water oxidation and oxygen release. The catalytic core of the PSII is a tetramanganese-calcium cluster (Mn₄O₅Ca), commonly referred to as the oxygen-evolving complex (OEC). The function of the OEC rests on its ability to cycle through five metastable states (Si, i = 0-4), transiently storing four oxidizing equivalents, and in so doing, facilitates the four electron water splitting reaction. While the latest crystallographic model of PSII gives an atomic picture of the OEC, the exact connectivity within the inorganic core and the S-state(s) that the X-ray model represents remain uncertain. In this Account, we describe our joint experimental and theoretical efforts to eliminate these ambiguities by combining the X-ray data with spectroscopic constraints and introducing computational modeling. We are developing quantum chemical methods to predict electron paramagnetic resonance (EPR) parameters for transition metal clusters, especially focusing on spin-projection approaches combined with density functional theory (DFT) calculations. We aim to resolve the geometric and electronic structures of all S-states, correlating their structural features with spectroscopic observations to elucidate reactivity. The sequence of manganese oxidations and concomitant charge compensation events via proton transfer allow us to rationalize the multielectron S-state cycle. EPR spectroscopy combined with theoretical calculations provides a unique window into the tetramangenese complex, in particular its protonation states and metal ligand sphere evolution, far beyond the scope of static techniques such as X-ray crystallography. This approach has led, for ex le, to a detailed understanding of the EPR signals in the S₂-state of the OEC in terms of two interconvertible, isoenergetic structures. These two structures differ in their valence distribution and spin multiplicity, which has important consequences for substrate binding and may explain its low barrier exchange with solvent water. New experimental techniques and innovative s le preparations are beginning to unravel the complex sequence of substrate uptake/inclusion, which is coupled to proton release. The introduction of specific site perturbations, such as replacing Ca²⁺ with Sr²⁺, provides discrete information about the ligand environment of the in idual Mn ions. In this way, we have identified a potential open coordination site for one Mn center, which may serve as a substrate binding site in the higher S-states, such as S₃ and S₄. In addition, we can now monitor the binding of the substrate water in the lower S-states (S₁ and S₂) using new EPR-detected NMR spectroscopies. These studies provided the first evidence that one of the substrates is subsumed into the complex itself and forms an oxo-bridge between two Mn ions. This result places important new restrictions on the mechanism of O-O bond formation. These new insights from nature's water splitting catalyst provide important criteria for the rational design of bioinspired synthetic catalysts.

Simulating the low-temperature, metastable electrochromism of Photosystem I: Applications to Thermosynechococcus vulcanus and Chroococcidiopsis thermalis

Publisher: AIP Publishing

Date: 28-09-2022

DOI: 10.1063/5.0100431

Abstract: Low-temperature, metastable electrochromism has been used as a tool to assign pigments in Photosystem I (PS I) from Thermosynechococcus vulcanus and both the white light and far-red light (FRL) forms of Chroococcidiopsis thermalis. We find that a minimum of seven pigments is required to satisfactorily model the electrochromism of PS I. Using our model, we provide a short list of candidates for the chlorophyll f pigment in FRL C. thermalis that absorbs at 756 nm, whose identity, to date, has proven to be controversial. Specifically, we propose the linker pigments A40 and B39 and two antenna pigments A26 and B24 as defined by crystal structure 1JB0. The pros and cons of these assignments are discussed, and we propose further experiments to better understand the functioning of FRL C. thermalis.

Independent relationship of left atrial size and mortality in patients with heart failure: an individual patient meta‐analysis of longitudinal data (MeRGE Heart Failure)

Publisher: Wiley

Date: 29-09-2009

DOI: 10.1093/EURJHF/HFP112

Abstract: Left atrial (LA) size is considered a marker of poor prognosis in heart failure (HF) patients. Prior studies have recruited relatively few subjects limiting their power to adequately analyse the interaction between LA size, left ventricular (LV) systolic and diastolic function, and prognosis. The MeRGE collaboration combines prospective data from 18 studies in HF patients. In this analysis of data from 1157 patients, the primary endpoint was death or hospitalization for worsening HF. In multivariate analysis (Cox proportion hazard model), LA area was associated with prognosis (HR 1.03 per cm(2), 95% CI 1.02, 1.05 P < 0.0001) independently of age, NYHA class, LV ejection fraction, and restrictive filling pattern (RFP). When LA area was used as a categorical variable, the HR associated with larger LA area (above median) was 1.4 (95% CI 1.13, 1.74) and when LA area index was used, the HR was 2.36 (95% CI 1.80, 3.08). When the patients with and without RFP were ided on the basis of either LA area or LA area index, significantly higher event rates were observed in those with larger LA area. Left atrial area is a powerful predictor of outcome among HF patients with predominantly impaired systolic function, and is independent of, and provides additional prognostic information beyond LV systolic and diastolic function.

Water oxidation in oxygenic photosynthesis studied by magnetic resonance techniques

Publisher: Wiley

Date: 02-12-2022

DOI: 10.1002/1873-3468.14543

Abstract: The understanding of light‐induced biological water oxidation in oxygenic photosynthesis is of great importance both for biology and (bio)technological applications. The chemically difficult multistep reaction takes place at a unique protein‐bound tetra‐manganese/calcium cluster in photosystem II whose structure has been elucidated by X‐ray crystallography (Umena et al. Nature 2011, 473 , 55). The cluster moves through several intermediate states in the catalytic cycle. A detailed understanding of these intermediates requires information about the spatial and electronic structure of the Mn 4 Ca complex the latter is only available from spectroscopic techniques. Here, the important role of Electron Paramagnetic Resonance (EPR) and related double resonance techniques (ENDOR, EDNMR), complemented by quantum chemical calculations, is described. This has led to the elucidation of the cluster's redox and protonation states, the valence and spin states of the manganese ions and the interactions between them, and contributed substantially to the understanding of the role of the protein surrounding, as well as the binding and processing of the substrate water molecules, the O‐O bond formation and dioxygen release. Based on these data, models for the water oxidation cycle are developed.

Defect structure and property consequence when small Li⁺ ions meet BaTiO3

Publisher: American Physical Society (APS)

Date: 31-08-2020

DOI: 10.1103/PHYSREVMATERIALS.4.084412

Charge separation in Photosystem II: A comparative and evolutionary overview

Publisher: Elsevier BV

Date: 2012

DOI: 10.1016/J.BBABIO.2011.07.012

Abstract: Our current understanding of the PSII reaction centre owes a great deal to comparisons to the simpler and better understood, purple bacterial reaction centre. Here we provide an overview of the similarities with a focus on charge separation and the electron acceptors. We go on to discuss some of the main differences between the two kinds of reaction centres that have been highlighted by the improving knowledge of PSII. We attempt to relate these differences to functional requirements of water splitting. Some are directly associated with that function, e.g. high oxidation potentials, while others are associated with regulation and protection against photodamage. The protective and regulatory functions are associated with the harsh chemistry performed during its normal function but also with requirements of the enzyme while it is undergoing assembly and repair. Key aspects of PSII reaction centre evolution are also addressed. This article is part of a Special Issue entitled: Photosystem II.

The S1 split signal of photosystem II; a tyrosine–manganese coupled interaction

Publisher: Elsevier BV

Date: 07-2009

DOI: 10.1016/J.BBABIO.2009.03.023

Abstract: Detailed optical and EPR analyses of states induced in dark-adapted PS II membranes by cryogenic illumination permit characterization and quantification of all pigment derived donors and acceptors, as well as optically silent (in the visible, near infrared) species which are EPR active. Near complete turnover formation of Q(A)((-)) is seen in all centers, but with variable efficiency, depending on the donor species. In minimally detergent-exposed PS II membranes, negligible (<5%) oxidation of chlorophyll or carotenoid centers occurs for illumination temperatures 5-20 K. An optically silent electron donor to P680(+) is observed with the same decay kinetics as the S(1) split signal. Cryogenic donors to P680(+) seen are: (i) transient (t(1/2) approximately 150 s) tyrosine related species, including 'split signals' ( approximately 15% total centers), (ii) reduced cytochrome b(559) ( approximately 30-50% centers), and (iii) an organic donor, possibly an amino acid side chain, ( approximately 30% centers).

Two Interconvertible Structures that Explain the Spectroscopic Properties of the Oxygen-Evolving Complex of Photosystem II in the S₂State

Publisher: Wiley

Date: 21-08-2012

DOI: 10.1002/ANIE.201204705

Abstract: Using models derived from the X-ray structure of photosystem II, it is shown that the oxygen evolving complex in the S(2) state exists in two energetically similar and interconvertible forms. A longstanding question regarding the spectroscopy of the catalyst is thus answered: one form corresponds to the multiline g=2.0 EPR signal (see picture, right O red, Mn purple, Ca yellow), and the other to the g≥4.1 signals (left).

D1 protein variants in Photosystem II from Thermosynechococcus elongatus studied by low temperature optical spectroscopy

Publisher: Elsevier BV

Date: 2010

DOI: 10.1016/J.BBABIO.2009.07.007

Abstract: In Photosystem II (PSII) from Thermosynechococcus elongatus, high-light intensity growth conditions induce the preferential expression of the psbA(3) gene over the psbA(1) gene. These genes encode for the D1 protein variants labeled D1:3 and D1:1, respectively. We have compared steady state absorption and photo-induced difference spectra at 50 ms) Q(A)(-) yield was approximately 95% in D1:3, and approximately 60% in D1:1, relative to green light excitation. (iv) For the D1:1 variant, the quantum efficiency of photo-induced oxidation of side-pathway donors was lower. These effects can be correlated with amino acid changes between the two D1 variants. The effects on the pheophytin Q(x) band can be attributed to the hydrogen bond from Glu130 in D1:3 to the 13(1)-keto of Pheo(D1), which is absent for Gln130 in D1:1. The reduced yield with red light in the D1:1 variant could be associated with either the Glu130Gln change, and/or the four changes near the binding site of P(D1), in particular Ser153Ala. Photo-induced Q(A)(-) formation with far red light is assigned to the direct optical excitation of a weakly absorbing charge transfer state of the reaction centre. We suggest that this state is blue-shifted in the D1:1 variant. A reduced efficiency for the oxidation of side-pathway donors in the D1:1 variant could be explained by a variation in the location and/or redox potential of P+.

