ORCID Profile
0000-0002-2146-9065
Current Organisations
Aristotle University of Thessaloniki
,
University of York
,
Max-Planck-Institut für Kohlenforschung
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Plant Cell and Molecular Biology | Other Physical Sciences | Structural Chemistry and Spectroscopy | Biological Physics
Expanding Knowledge in the Physical Sciences | Climate Change Mitigation Strategies | Expanding Knowledge in the Biological Sciences |
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
Publisher: American Chemical Society (ACS)
Date: 21-01-2009
DOI: 10.1021/OM800999A
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.
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.
Publisher: American Chemical Society (ACS)
Date: 26-05-2007
DOI: 10.1021/CT700043