Highly Efficient Visible Light Catalysts Driven by Ti ³⁺ -V O -2Ti ⁴⁺ -N ³⁻ Defect Clusters

Publisher: Wiley

Date: 19-11-2019

DOI: 10.1002/CNMA.201800400

Zero-emission multivalorization of light alcohols with self-separable pure H2 fuel

Publisher: Elsevier BV

Date: 09-2021

DOI: 10.1016/J.APCATB.2021.120212

The electronic structures of the S2 states of the oxygen-evolving complexes of photosystem II in plants and cyanobacteria in the presence and absence of methanol

Publisher: Elsevier BV

Date: 07-2011

DOI: 10.1016/J.BBABIO.2011.03.002

Abstract: The electronic properties of the Mn(4)O(x)Ca cluster in the S(2) state of the oxygen-evolving complex (OEC) were studied using X- and Q-band EPR and Q-band (55)Mn-ENDOR using photosystem II preparations isolated from the thermophilic cyanobacterium T. elongatus and higher plants (spinach). The data presented here show that there is very little difference between the two species. Specifically it is shown that: (i) only small changes are seen in the fitted isotropic hyperfine values, suggesting that there is no significant difference in the overall spin distribution (electronic coupling scheme) between the two species (ii) the inferred fine-structure tensor of the only Mn(III) ion in the cluster is of the same magnitude and geometry for both species types, suggesting that the Mn(III) ion has the same coordination sphere in both s le preparations and (iii) the data from both species are consistent with only one structural model available in the literature, namely the Siegbahn structure [Siegbahn, P. E. M. Accounts Chem. Res.2009, 42, 1871-1880, Pantazis, D. A. et al., Phys. Chem. Chem. Phys.2009, 11, 6788-6798]. These measurements were made in the presence of methanol because it confers favorable magnetic relaxation properties to the cluster that facilitate pulse-EPR techniques. In the absence of methanol the separation of the ground state and the first excited state of the spin system is smaller. For cyanobacteria this effect is minor but in plant PS II it leads to a break-down of the S(T)=½ spin model of the S(2) state. This suggests that the methanol-OEC interaction is species dependent. It is proposed that the effect of small organic solvents on the electronic structure of the cluster is to change the coupling between the outer Mn (Mn(A)) and the other three Mn ions that form the trimeric part of the cluster (Mn(B), Mn(C), Mn(D)), by perturbing the linking bis-μ-oxo bridge. The flexibility of this bridging unit is discussed with regard to the mechanism of O-O bond formation.

What Are the Oxidation States of Manganese Required To Catalyze Photosynthetic Water Oxidation?

Publisher: Elsevier BV

Date: 07-2012

DOI: 10.1016/J.BPJ.2012.05.031

Oxygen‐evolving Photosystem II

Publisher: Wiley

Date: 14-03-2014

DOI: 10.1002/9781119951438.EIBC2166

Abstract: Photosystem II is the enzyme that catalyzes the thermodynamically demanding splitting of water, yielding dioxygen, protons, and electrons, a sunlight‐driven reaction that forms the foundation of oxygenic photosynthesis. The latest results from high‐resolution crystallographic models have led to a refined view of the overall structure of the enzyme and the arrangement of the cofactors involved in light harvesting, charge separation, and electron transfer. In addition, combined efforts from protein crystallography, spectroscopy, and computational chemistry have greatly improved the understanding of the tetramanganese–calcium cluster of the oxygen‐evolving complex, the site of water oxidation. The most important features of the geometric and electronic structures of the oxygen‐evolving complex for the earlier reaction cycle intermediates are now sufficiently well understood such that connections between several structural features and spectroscopic observables can be made with confidence. Advanced spectroscopic techniques have also identified possible sites where the substrate water binds. Although the details of the actual mechanism of biological water oxidation remain elusive, experimental and theoretical studies impose an increasing number of constraints that significantly limit the number of mechanistic possibilities for O–O bond formation.

Oxygen-deficient photostable Cu2O for enhanced visible light photocatalytic activity

Publisher: Royal Society of Chemistry (RSC)

Date: 2018

DOI: 10.1039/C7NR08388B

Abstract: Large-scale synthesis of O-deficient Cu 2 O with outstanding visible photoactivity is shown.

Crystalline Germanium(I) and Tin(I) Centered Radical Anions

Publisher: Wiley

Date: 23-03-2022

DOI: 10.1002/ANIE.202201248

Abstract: An isostructural series of heavy Group 14 E(I) radical anions (Ge, Sn, Pb), stabilized by a bulky xanthene‐based diamido ligand are reported. The radical anions were synthesised by the one‐electron reduction of their corresponding E(II) precursor complexes with sodium naphthalenide in THF, yielding the radical anions as charge‐separated sodium salts. The series of main group radicals have been comprehensively characterized by EPR spectroscopy, X‐ray crystallography and DFT analysis, which reveal that in all cases, the spin density of the unpaired electron almost exclusively resides in a p ‐orbital of π symmetry located on the Group 14 center.

The Semiquinone-Iron Complex of Photosystem II: Structural Insights from ESR and Theoretical Simulation; Evidence that the Native Ligand to the Non-Heme Iron Is Carbonate

Publisher: Elsevier BV

Date: 10-2009

DOI: 10.1016/J.BPJ.2009.06.033

Pulse Double-Resonance EPR Techniques for the Study of Metallobiomolecules

Publisher: Elsevier

Date: 2015

DOI: 10.1016/BS.MIE.2015.08.016

Abstract: Electron paramagnetic resonance (EPR) spectroscopy exploits an intrinsic property of matter, namely the electron spin and its related magnetic moment. This can be oriented in a magnetic field and thus, in the classical limit, acts like a little bar magnet. Its moment will align either parallel or antiparallel to the field, giving rise to different energies (termed Zeeman splitting). Transitions between these two quantized states can be driven by incident microwave frequency radiation, analogous to NMR experiments, where radiofrequency radiation is used. However, the electron Zeeman interaction alone provides only limited information. Instead, much of the usefulness of EPR is derived from the fact that the electron spin also interacts with its local magnetic environment and thus can be used to probe structure via detection of nearby spins, e.g., NMR-active magnetic nuclei and/or other electron spin(s). The latter is exploited in spin labeling techniques, an exciting new area in the development of noncrystallographic protein structure determination. Although these interactions are often smaller than the linewidth of the EPR experiment, sophisticated pulse EPR methods allow their detection. A number of such techniques are well established today and can be broadly described as double-resonance methods, in which the electron spin is used as a reporter. Below we give a brief description of pulse EPR methods, particularly their implementation at higher magnetic fields, and how to best exploit them for studying metallobiomolecules.

The FTIR spectra of glycine and glycylglycine zwitterions isolated in alkali halide matrices

Publisher: Elsevier BV

Date: 06-2005

DOI: 10.1016/J.CHEMPHYS.2004.12.011

Identification of the Q Y Excitation of the Primary Electron Acceptor of Photosystem II: CD Determination of Its Coupling Environment

Publisher: American Chemical Society (ACS)

Date: 17-08-2009

DOI: 10.1021/JP808796X

Abstract: Low-temperature absorption and CD spectra, measured simultaneously, are reported from Photosystem II (PS II) reduced with sodium dithionite. Spectra were obtained using PS II core complexes before and after photoaccumulation of Pheo(D1)(-), the anion of the primary acceptor. For plant PS II, Pheo(D1)(-) was generated under conditions in which the primary plastoquinone was present as an anion (Q(A)(-)) and as a modified species taken to be the neutral doubly reduced hydroquinone (Q(A)H(2)). The bleaches observed upon Pheo(D1)(-) formation in the presence of Q(A)(-) are shifted to the blue compared those in the presence of Q(A)H(2). This is attributed to the influence of the charge on Q(A)(-), and this effect mirrors the well-known electrochromic effect of Q(A)(-) on the neutral pigments. The absorption bleaches induced by Pheo(D1) reduction are species dependent. Structured changes of the CD in the 680-690 nm spectral region are seen upon photoaccumulation of Pheo(D1)(-) in PS II from plant, Synechocystis and Thermosynechococcus vulcanus. These CD changes are shown to be consistent with the overall electronic assignments of Raszewski et al. [Raszewski et al. Biophys. J. 2008, 95, 105], which place the dominant Pheo(D1) excitation near 672 nm. CD changes associated with Pheo(D1) reduction are modeled to arise from the shift and intensity changes of two CD features: one predominately of Chl(D1) character, the other predominately Pheo(D2) in character. The assignments are also shown to account for the Q(Y) absorption changes in s les where the quinone is its charged (Q(A)(-)) and neutral (Q(A)H(2)) states.

Biomolecular EPR Meets NMR at High Magnetic Fields

Publisher: MDPI AG

Date: 06-11-2018

DOI: 10.3390/MAGNETOCHEMISTRY4040050

Abstract: In this review on advanced biomolecular EPR spectroscopy, which addresses both the EPR and NMR communities, considerable emphasis is put on delineating the complementarity of NMR and EPR regarding the measurement of interactions and dynamics of large molecules embedded in fluid-solution or solid-state environments. Our focus is on the characterization of protein structure, dynamics and interactions, using sophisticated EPR spectroscopy methods. New developments in pulsed microwave and sweepable cryomagnet technology as well as ultrafast electronics for signal data handling and processing have pushed the limits of EPR spectroscopy to new horizons reaching millimeter and sub-millimeter wavelengths and 15 T Zeeman fields. Expanding traditional applications to paramagnetic systems, spin-labeling of biomolecules has become a mainstream multifrequency approach in EPR spectroscopy. In the high-frequency/high-field EPR region, sub-micromolar concentrations of nitroxide spin-labeled molecules are now sufficient to characterize reaction intermediates of complex biomolecular processes. This offers promising analytical applications in biochemistry and molecular biology where s le material is often difficult to prepare in sufficient concentration for NMR characterization. For multifrequency EPR experiments on frozen solutions typical s le volumes are of the order of 250 μL (S-band), 150 μL (X-band), 10 μL (Q-band) and 1 μL (W-band). These are orders of magnitude smaller than the s le volumes required for modern liquid- or solid-state NMR spectroscopy. An important additional advantage of EPR over NMR is the ability to detect and characterize even short-lived paramagnetic reaction intermediates (down to a lifetime of a few ns). Electron–nuclear and electron–electron double-resonance techniques such as electron–nuclear double resonance (ENDOR), ELDOR-detected NMR, PELDOR (DEER) further improve the spectroscopic selectivity for the various magnetic interactions and their evolution in the frequency and time domains. PELDOR techniques applied to frozen-solution s les of doubly spin-labeled proteins allow for molecular distance measurements ranging up to about 100 Å. For disordered frozen-solution s les high-field EPR spectroscopy allows greatly improved orientational selection of the molecules within the laboratory axes reference system by means of the anisotropic electron Zeeman interaction. Single-crystal resolution is approached at the canonical g-tensor orientations—even for molecules with very small g-anisotropies. Unique structural, functional, and dynamic information about molecular systems is thus revealed that can hardly be obtained by other analytical techniques. On the other hand, the limitation to systems with unpaired electrons means that EPR is less widely used than NMR. However, this limitation also means that EPR offers greater specificity, since ordinary chemical solvents and matrices do not give rise to EPR in contrast to NMR spectra. Thus, multifrequency EPR spectroscopy plays an important role in better understanding paramagnetic species such as organic and inorganic radicals, transition metal complexes as found in many catalysts or metalloenzymes, transient species such as light-generated spin-correlated radical pairs and triplets occurring in protein complexes of photosynthetic reaction centers, electron-transfer relays, etc. Special attention is drawn to high-field EPR experiments on photosynthetic reaction centers embedded in specific sugar matrices that enable organisms to survive extreme dryness and heat stress by adopting an anhydrobiotic state. After a more general overview on methods and applications of advanced multifrequency EPR spectroscopy, a few representative ex les are reviewed to some detail in two Case Studies: (I) High-field ELDOR-detected NMR (EDNMR) as a general method for electron–nuclear hyperfine spectroscopy of nitroxide radical and transition metal containing systems (II) High-field ENDOR and EDNMR studies of the Oxygen Evolving Complex (OEC) in Photosystem II, which performs water oxidation in photosynthesis, i.e., the light-driven splitting of water into its elemental constituents, which is one of the most important chemical reactions on Earth.

Characterization of Oxygen Bridged Manganese Model Complexes Using Multifrequency ¹⁷O-Hyperfine EPR Spectroscopies and Density Functional Theory

Publisher: American Chemical Society (ACS)

Date: 26-08-2015

DOI: 10.1021/ACS.JPCB.5B04614

Abstract: Multifrequency pulsed EPR data are reported for a series of oxygen bridged (μ-oxo/μ-hydroxo) bimetallic manganese complexes where the oxygen is labeled with the magnetically active isotope (17)O (I = 5/2). Two synthetic complexes and two biological metallocofactors are examined: a planar bis-μ-oxo bridged complex and a bent, bis-μ-oxo-μ-carboxylato bridge complex the dimanganese catalase, which catalyzes the dismutation of H2O2 to H2O and O2, and the recently identified manganese/iron cofactor of the R2lox protein, a homologue of the small subunit of the ribonuclotide reductase enzyme (class 1c). High field (W-band) hyperfine EPR spectroscopies are demonstrated to be ideal methods to characterize the (17)O magnetic interactions, allowing a magnetic fingerprint for the bridging oxygen ligand to be developed. It is shown that the μ-oxo bridge motif displays a small positive isotropic hyperfine coupling constant of about +5 to +7 MHz and an anisotropic/dipolar coupling of -9 MHz. In addition, protonation of the bridge is correlated with an increase of the hyperfine coupling constant. Broken symmetry density functional theory is evaluated as a predictive tool for estimating hyperfine coupling of bridging species. Experimental and theoretical results provide a framework for the characterization of the oxygen bridge in Mn metallocofactor systems, including the water oxidizing cofactor of photosystem II, allowing the substrate/solvent interface to be examined throughout its catalytic cycle.

Spectral characteristics of PS II reaction centres: as isolated preparations and when integral to PS II core complexes

Publisher: Springer Science and Business Media LLC

Date: 29-07-2008

DOI: 10.1007/S11120-008-9328-8

Abstract: The discovery that the native PS II enzyme undergoes charge separation via an absorption extending to 730 nm has led us to re-examine the low-temperature absorption spectra of Nanba-Satoh PS II reaction centre preparations with particular focus on the long wavelength region. It is shown that these preparations do not exhibit absorption in the 700-730 nm region at 1.7 K. Absorption in the Nanba-Satoh type preparations analogous to the 'red tail' as observed in functional PS II core complexes is likely shifted to higher energy by >20 nm. Spectral changes associated with the stable reduction of pheo(a) in chemically treated reaction centre preparations are also revisited. Dithionite treatment of PS II preparations in the dark leads to changes of pigment-pigment and/or pigment-protein interactions, as evidenced by changes in absorption and CD spectra. Absorption and CD changes associated with stable Pheo(D1) photo-reduction in PS II core complexes and Nanba-Satoh preparations are compared. For Nanba-Satoh preparations, Q(y) bleaches are approximately 3x broader than in PS II core complexes and are blue-shifted by approximately 4 nm. These data are discussed in terms of current models of PS II, and suggest a need to consider protein-induced changes of some electronic properties of reaction centre pigments.

What Can We Learn from a Biomimetic Model of Nature’s Oxygen-Evolving Complex?

Publisher: American Chemical Society (ACS)

Date: 14-03-2017

DOI: 10.1021/ACS.INORGCHEM.6B02777

Abstract: A recently reported synthetic complex with a Mn

Artificial photosynthesis: understanding water splitting in nature

Publisher: The Royal Society

Date: 06-06-2015

DOI: 10.1098/RSFS.2015.0009

Abstract: In the context of a global artificial photosynthesis (GAP) project, we review our current work on nature's water splitting catalyst. In a recent report (Cox et al . 2014 Science 345, 804–808 ( doi:10.1126/science.1254910 )), we showed that the catalyst—a Mn 4 O 5 Ca cofactor—converts into an ‘activated’ form immediately prior to the O–O bond formation step. This activated state, which represents an all Mn IV complex, is similar to the structure observed by X-ray crystallography but requires the coordination of an additional water molecule. Such a structure locates two oxygens, both derived from water, in close proximity, which probably come together to form the product O 2 molecule. We speculate that formation of the activated catalyst state requires inherent structural flexibility. These features represent new design criteria for the development of biomimetic and bioinspired model systems for water splitting catalysts using first-row transition metals with the aim of delivering globally deployable artificial photosynthesis technologies.

Free Radical Generation from High-Frequency Electromechanical Dissociation of Pure Water

Publisher: American Chemical Society (ACS)

Date: 26-05-2020

DOI: 10.1021/ACS.JPCLETT.0C01227

A Chiral Lanthanide Tag for Stable and Rigid Attachment to Single Cysteine Residues in Proteins for NMR, EPR and Time‐Resolved Luminescence Studies

Publisher: Wiley

Date: 29-07-2021

DOI: 10.1002/CHEM.202101143

Abstract: A lanthanide‐binding tag site‐specifically attached to a protein presents a tool to probe the protein by multiple spectroscopic techniques, including nuclear magnetic resonance, electron paramagnetic resonance and time‐resolved luminescence spectroscopy. Here a new stable chiral Ln III tag, referred to as C12 , is presented for spontaneous and quantitative reaction with a cysteine residue to generate a stable thioether bond. The synthetic protocol of the tag is relatively straightforward, and the tag is stable for storage and shipping. It displays greatly enhanced reactivity towards selenocysteine, opening a route towards selective tagging of selenocysteine in proteins containing cysteine residues. Loaded with Tb III or Tm III ions, the C12 tag readily generates pseudocontact shifts (PCS) in protein NMR spectra. It produces a relatively rigid tether between lanthanide and protein, which is beneficial for interpretation of the PCSs by single magnetic susceptibility anisotropy tensors, and it is suitable for measuring distance distributions in double electron–electron resonance experiments. Upon reaction with cysteine or other thiol compounds, the Tb III complex exhibits a 100‐fold enhancement in luminescence quantum yield, affording a highly sensitive turn‐on luminescence probe for time‐resolved FRET assays and enzyme reaction monitoring.

Anywhere, anytime, with any device: scenario-based mobile learning in biomedical sciences

Publisher: Inderscience Publishers

Date: 2013

DOI: 10.1504/IJMLO.2013.055617

The Basic Properties of the Electronic Structure of the Oxygen-evolving Complex of Photosystem II Are Not Perturbed by Ca2+ Removal

Publisher: Elsevier BV

Date: 07-2012

DOI: 10.1074/JBC.M112.365288

The syntheses, structures and spectroelectrochemical properties of 6-oxo-verdazyl derivatives bearing surface anchoring groups

Publisher: Royal Society of Chemistry (RSC)

Date: 2022

DOI: 10.1039/D1TC05495C

Abstract: The electronic properties of 6-oxo-verdazyl radicals are tuned by substituents at the 1- and 5-positions, with frontier orbital analysis indicating that each system should display constructive quantum interference in a molecular junction.

BiOCl Nanoflowers with High Levels of Oxygen Vacancy for Photocatalytic CO₂ Reduction

Publisher: American Chemical Society (ACS)

Date: 28-02-2023

DOI: 10.1021/ACSANM.2C05364

Photo-catalytic oxidation of a di-nuclear manganese centre in an engineered bacterioferritin ‘reaction centre’

Publisher: Elsevier BV

Date: 09-2009

DOI: 10.1016/J.BBABIO.2009.04.011

Abstract: Photosynthesis involves the conversion of light into chemical energy through a series of electron transfer reactions within membrane-bound pigment rotein complexes. The Photosystem II (PSII) complex in plants, algae and cyanobacteria catalyse the oxidation of water to molecular O2. The complexity of PSII has thus far limited attempts to chemically replicate its function. Here we introduce a reverse engineering approach to build a simple, light-driven photo-catalyst based on the organization and function of the donor side of the PSII reaction centre. We have used bacterioferritin (BFR) (cytochrome b1) from Escherichia coli as the protein scaffold since it has several, inherently useful design features for engineering light-driven electron transport. Among these are: (i.) a di-iron binding site (ii.) a potentially redox-active tyrosine residue and (iii.) the ability to dimerise and form an inter-protein heme binding pocket within electron tunnelling distance of the di-iron binding site. Upon replacing the heme with the photoactive zinc-chlorin e6 (ZnCe6) molecule and the di-iron binding site with two manganese ions, we show that the two Mn ions bind as a weakly coupled di-nuclear Mn2II,II centre, and that ZnCe6 binds in stoichiometric amounts of 1:2 with respect to the dimeric form of BFR. Upon illumination the bound ZnCe6 initiates electron transfer, followed by oxidation of the di-nuclear Mn centre possibly via one of the inherent tyrosine residues in the vicinity of the Mn cluster. The light dependent loss of the MnII EPR signals and the formation of low field parallel mode Mn EPR signals are attributed to the formation of MnIII species. The formation of the MnIII is concomitant with consumption of oxygen. Our model is the first artificial reaction centre developed for the photo-catalytic oxidation of a di-metal site within a protein matrix which potentially mimics water oxidation centre (WOC) photo-assembly.

ELDOR-detected NMR: A general and robust method for electron-nuclear hyperfine spectroscopy?

Publisher: Elsevier BV

Date: 07-2017

DOI: 10.1016/J.JMR.2017.04.006

Abstract: ELDOR-detected NMR (EDNMR) performed at higher magnetic fields is becoming an increasingly popular alternative to conventional ENDOR for the characterization of electron-nuclear hyperfine interactions owing to its enhanced sensitivity. However there are two key problems that limit its widespread adoption, with factors controlling: (i) lineshape distortions and (ii) overall spectral resolution, still largely understood only at a qualitative level. Indeed highly anisotropic (dipolar) coupled species are particularly problematic in the EDNMR experiment. Nor is it clear as to whether line intensities measured in EDNMR can provide quantitative information. Here we describe how all these problems can be overcome for a nitroxide radical as model system. We introduce a simulation procedure rotocol for the simulation of EDNMR line-shapes collected over a range of high turning angle (HTA) pulse lengths. It is shown that spectral line-shapes can be robustly reproduced and that the intensities of spectral lines and the spin nutation behavior can be quantitatively assessed. This broadens the scope of the EDNMR experiment as a generally applicable, quantitative double resonance method.

Structural adaptations of photosynthetic complex I enable ferredoxin-dependent electron transfer

Publisher: American Association for the Advancement of Science (AAAS)

Date: 18-01-2019

DOI: 10.1126/SCIENCE.AAU3613

Abstract: Photosynthetic organisms use light to fix carbon dioxide in a process that requires both chemical reducing equivalents and adenosine triphosphate (ATP). Balancing the ratio of these inputs is accomplished by a short circuit in electron flow through photosynthetic complex I, a proton pump that contributes to ATP production but does not increase net reducing equivalents in the cell. Schuller et al. solved a cryo–electron microscopy structure of photosynthetic complex I (see the Perspective by Brandt) and went on to reconstitute electron transfer using the electron carrier protein ferredoxin. Science , this issue p. 257 see also p. 230

W-band ELDOR-detected NMR (EDNMR) spectroscopy as a versatile technique for the characterisation of transition metal–ligand interactions

Publisher: Informa UK Limited

Date: 09-09-2013

DOI: 10.1080/00268976.2013.830783

Electronic Structure of a Weakly Antiferromagnetically Coupled Mn^IIMn^III Model Relevant to Manganese Proteins: A Combined EPR, ⁵⁵Mn-ENDOR, and DFT Study

Publisher: American Chemical Society (ACS)

Date: 11-08-2011

DOI: 10.1021/IC200767E

Abstract: An analysis of the electronic structure of the [Mn(II)Mn(III)(μ-OH)-(μ-piv)(2)(Me(3)tacn)(2)](ClO(4))(2) (PivOH) complex is reported. It displays features that include: (i) a ground 1/2 spin state (ii) a small exchange (J) coupling between the two Mn ions (iii) a mono-μ-hydroxo bridge, bis-μ-carboxylato motif and (iv) a strongly coupled, terminally bound N ligand to the Mn(III). All of these features are observed in structural models of the oxygen evolving complex (OEC). Multifrequency electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) measurements were performed on this complex, and the resultant spectra simulated using the Spin Hamiltonian formalism. The strong field dependence of the (55)Mn-ENDOR constrains the (55)Mn hyperfine tensors such that a unique solution for the electronic structure can be deduced. Large hyperfine anisotropy is required to reproduce the EPR/ENDOR spectra for both the Mn(II) and Mn(III) ions. The large effective hyperfine tensor anisotropy of the Mn(II), a d(5) ion which usually exhibits small anisotropy, is interpreted within a formalism in which the fine structure tensor of the Mn(III) ion strongly perturbs the zero-field energy levels of the Mn(II)Mn(III) complex. An estimate of the fine structure parameter (d) for the Mn(III) of -4 cm(-1) was made, by assuming the intrinsic anisotropy of the Mn(II) ion is small. The magnitude of the fine structure and intrinsic (onsite) hyperfine tensor of the Mn(III) is consistent with the known coordination environment of the Mn(III) ion as seen from its crystal structure. Broken symmetry density functional theory (DFT) calculations were performed on the crystal structure geometry. DFT values for both the isotropic and the anisotropic components of the onsite (intrinsic) hyperfine tensors match those inferred from the EPR/ENDOR simulations described above, to within 5%. This study demonstrates that DFT calculations provide reliable estimates for spectroscopic observables of mixed valence Mn complexes, even in the limit where the description of a well isolated S = 1/2 ground state begins to break down.

Chemical flexibility of heterobimetallic Mn/Fe cofactors: R2lox and R2c proteins

Publisher: Elsevier BV

Date: 11-2019

DOI: 10.1074/JBC.RA119.010570

Effect of Ca²⁺/Sr²⁺ Substitution on the Electronic Structure of the Oxygen-Evolving Complex of Photosystem II: A Combined Multifrequency EPR, ⁵⁵Mn-ENDOR, and DFT Study

Publisher: American Chemical Society (ACS)

Date: 22-02-2011

DOI: 10.1021/JA110145V

Abstract: The electronic structures of the native Mn(4)O(x)Ca cluster and the biosynthetically substituted Mn(4)O(x)Sr cluster of the oxygen evolving complex (OEC) of photosystem II (PSII) core complexes isolated from Thermosynechococcus elongatus, poised in the S(2) state, were studied by X- and Q-band CW-EPR and by pulsed Q-band (55)Mn-ENDOR spectroscopy. Both wild type and tyrosine D less mutants grown photoautotrophically in either CaCl(2) or SrCl(2) containing media were measured. The obtained CW-EPR spectra of the S(2) state displayed the characteristic, clearly noticeable differences in the hyperfine pattern of the multiline EPR signal [Boussac et al. J. Biol. Chem.2004, 279, 22809-22819]. In sharp contrast, the manganese ((55)Mn) ENDOR spectra of the Ca and Sr forms of the OEC were remarkably similar. Multifrequency simulations of the X- and Q-band CW-EPR and (55)Mn-pulsed ENDOR spectra using the Spin Hamiltonian formalism were performed to investigate this surprising result. It is shown that (i) all four manganese ions contribute to the (55)Mn-ENDOR spectra (ii) only small changes are seen in the fitted isotropic hyperfine values for the Ca(2+) and Sr(2+) containing OEC, suggesting that there is no change in the overall spin distribution (electronic coupling scheme) upon Ca(2+)/Sr(2+) substitution (iii) the changes in the CW-EPR hyperfine pattern can be explained by a small decrease in the anisotropy of at least two hyperfine tensors. It is proposed that modifications at the Ca(2+) site may modulate the fine structure tensor of the Mn(III) ion. DFT calculations support the above conclusions. Our data analysis also provides strong support for the notion that in the S(2) state the coordination of the Mn(III) ion is square-pyramidal (5-coordinate) or octahedral (6-coordinate) with tetragonal elongation. In addition, it is shown that only one of the currently published OEC models, the Siegbahn structure [Siegbahn, P. E. M. Acc. Chem. Res.2009, 42, 1871-1880, Pantazis, D. A. et al. Phys. Chem. Chem. Phys.2009, 11, 6788-6798], is consistent with all data presented here. These results provide important information for the structure of the OEC and the water-splitting mechanism. In particular, the 5-coordinate Mn(III) is a potential site for substrate 'water' (H(2)O, OH(-)) binding. Its location within the cuboidal structural unit, as opposed to the external 'dangler' position, may have important consequences for the mechanism of O-O bond formation.

Enhanced synthesis of oxo-verdazyl radicals bearing sterically-and electronically-diverse C3-substituents

Publisher: Royal Society of Chemistry (RSC)

Date: 2021

DOI: 10.1039/D1OB01946E

Abstract: Gram-scale synthesis of functionalised aryl substituted tetrazinanones for phosgene- and hydrazine-free preparation of 6-oxo-verdazyls.

One- and two-electron reductions of a bulky BODIPY compound

Publisher: Royal Society of Chemistry (RSC)

Date: 2023

DOI: 10.1039/D3DT02048G

Abstract: The redox chemistry of a bulky BODIPY compound has been investigated with products from formal one- and two-electron reduction processes isolated and characterised in the solid state.

In Situ EPR Characterization of a Cobalt Oxide Water Oxidation Catalyst at Neutral pH

Publisher: MDPI AG

Date: 06-11-2019

DOI: 10.3390/CATAL9110926

Abstract: Here we report an in situ electron paramagnetic resonance (EPR) study of a low-cost, high-stability cobalt oxide electrodeposited material (Co-Pi) that oxidizes water at neutral pH and low over-potential, representing a promising system for future large-scale water splitting applications. Using CW X-band EPR we can follow the film formation from a Co(NO3)2 solution in phosphate buffer and quantify Co uptake into the catalytic film. As deposited, the film shows predominantly a Co(II) EPR signal, which converts into a Co(IV) signal as the electrode potential is increased. A purpose-built spectroelectrochemical cell allowed us to quantify the extent of Co(II) to Co(IV) conversion as a function of potential bias under operating conditions. Consistent with its role as an intermediate, Co(IV) is formed at potentials commensurate with electrocatalytic O2 evolution (+1.2 V, vs. SHE). The EPR resonance position of the Co(IV) species shifts to higher fields as the potential is increased above 1.2 V. Such a shift of the Co(IV) signal may be assigned to changes in the local Co structure, displaying a more distorted ligand field or more ligand radical character, suggesting it is this subset of sites that represents the catalytically ‘active’ component. The described spectroelectrochemical approach provides new information on catalyst function and reaction pathways of water oxidation.

Dielectric Coupler for General Purpose Q-Band EPR Cavity

Publisher: Springer Science and Business Media LLC

Date: 19-08-2021

DOI: 10.1007/S00723-021-01404-4

Abstract: Here, we report on a robust and efficient mechanism for tuning the microwave coupling of a Q-band (34 GHz), general purpose, cylindrical EPR cavity operating in the TE 011 mode. This novel mechanism allows for both the adjustment of the cavity’s coupling over a wide frequency range, as well as its bandwidth from that of a high-Q cavity (about 10 MHz), to a broadband cavity (above 1 GHz). The coupling element consists of a dielectric plate fixed onto a movable waveguide short that allows for two modes of operation. In the first mode, the dielectric plate does not influence the resonance properties of the coupling iris and allows for precise, critical coupling of the high-Q cavity. In the second mode, the dielectric plate is positioned in front of the coupling iris, varying the iris’ resonance properties and allowing very strong overcoupling to be achieved. This mechanism can be generalized for other types of EPR cavities, in particular at high microwave frequencies.

Short-range ENDOR distance measurements between Gd(iii) and trifluoromethyl labels in proteins

Publisher: Royal Society of Chemistry (RSC)

Date: 2022

DOI: 10.1039/D2CP02889A

Abstract: Using Gd 3+ paramagnetic and trifluoro nuclear tags allows measuring 10–20 Å distances in proteins with improved sensitivity and faster accumulation.

Detection of the Water-Binding Sites of the Oxygen-Evolving Complex of Photosystem II Using W-Band ¹⁷O Electron–Electron Double Resonance-Detected NMR Spectroscopy

Publisher: American Chemical Society (ACS)

Date: 27-09-2012

DOI: 10.1021/JA3053267

Abstract: Water binding to the Mn(4)O(5)Ca cluster of the oxygen-evolving complex (OEC) of Photosystem II (PSII) poised in the S(2) state was studied via H(2)(17)O- and (2)H(2)O-labeling and high-field electron paramagnetic resonance (EPR) spectroscopy. Hyperfine couplings of coordinating (17)O (I = 5/2) nuclei were detected using W-band (94 GHz) electron-electron double resonance (ELDOR) detected NMR and Davies/Mims electron-nuclear double resonance (ENDOR) techniques. Universal (15)N (I = ½) labeling was employed to clearly discriminate the (17)O hyperfine couplings that overlap with (14)N (I = 1) signals from the D1-His332 ligand of the OEC (Stich Biochemistry 2011, 50 (34), 7390-7404). Three classes of (17)O nuclei were identified: (i) one μ-oxo bridge (ii) a terminal Mn-OH/OH(2) ligand and (iii) Mn/Ca-H(2)O ligand(s). These assignments are based on (17)O model complex data, on comparison to the recent 1.9 Å resolution PSII crystal structure (Umena Nature 2011, 473, 55-60), on NH(3) perturbation of the (17)O signal envelope and density functional theory calculations. The relative orientation of the putative (17)O μ-oxo bridge hyperfine tensor to the (14)N((15)N) hyperfine tensor of the D1-His332 ligand suggests that the exchangeable μ-oxo bridge links the outer Mn to the Mn(3)O(3)Ca open-cuboidal unit (O4 and O5 in the Umena et al. structure). Comparison to literature data favors the Ca-linked O5 oxygen over the alternative assignment to O4. All (17)O signals were seen even after very short (≤15 s) incubations in H(2)(17)O suggesting that all exchange sites identified could represent bound substrate in the S(1) state including the μ-oxo bridge. (1)H/(2)H (I = ½, 1) ENDOR data performed at Q- (34 GHz) and W-bands complement the above findings. The relatively small (1)H/(2)H couplings observed require that all the μ-oxo bridges of the Mn(4)O(5)Ca cluster are deprotonated in the S(2) state. Together, these results further limit the possible substrate water-binding sites and modes within the OEC. This information restricts the number of possible reaction pathways for O-O bond formation, supporting an oxo/oxyl coupling mechanism in S(4).

Ultrabroadband plasmon driving selective photoreforming of methanol under ambient conditions

Publisher: Proceedings of the National Academy of Sciences

Date: 12-01-2023

DOI: 10.1073/PNAS.2212075120

Abstract: Liquid methanol has the potential to be the hydrogen energy carrier and storage medium for the future green economy. However, there are still many challenges before zero-emission, affordable molecular H 2 can be extracted from methanol with high performance. Here, we present noble-metal-free Cu–WC/W plasmonic nanohybrids which exhibit unsurpassed solar H 2 extraction efficiency from pure methanol of 2,176.7 µmol g −1 h −1 at room temperature and normal pressure. Macro-to-micro experiments and simulations unveil that local reaction microenvironments are generated by the coperturbation of WC/W’s lattice strain and infrared-plasmonic electric field. It enables spontaneous but selective zero-emission reaction pathways. Such microenvironments are found to be highly cooperative with solar-broadband-plasmon-excited charge carriers flowing from Cu to WC surfaces for efficient stable CH 3 OH plasmonic reforming with C 3 -dominated liquid products and 100% selective gaseous H 2 . Such high efficiency, without any CO x emission, can be sustained for over a thousand-hour operation without obvious degradation.

Sb-Substituted Cs₂AgBiBr₆/g-C₃N₄ Composite for Photocatalytic C(sp³)–H Bond Activation in Toluene

Publisher: American Chemical Society (ACS)

Date: 03-04-2023

DOI: 10.1021/ACS.CHEMMATER.2C03390

Reflections on substrate water and dioxygen formation

Publisher: Elsevier BV

Date: 08-2013

DOI: 10.1016/J.BBABIO.2013.01.013

Abstract: This brief article aims at presenting a concise summary of all experimental findings regarding substrate water-binding to the Mn4CaO5 cluster in photosystem II. Mass spectrometric and spectroscopic results are interpreted in light of recent structural information of the water oxidizing complex obtained by X-ray crystallography, spectroscopy and theoretical modeling. Within this framework current proposals for the mechanism of photosynthetic water-oxidation are evaluated. This article is part of a Special Issue entitled: Metals in Bioenergetics and Biomimetics Systems.

Grandparents fostering orphans: influences of protective factors on their health and well-being.

Publisher: Informa UK Limited

Date: 10-07-2014

DOI: 10.1080/07399332.2014.916294

Abstract: In this study the authors explore the impact of protective factors on the health and well-being of grandmothers who are primary caregivers. Although researchers in Africa have studied grandparents who assume primary caregiving responsibilities, it is rare that they do so from a strength perspective, hence the need to examine the utility of personal, social, and environmental assets on caregiving. Grandmothers are the primary caregivers of orphaned children due to HIV and AIDS deaths thus it becomes pertinent to establish how they are coping without the provision of social security. The results of this study will be beneficial to all stakeholders interested in the welfare of elders with similar responsibilities. Knowledge about the health and well-being of grandmothers who are caregivers will assist public service and private sectors to formulate viable policies concerning elderly carers who foster orphans, particularly in countries with high HIV prevalence.

Newspapers as tools to promote national agenda: How Chinese Communist Party newspapers frame images of the South China Sea disputes for national and international audiences

Publisher: SAGE Publications

Date: 27-03-2022

DOI: 10.1177/17427665221081945

Abstract: Using both quantitative and qualitative content analysis, this study investigates how two Chinese Communist Party newspapers frame the same story to international and national audiences. The empirical findings illustrate how propaganda techniques originally developed and applied in Western and democratic countries have been adopted and refined by newspapers in a state-run Communist press environment to create frames that best align with the cultural and political predispositions of domestic and international readers. The findings suggest Chinese authorities understand Western communication theory and appreciate how that theory can be applied to disseminate messages to both foreign and domestic audiences.

Defect engineering enhances plasmonic-hot electrons exploitation for CO₂ reduction over polymeric catalysts

Publisher: Royal Society of Chemistry (RSC)

Date: 2023

DOI: 10.1039/D3NH00348E

A First-Principles Approach to the Calculation of the on-Site Zero-Field Splitting in Polynuclear Transition Metal Complexes

Publisher: American Chemical Society (ACS)

Date: 23-10-2014

DOI: 10.1021/IC502081C

Abstract: The interpretation of electron paramagnetic resonance spectra of polynuclear transition metal complexes in terms of in idual contributions from each paramagnetic center can be greatly facilitated by the availability of theoretical methods that enable the reliable prediction of local spectroscopic parameters. In this work we report an approach that enables the application of multireference ab initio methods for the calculation of local zero field splitting tensors, one of the leading terms in the spin Hamiltonian for exchange-coupled systems of high nuclearity. The method referred to as local complete active space configuration interaction (L-CASCI) represents a multireference calculation with an active space composed of local orbitals of the center of interest. By successive permutation of the active space to include the localized orbitals corresponding to a particular center of the complex, all on-site parameters can be easily obtained at a high-level of theory with a corresponding low computational cost. Benchmark calculations on synthetic complexes confirm the validity of the approach. As an ex le of the applicability of the L-CASCI method to large systems, we determine the local anisotropy of the Mn(III) ion of the tetranuclear manganese cluster of photosystem II in both structural forms of its S2 state.

Selective N2/H2O adsorption onto 2D amphiphilic amorphous photocatalysts for ambient gas-phase nitrogen fixation

Publisher: Elsevier BV

Date: 10-2021

DOI: 10.1016/J.APCATB.2021.120240

Water oxidation in photosystem II

Publisher: Springer Science and Business Media LLC

Date: 11-06-2019

DOI: 10.1007/S11120-019-00648-3

Five-coordinate Mn ^IV intermediate in the activation of nature’s water splitting cofactor

Publisher: Proceedings of the National Academy of Sciences

Date: 07-08-2019

DOI: 10.1073/PNAS.1817526116

Abstract: Recent results have shown that nature’s water splitting catalyst inserts an additional water molecule into what appears to be a solvent inaccessible site late in its reaction cycle. The emerging consensus of the field is that this water molecule is one of the substrates of the reaction. Here, we show that this water molecule does not come directly from solvent. It instead represents an earlier bound water, which is inserted into this site via facile structural tautomerism. The trigger for this process is cofactor oxidation. This then allows an additional water to bind from solvent to a more open site of the cofactor. In this way the cofactor carefully regulates water uptake, preventing water insertion earlier in the reaction cycle.

Electronic structure of the oxygen-evolving complex in photosystem II prior to O-O bond formation

Publisher: American Association for the Advancement of Science (AAAS)

Date: 15-08-2014

DOI: 10.1126/SCIENCE.1254910

Abstract: Plants transform water into the oxygen we breathe using a protein-bound cluster of four manganese (Mn) ions and a calcium ion. Cox et al. now establish the precise electronic structure in that cluster immediately before formation of the O-O bond (see the Perspective by Britt and Oyala). Using the technique of electron paramagnetic resonance spectroscopy, they confirm a hypothesis that all four Mn ions are octahedrally coordinated and in the 4+ oxidation state. Such clues to the efficiency of the photosynthetic process, so essential to life on Earth, may also facilitate the development of artificial waters-plitting catalysts. Science , this issue p. 804 see also p. 736

Redox‐Addressable Single‐Molecule Junctions Incorporating a Persistent Organic Radical**

Publisher: Wiley

Date: 05-04-2022

DOI: 10.1002/ANGE.202116985

Abstract: Integrating radical (open‐shell) species into non‐cryogenic nanodevices is key to unlocking the potential of molecular electronics. While many efforts have been devoted to this issue, in the absence of a chemical/electrochemical potential the open‐shell character is generally lost in contact with the metallic electrodes. Herein, single‐molecule devices incorporating a 6‐oxo‐verdazyl persistent radical have been fabricated using break‐junction techniques. The open‐shell character is retained at room temperature, and electrochemical gating permits in situ reduction to a closed‐shell anionic state in a single‐molecule transistor configuration. Furthermore, electronically driven rectification arises from bias‐dependent alignment of the open‐shell resonances. The integration of radical character, transistor‐like switching, and rectification in a single molecular component paves the way to further studies of the electronic, magnetic, and thermoelectric properties of open‐shell species.

Perspectives on integer programming for time-dependent models

Publisher: Springer Science and Business Media LLC

Date: 10-05-2019

DOI: 10.1007/S11750-019-00514-4

The first tyrosyl radical intermediate formed in the S2–S3 transition of photosystem II

Publisher: Royal Society of Chemistry (RSC)

Date: 2014

DOI: 10.1039/C4CP00696H

Abstract: The EPR "split signals" represent key intermediates of the S-state cycle where the redox active D1-Tyr161 (YZ) has been oxidized by the reaction center of the photosystem II enzyme to its tyrosyl radical form, but the successive oxidation of the Mn4CaO5 cluster has not yet occurred (SiYZ˙). Here we focus on the S2YZ˙ state, which is formed en route to the final metastable state of the catalyst, the S3 state, the state which immediately precedes O-O bond formation. Quantum chemical calculations demonstrate that both isomeric forms of the S2 state, the open and closed cubane isomers, can form states with an oxidized YZ˙ residue without prior deprotonation of the Mn4CaO5 cluster. The two forms are expected to lie close in energy and retain the electronic structure and magnetic topology of the corresponding S2 state of the inorganic core. As expected, tyrosine oxidation results in a proton shift towards His190. Analysis of the electronic rearrangements that occur upon formation of the tyrosyl radical suggests that a likely next step in the catalytic cycle is the deprotonation of a terminal water ligand (W1) of the Mn4CaO5 cluster. Diamagnetic metal ion substitution is used in our calculations to obtain the molecular g-tensor of YZ˙. It is known that the gx value is a sensitive probe not only of the extent of the proton shift between the tyrosine-histidine pair, but also of the polarization environment of the tyrosine, especially about the phenolic oxygen. It is shown for PSII that this environment is determined by the Ca(2+) ion, which locates two water molecules about the phenoxyl oxygen, indirectly modulating the oxidation potential of YZ.

EPR Spectroscopy and the Electronic Structure of the Oxygen-Evolving Complex of Photosystem II

Publisher: Springer Science and Business Media LLC

Date: 05-02-2013

DOI: 10.1007/S00723-012-0437-3

Oxygen‐evolving Photosystem II

Publisher: Wiley

Date: 14-03-2014

DOI: 10.1002/9781119951438.EIBC2166

Abstract: Photosystem II is the enzyme that catalyzes the thermodynamically demanding splitting of water, yielding dioxygen, protons, and electrons, a sunlight‐driven reaction that forms the foundation of oxygenic photosynthesis. The latest results from high‐resolution crystallographic models have led to a refined view of the overall structure of the enzyme and the arrangement of the cofactors involved in light harvesting, charge separation, and electron transfer. In addition, combined efforts from protein crystallography, spectroscopy, and computational chemistry have greatly improved the understanding of the tetramanganese–calcium cluster of the oxygen‐evolving complex, the site of water oxidation. The most important features of the geometric and electronic structures of the oxygen‐evolving complex for the earlier reaction cycle intermediates are now sufficiently well understood such that connections between several structural features and spectroscopic observables can be made with confidence. Advanced spectroscopic techniques have also identified possible sites where the substrate water binds. Although the details of the actual mechanism of biological water oxidation remain elusive, experimental and theoretical studies impose an increasing number of constraints that significantly limit the number of mechanistic possibilities for O–O bond formation.

On the assignment of PSHB in D1/D2/ cytb559 reaction centers

Publisher: Elsevier BV

Date: 02-2010

DOI: 10.1016/J.PHPRO.2010.01.227

The D1-173 amino acid is a structural determinant of the critical interaction between D1-Tyr161 (TyrZ) and D1-His190 in Photosystem II

Publisher: Elsevier BV

Date: 12-2014

DOI: 10.1016/J.BBABIO.2014.08.008

Abstract: The main cofactors of Photosystem II (PSII) are borne by the D1 and D2 subunits. In the thermophilic cyanobacterium Thermosynechococcus elongatus, three psbA genes encoding D1 are found in the genome. Among the 344 residues constituting the mature form of D1, there are 21 substitutions between PsbA1 and PsbA3, 31 between PsbA1 and PsbA2, and 27 between PsbA2 and PsbA3. In a previous study (Sugiura et al., J. Biol. Chem. 287 (2012), 13336-13347) we found that the oxidation kinetics and spectroscopic properties of TyrZ were altered in PsbA2-PSII when compared to PsbA(1/3)-PSII. The comparison of the different amino acid sequences identified the residues Cys144 and Pro173 found in PsbA1 and PsbA3, as being substituted in PsbA2 by Pro144 and Met173, and thus possible candidates accounting for the changes in the geometry and/or the environment of the TyrZ/His190 phenol/imidizol motif. Indeed, these amino acids are located upstream of the α-helix bearing TyrZ and between the two α-helices bearing TyrZ and its hydrogen-bonded partner, D1/His190. Here, site-directed mutants of PSII, PsbA3/Pro173Met and PsbA2/Met173Pro, were analyzed using X- and W-band EPR and UV-visible time-resolved absorption spectroscopy. The Pro173Met substitution in PsbA2-PSII versus PsbA3-PSII is shown to be the main structural determinant of the previously described functional differences between PsbA2-PSII and PsbA3-PSII. In PsbA2-PSII and PsbA3/Pro173Met-PSII, we found that the oxidation of TyrZ by P680+● was specifically slowed during the transition between S-states associated with proton release. We thus propose that the increase of the electrostatic charge of the Mn4CaO5 cluster in the S2 and S3 states could weaken the strength of the H-bond interaction between TyrZ● and D1/His190 in PsbA2 versus PsbA3 and/or induce structural modification(s) of the water molecules network around TyrZ.

Homologous expression of the nrdF gene of Corynebacterium ammoniagenes strain ATCC 6872 generates a manganese-metallocofactor (R2F) and a stable tyrosyl radical (Y•) involved in ribonucleotide reducti

Publisher: Wiley

Date: 26-10-2010

DOI: 10.1111/J.1742-4658.2010.07885.X

Abstract: Ribonucleotide reduction, the unique step in the pathway to DNA synthesis, is catalyzed by enzymes via radical-dependent redox chemistry involving an array of erse metallocofactors. The nucleotide reduction gene (nrdF) encoding the metallocofactor containing small subunit (R2F) of the Corynebacterium ammoniagenes ribonucleotide reductase was reintroduced into strain C. ammoniagenes ATCC 6872. Efficient homologous expression from plasmid pOCA2 using the tac-promotor enabled purification of R2F to homogeneity. The chromatographic protocol provided native R2F with a high ratio of manganese to iron (30:1), high activity (69 μmol 2'-deoxyribonucleotide·mg⁻¹ ·min⁻¹) and distinct absorption at 408 nm, characteristic of a tyrosyl radical (Y˙), which is sensitive to the radical scavenger hydroxyurea. A novel enzyme assay revealed the direct involvement of Y˙ in ribonucleotide reduction because 0.2 nmol 2'-deoxyribonucleotide was formed, driven by 0.4 nmol Y˙ located on R2F. X-band electron paramagnetic resonance spectroscopy demonstrated a tyrosyl radical at an effective g-value of 2.004. Temperature dependent X/Q-band EPR studies revealed that this radical is coupled to a metallocofactor. Similarities of the native C. ammoniagenes ribonucleotide reductase to the in vitro activated Escherichia coli class Ib enzyme containing a dimanganese(III)-tyrosyl metallocofactor are discussed.

Solvent water interactions within the active site of the membrane type I matrix metalloproteinase

Publisher: Royal Society of Chemistry (RSC)

Date: 2017

DOI: 10.1039/C7CP05572B

Abstract: EPR, MCD, crystallographic and QM/MM investigation reveals the role of the 1st solvation shell waters in the catalytic mechanism of MT1-MMP.

Disubstituted Aminoanthraquinone-Based Photoinitiators for Free Radical Polymerization and Fast 3D Printing under Visible Light

Publisher: American Chemical Society (ACS)

Date: 06-12-2018

DOI: 10.1021/ACS.MACROMOL.8B02145

Metal-free ribonucleotide reduction powered by a DOPA radical in Mycoplasma pathogens

Publisher: Springer Science and Business Media LLC

Date: 31-10-2018

DOI: 10.1038/S41586-018-0653-6

Ammonia binding to the oxygen-evolving complex of photosystem II identifies the solvent-exchangeable oxygen bridge (μ-oxo) of the manganese tetramer

Publisher: Proceedings of the National Academy of Sciences

Date: 10-09-2013

DOI: 10.1073/PNAS.1304334110

Abstract: The assignment of the two substrate water sites of the tetra-manganese penta-oxygen calcium (Mn 4 O 5 Ca) cluster of photosystem II is essential for the elucidation of the mechanism of biological O-O bond formation and the subsequent design of bio-inspired water-splitting catalysts. We recently demonstrated using pulsed EPR spectroscopy that one of the five oxygen bridges (μ-oxo) exchanges unusually rapidly with bulk water and is thus a likely candidate for one of the substrates. Ammonia, a water analog, was previously shown to bind to the Mn 4 O 5 Ca cluster, potentially displacing a water/substrate ligand [Britt RD, et al. (1989) J Am Chem Soc 111(10):3522–3532]. Here we show by a combination of EPR and time-resolved membrane inlet mass spectrometry that the binding of ammonia perturbs the exchangeable μ-oxo bridge without drastically altering the binding/exchange kinetics of the two substrates. In combination with broken-symmetry density functional theory, our results show that ( i ) the exchangable μ-oxo bridge is O5 {using the labeling of the current crystal structure [Umena Y, et al. (2011) Nature 473(7345):55–60]} ( ii ) ammonia displaces a water ligand to the outer manganese (Mn A4 -W1) and ( iii ) as W1 is trans to O5, ammonia binding elongates the Mn A4 -O5 bond, leading to the perturbation of the μ-oxo bridge resonance and to a small change in the water exchange rates. These experimental results support O-O bond formation between O5 and possibly an oxyl radical as proposed by Siegbahn and exclude W1 as the second substrate water.

Effects of formate binding on the quinone–iron electron acceptor complex of photosystem II

Publisher: Elsevier BV

Date: 02-2011

DOI: 10.1016/J.BBABIO.2010.10.019

Abstract: EPR was used to study the influence of formate on the electron acceptor side of photosystem II (PSII) from Thermosynechococcus elongatus. Two new EPR signals were found and characterized. The first is assigned to the semiquinone form of Q(B) interacting magnetically with a high spin, non-heme-iron (Fe²(+), S=2) when the native bicarbonate/carbonate ligand is replaced by formate. This assignment is based on several experimental observations, the most important of which were: (i) its presence in the dark in a significant fraction of centers, and (ii) the period-of-two variations in the concentration expected for Q(B)(•-) when PSII underwent a series of single-electron turnovers. This signal is similar but not identical to the well-know formate-modified EPR signal observed for the Q(A)(•-)Fe²(+) complex (W.F.J. Vermaas and A.W. Rutherford, FEBS Lett. 175 (1984) 243-248). The formate-modified signals from Q(A)(•-)Fe²(+) and Q(B)(•-)Fe²(+) are also similar to native semiquinone-iron signals (Q(A)(•-)Fe²(+)/Q(B)(•-)Fe²(+)) seen in purple bacterial reaction centers where a glutamate provides the carboxylate ligand to the iron. The second new signal was formed when Q(A)(•-) was generated in formate-inhibited PSII when the secondary acceptor was reduced by two electrons. While the signal is reminiscent of the formate-modified semiquinone-iron signals, it is broader and its main turning point has a major sub-peak at higher field. This new signal is attributed to the Q(A)(•-)Fe²(+) with formate bound but which is perturbed when Q(B) is fully reduced, most likely as Q(B)H₂ (or possibly Q(B)H(•-) or Q(B)(²•-)). Flash experiments on formate-inhibited PSII monitoring these new EPR signals indicate that the outcome of charge separation on the first two flashes is not greatly modified by formate. However on the third flash and subsequent flashes, the modified Q(A)(•-)Fe²(+)Q(B)H₂ signal is trapped in the EPR experiment and there is a marked decrease in the quantum yield of formation of stable charge pairs. The main effect of formate then appears to be on Q(B)H₂ exchange and this agrees with earlier studies using different methods.

Semiquinone–Iron Complex of Photosystem II: EPR Signals Assigned to the Low-Field Edge of the Ground State Doublet of Q_A^•–Fe²⁺ and Q_B^•–Fe^2+<

Publisher: American Chemical Society (ACS)

Date: 16-06-2011

DOI: 10.1021/BI200313P

Abstract: The quinone-iron complex of the electron acceptor complex of Photosystem II was studied by EPR spectroscopy in Thermosynechococcus elongatus. New g ∼ 2 features belonging to the EPR signal of the semiquinone forms of the primary and secondary quinone, i.e., Q(A)(•-)Fe(2+) and Q(B)(•-)Fe(2+), respectively, are reported. In previous studies, these signals were missed because they were obscured by the EPR signal arising from the stable tyrosyl radical, TyrD(•). When the TyrD(•) signal was removed, either by chemical reduction or by the use of a mutant lacking TyrD, the new signals dominated the spectrum. For Q(A)(•-)Fe(2+), the signal was formed by illumination at 77 K or by sodium dithionite reduction in the dark. For Q(B)(•-)Fe(2+), the signal showed the characteristic period-of-two variations in its intensity when generated by a series of laser flashes. The new features showed relaxation characteristics comparable to those of the well-known features of the semiquinone-iron complexes and showed a temperature dependence consistent with an assignment to the low-field edge of the ground state doublet of the spin system. Spectral simulations are consistent with this assignment and with the current model of the spin system. The signal was also present in Q(B)(•-)Fe(2+) in plant Photosystem II, but in plants, the signal was not detected in the Q(A)(•-)Fe(2+) state.

Recent developments in biological water oxidation

Publisher: Elsevier BV

Date: 04-2016

DOI: 10.1016/J.CBPA.2016.02.007

Abstract: Rapid progress has been made in the last five years towards resolution of the structure of nature's water splitting catalyst - a Mn4O5Ca cofactor embedded in Photosystem II - especially in the field of X-ray crystallography. In addition, recent magnetic resonance data have allowed the structure of the cofactor to be accessed in its last metastable intermediate state, prior to O-O bond formation. This activated form of the catalyst is geometrically similar to that seen by X-ray crystallography, which represents the resting state of the cofactor, but requires the coordination of an additional water molecule to the cofactor, rendering all Mn ions six coordinate. Importantly, it locates two water derived, Mn bound oxygen ligands in close proximity. It is these two oxygen ligands that likely form the product O2 molecule, as proposed earlier by quantum chemical modeling. Current views on the molecular level events that facilitate catalyst activation, that is, catalyst/substrate deprotonation, Mn oxidation and water molecule insertion are briefly described.

The primary donor of far-red photosystem II: ChlD1 or PD2?

Publisher: Elsevier BV

Date: 10-2020

DOI: 10.1016/J.BBABIO.2020.148248

A Tyrosyl−Dimanganese Coupled Spin System is the Native Metalloradical Cofactor of the R2F Subunit of the Ribonucleotide Reductase of Corynebacterium ammoniagenes

Publisher: American Chemical Society (ACS)

Date: 22-07-2010

DOI: 10.1021/JA1036995

Abstract: The X-ray crystallographic structure of the native R2F subunit of the ribonucleotide reductase (RNR) of Corynebacterium ammoniagenes ATCC 6872 is reported, with a resolution of 1.36 A. The metal site contains an oxo/hydroxo-bridged manganese dimer, located near a tyrosine residue (Y115). The coordination of the manganese dimer and its distance to a nearby tyrosine residue resemble the di-iron metalloradical cofactor of class I RNR from Escherichia coli . Multifrequency EPR measurements of the highly active C. ammoniagenes R2F subunit show that the metal site contains a ferromagnetically exchange-coupled Mn(III)Mn(III) dimer weakly coupled to a tyrosyl radical. A mechanism for the metalloradical cofactor (Mn(III)Mn(III)Y(*)) generation is proposed. H(2)O(2) (HO(2)(-)) instead of O(2) is hypothesized as physiological oxidant for the Mn dimer which in turn oxidizes the tyrosine Y115. Changes in the ligand sphere of both manganese ions during metalloradical generation direct the complex formation of this cofactor, disfavoring alternate reaction pathways such as H(2)O(2) dismutation, as observed for manganese catalase, a structural analogue of the R2F metal site. The presented results demonstrate the importance of manganese for radical formation in this RNR and confirm the assignment of this enzyme to class Ib.

A five-coordinate Mn(iv) intermediate in biological water oxidation: spectroscopic signature and a pivot mechanism for water binding

Publisher: Royal Society of Chemistry (RSC)

Date: 2016

DOI: 10.1039/C5SC03124A

Abstract: The identification of a unique intermediate in biological water oxidation establishes the water binding mechanism in the S 2 to S 3 state transition.

New Perspectives on Photosystem II Reaction Centres

Publisher: CSIRO Publishing

Date: 2020

DOI: 10.1071/CH19478

Abstract: We apply the differential optical spectroscopy techniques of circular polarisation of luminescence (CPL) and magnetic CPL (MCPL) to the study of isolated reaction centres (RCs) of photosystem II (PS II). The data and subsequent analysis provide insights into aspects of the RC chromophore site energies, exciton couplings, and heterogeneities. CPL measurements are able to identify weak luminescence associated with the unbound chlorophyll-a (Chl-a) present in the s le. The overall sign and magnitude of the CPL observed relates well to the circular dichroism (CD) of the s le. Both CD and CPL are reasonably consistent with modelling of the RC exciton structure. The MCPL observed for the free Chl-a luminescence component in the RC s les is also easily understandable, but the MCPL seen near 680nm at 1.8K is anomalous, appearing to have a narrow, strongly negative component. A negative sign is inconsistent with MCPL of (exciton coupled) Qy states of either Chl-a or pheophytin-a (Pheo-a). We propose that this anomaly may arise as a result of the luminescence from a transient excited state species created following photo-induced charge separation within the RC. A comparison of CD spectra and modelling of RC preparations having a different number of pigments suggests that the non-conservative nature of the CD spectra observed is associated with the ‘special pair’ pigments PD1 and PD2.

Defect engineering for creating and enhancing bulk photovoltaic effect in centrosymmetric materials

Publisher: Royal Society of Chemistry (RSC)

Date: 2021

DOI: 10.1039/D1TA02699B

Abstract: A defect-engineering strategy is reported to enhance the photovoltaic performance of BiNbO 4 . Synergetic effects: enhanced light absorption, ferroelectric-like depolarization and interfacial polarization on BiNbO 4 homojunction lead to an increase in the photovoltaic effects.

Structure, ligands and substrate coordination of the oxygen-evolving complex of photosystem II in the S2 state: a combined EPR and DFT study

Publisher: Royal Society of Chemistry (RSC)

Date: 2014

DOI: 10.1039/C3CP55017F

Abstract: The S2 state of the oxygen-evolving complex of photosystem II, which consists of a Mn4O5Ca cofactor, is EPR-active, typically displaying a multiline signal, which arises from a ground spin state of total spin ST = 1/2. The precise appearance of the signal varies amongst different photosynthetic species, preparation and solvent conditions/compositions. Over the past five years, using the model species Thermosynechococcus elongatus, we have examined modifications that induce changes in the multiline signal, i.e. Ca(2+)/Sr(2+)-substitution and the binding of ammonia, to ascertain how structural perturbations of the cluster are reflected in its magnetic/electronic properties. This refined analysis, which now includes high-field (W-band) data, demonstrates that the electronic structure of the S2 state is essentially invariant to these modifications. This assessment is based on spectroscopies that examine the metal centres themselves (EPR, (55)Mn-ENDOR) and their first coordination sphere ligands ((14)N/(15)N- and (17)O-ESEEM, -HYSCORE and -EDNMR). In addition, extended quantum mechanical models from broken-symmetry DFT now reproduce all EPR, (55)Mn and (14)N experimental magnetic observables, with the inclusion of second coordination sphere ligands being crucial for accurately describing the interaction of NH3 with the Mn tetramer. These results support a mechanism of multiline heterogeneity reported for species differences and the effect of methanol [Biochim. Biophys. Acta, Bioenerg., 2011, 1807, 829], involving small changes in the magnetic connectivity of the solvent accessible outer MnA4 to the cuboidal unit Mn3O3Ca, resulting in predictable changes of the measured effective (55)Mn hyperfine tensors. Sr(2+) and NH3 replacement both affect the observed (17)O-EDNMR signal envelope supporting the assignment of O5 as the exchangeable μ-oxo bridge and it acting as the first site of substrate inclusion.

Redox‐Addressable Single‐Molecule Junctions Incorporating a Persistent Organic Radical**

Publisher: Wiley

Date: 05-04-2022

DOI: 10.1002/ANIE.202116985

Abstract: Integrating radical (open‐shell) species into non‐cryogenic nanodevices is key to unlocking the potential of molecular electronics. While many efforts have been devoted to this issue, in the absence of a chemical/electrochemical potential the open‐shell character is generally lost in contact with the metallic electrodes. Herein, single‐molecule devices incorporating a 6‐oxo‐verdazyl persistent radical have been fabricated using break‐junction techniques. The open‐shell character is retained at room temperature, and electrochemical gating permits in situ reduction to a closed‐shell anionic state in a single‐molecule transistor configuration. Furthermore, electronically driven rectification arises from bias‐dependent alignment of the open‐shell resonances. The integration of radical character, transistor‐like switching, and rectification in a single molecular component paves the way to further studies of the electronic, magnetic, and thermoelectric properties of open‐shell species.

Nitrite Dismutase Reaction Mechanism: Kinetic and Spectroscopic Investigation of the Interaction between Nitrophorin and Nitrite

Publisher: American Chemical Society (ACS)

Date: 24-03-2015

DOI: 10.1021/JA512938U

Abstract: Nitrite is an important metabolite in the physiological pathways of NO and other nitrogen oxides in both enzymatic and nonenzymatic reactions. The ferric heme b protein nitrophorin 4 (NP4) is capable of catalyzing nitrite disproportionation at neutral pH, producing NO. Here we attempt to resolve its disproportionation mechanism. Isothermal titration calorimetry of a gallium(III) derivative of NP4 demonstrates that the heme iron coordinates the first substrate nitrite. Contrary to previous low-temperature EPR measurements, which assigned the NP4-nitrite complex electronic configuration solely to a low-spin (S = 1/2) species, electronic absorption and resonance Raman spectroscopy presented here demonstrate that the NP4-NO2(-) cofactor exists in a high-spin/low-spin equilibrium of 7:3 which is in fast exchange in solution. Spin-state interchange is taken as evidence for dynamic NO2(-) coordination, with the high-spin configuration (S = 5/2) representing the reactive species. Subsequent kinetic measurements reveal that the dismutation reaction proceeds in two discrete steps and identify an {FeNO}(7) intermediate species. The first reaction step, generating the {FeNO}(7) intermediate, represents an oxygen atom transfer from the iron bound nitrite to a second nitrite molecule in the protein pocket. In the second step this intermediate reduces a third nitrite substrate yielding two NO molecules. A nearby aspartic acid residue side-chain transiently stores protons required for the reaction, which is crucial for NPs' function as nitrite dismutase.

Microwave Dielectric Materials with Defect-Dipole Clusters Induced Colossal Permittivity and Ultra-low Loss

Publisher: American Chemical Society (ACS)

Date: 04-11-2021

DOI: 10.1021/ACSAELM.1C00236

Intramolecular light induced activation of a Salen–MnIII complex by a ruthenium photosensitizer

Publisher: Royal Society of Chemistry (RSC)

Date: 2010

DOI: 10.1039/C0CC01710H

Abstract: We have designed a molecular system consisting of a heteroleptic [Ru(bpy)(2)L](2+) chromophore covalently linked to a Mn(III)-Salen unit. We demonstrate the light induced oxidation of the Mn(III) center in this putative photo-catalyst assembly to a Mn(IV) high spin intermediate. Both oxidation states have been characterized by transient absorption and EPR techniques.

Prognostic Implications of Left Ventricular Dilation in Patients With Nonischemic Heart Failure: Interactions With Restrictive Filling Pattern and Mitral Regurgitation

Publisher: Wiley

Date: 18-04-2012

DOI: 10.1111/J.1751-7133.2011.00281.X

Abstract: The aim of this study was to evaluate whether small left ventricular (LV) volumes increase the negative prognostic impact of a restrictive filling pattern (RFP) and that of mitral regurgitation (MR) in patients with nonischemic heart failure (HF). The Meta-analysis Research Group in Echocardiography (MeRGE) is a meta-analysis that collated in idual patient data from several prospective echocardiography outcome studies. This analysis was restricted to 10 studies and 601 patients with nonischemic HF. The role of MR was tested in a subgroup of 252 patients. A total of 106 deaths occurred during a median follow-up of 32 months. At multivariate analysis, RFP (hazard ratio [HR], 4.16 95% confidence interval [CI], 1.54-11.23 P=.005) and New York Heart Association class III or IV (HR, 2.15 95% CI, 1.33-3.47 P=.001) were the independent predictors of poor prognosis, and there was no statistically significant interaction between LV dilation and RFP. Moderate/severe MR was associated with poorer outcome in the group of patients with normal volumes, whereas it was not a significant predictor of mortality in patients with any degree of LV dilation. In patients with nonischemic HF, RFP is the most important indicator of poor prognosis, irrespective of the degree of LV dilation. Normal LV volumes increase the negative prognostic impact of moderate to severe MR.

Hole-Pinned Defect Clusters for a Large Dielectric Constant up to GHz in Zinc and Niobium Codoped Rutile SnO₂

Publisher: American Chemical Society (ACS)

Date: 02-11-2021

DOI: 10.1021/ACSAMI.1C09632

Ketyl Radicals Generated from Magnesium(I) Compounds: Useful Reagents for C−C Bond Forming Reactions

Publisher: Wiley

Date: 16-03-2023

DOI: 10.1002/CHEM.202300135

Abstract: The aryl ketones, 9‐fluorenone (fluor), 9‐xanthenone (xanth) and 9,10‐anthraquinone (anth), were reacted with β‐diketiminato dimagnesium(I) compounds, [{( Ar Nacnac)Mg} 2 ] ( Ar Nacnac=[HC(MeCNAr) 2 ] − , Ar=mesityl (Mes) or 2,6‐diisopropylphenyl (Dip)). This gave stable magnesium ketyl complexes which are monomeric, [( Ar Nacnac)(DMAP)Mg(fluor⋅)] (Ar=Mes or Dip, DMAP=4‐dimethylaminopyridine) and [( Mes Nacnac)Mg(xanth⋅)(xanth)] dimeric, [{( Mes Nacnac)Mg(μ‐fluor⋅)} 2 ], or tetrameric, [{( Dip Nacnac)Mg(μ‐anth⋅)} 4 ]. In contrast, di‐2‐pyridylketone (OCPy 2 ) is doubly reduced with [{( Xyl Nacnac)Mg} 2 ] (Xyl=xylyl) to give a diamagnetic alkoxy/amido complex, [{( Xyl Nacnac)Mg} 2 (μ‐OCPy 2 )]. These complexes have been characterized by X‐ray crystallography, and in three cases, EPR spectroscopy. Regioselective C−C hetero‐coupling reactions between magnesium ketyls and phenanthroline (phen) have yielded the alkoxy compounds, [( Mes Nacnac)Mg(OCR 2 ‐2‐phen)] (OCR 2 =xanth, OCPh 2 or OC(Ph)(2‐Me−Ph)). In addition, homo‐ C−C couplings of the enones, chalcone (chalc) and dibenziylideneactetone (DBA), using magnesium(I) reducing agents, have afforded dimagnesium enolates, [{( Ar Nacnac)(THF)Mg} 2 (μ‐chalc 2 )] (Ar=Mes or Xyl) and [{( Dip Nacnac)(THF)Mg} 2 (μ‐DBA 2 )]. A pinacol coupling reaction between [{( Dip Nacnac)Mg} 2 ] and 2‐adamantanone (OAd) yielded [{( Dip Nacnac)(OAd)Mg} 2 (μ‐OAd 2 )], presumably via a ketyl intermediate. This study further highlights the utility magnesium(I) compounds have as selective reducing agents in organic transformations.

Crystalline Germanium(I) and Tin(I) Centered Radical Anions

Publisher: Wiley

Date: 23-03-2022

DOI: 10.1002/ANGE.202201248

Abstract: An isostructural series of heavy Group 14 E(I) radical anions (Ge, Sn, Pb), stabilized by a bulky xanthene‐based diamido ligand are reported. The radical anions were synthesised by the one‐electron reduction of their corresponding E(II) precursor complexes with sodium naphthalenide in THF, yielding the radical anions as charge‐separated sodium salts. The series of main group radicals have been comprehensively characterized by EPR spectroscopy, X‐ray crystallography and DFT analysis, which reveal that in all cases, the spin density of the unpaired electron almost exclusively resides in a p ‐orbital of π symmetry located on the Group 14 center.

Substituent Effects on Photoinitiation Ability of Monoaminoanthraquinone-Based Photoinitiating Systems for Free Radical Photopolymerization under LEDs

Publisher: Wiley

Date: 08-05-2020

DOI: 10.1002/MARC.202000166

Redox-dependent Ligand Switching in a Sensory Heme-binding GAF Domain of the Cyanobacterium Nostoc sp. PCC7120

Publisher: Elsevier BV

Date: 07-2015

DOI: 10.1074/JBC.M115.654087

Divergent assembly mechanisms of the manganese/iron cofactors in R2lox and R2c proteins

Publisher: Elsevier BV

Date: 09-2016

DOI: 10.1016/J.JINORGBIO.2016.04.019

Abstract: A manganese/iron cofactor which performs multi-electron oxidative chemistry is found in two classes of ferritin-like proteins, the small subunit (R2) of class Ic ribonucleotide reductase (R2c) and the R2-like ligand-binding oxidase (R2lox). It is unclear how a heterodimeric Mn/Fe metallocofactor is assembled in these two related proteins as opposed to a homodimeric Fe/Fe cofactor, especially considering the structural similarity and proximity of the two metal-binding sites in both protein scaffolds and the similar first coordination sphere ligand preferences of Mn

Metal oxidation states in biological water splitting

Publisher: Royal Society of Chemistry (RSC)

Date: 2015

DOI: 10.1039/C4SC03720K

Abstract: A central question in biological water splitting concerns the oxidation states of the manganese ions that comprise the oxygen-evolving complex of photosystem II.

Spin State as a Marker for the Structural Evolution of Nature’s Water-Splitting Catalyst

Publisher: American Chemical Society (ACS)

Date: 23-12-2015

DOI: 10.1021/ACS.INORGCHEM.5B02578

Abstract: In transition-metal complexes, the geometric structure is intimately connected with the spin state arising from magnetic coupling between the paramagnetic ions. The tetramanganese-calcium cofactor that catalyzes biological water oxidation in photosystem II cycles through five catalytic intermediates, each of which adopts a specific geometric and electronic structure and is thus characterized by a specific spin state. Here, we review spin-structure correlations in Nature's water-splitting catalyst. The catalytic cycle of the Mn4O5Ca cofactor can be described in terms of spin-dependent reactivity. The lower "inactive" S states of the catalyst, S0 and S1, are characterized by low-spin ground states, SGS = 1/2 and SGS = 0. This is connected to the "open cubane" topology of the inorganic core in these states. The S2 state exhibits structural and spin heterogeneity in the form of two interconvertible isomers and is identified as the spin-switching point of the catalytic cycle. The first S2 state form is an open cubane structure with a low-spin SGS = 1/2 ground state, whereas the other represents the first appearance of a closed cubane topology in the catalytic cycle that is associated with a higher-spin ground state of SGS = 5/2. It is only this higher-spin form of the S2 state that progresses to the "activated" S3 state of the catalyst. The structure of this final metastable catalytic state was resolved in a recent report, showing that all manganese ions are six-coordinate. The magnetic coupling is dominantly ferromagnetic, leading to a high-spin ground state of SGS = 3. The ability of the Mn4O5Ca cofactor to adopt two distinct structural and spin-state forms in the S2 state is critical for water binding in the S3 state, allowing spin-state crossing from the inactive, low-spin configuration of the catalyst to the activated, high-spin configuration. Here we describe how an understanding of the magnetic properties of the catalyst in all S states has allowed conclusions on the catalyst function to be reached. A summary of recent literature results is provided that constrains the sequence of molecular level events: catalyst/substrate deprotonation, manganese oxidation, and water molecule insertion.

Structured near-infrared Magnetic Circular Dichroism spectra of the Mn4CaO5 cluster of PSII in T. vulcanus are dominated by Mn(IV) d-d ‘spin-flip’ transitions

Publisher: Elsevier BV

Date: 02-2018

DOI: 10.1016/J.BBABIO.2017.10.004

Abstract: Photosystem II passes through four metastable S-states in catalysing light-driven water oxidation. Variable temperature variable field (VTVH) Magnetic Circular Dichroism (MCD) spectra in PSII of Thermosynochococcus (T.) vulcanus for each S-state are reported. These spectra, along with assignments, provide a new window into the electronic and magnetic structure of Mn

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Max-Planck Institute For Chemical Energy Conversion

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Discovery Projects - Grant ID: DP200100338

Start Date: 06-2020

End Date: 03-2024

Amount: $390,000.00

Funder: Australian Research Council

ARC Future Fellowships - Grant ID: FT140100834

Start Date: 2016

End Date: 01-2021

Amount: $772,104.00

Funder: Australian Research Council

Linkage Infrastructure, Equipment And Facilities - Grant ID: LE180100026

Start Date: 04-2018

End Date: 12-2020

Amount: $178,839.00

Funder: Australian Research Council

Linkage Infrastructure, Equipment And Facilities - Grant ID: LE230100048

Start Date: 03-2023

End Date: 03-2024

Amount: $570,702.00

Funder: Australian Research Council

Linkage Infrastructure, Equipment And Facilities - Grant ID: LE170100023

Start Date: 03-2017

End Date: 06-2018

Amount: $650,000.00

Funder: Australian Research Council

Discovery Projects - Grant ID: DP210100088

Start Date: 05-2021

End Date: 05-2024

Amount: $463,000.00

Funder: Australian Research Council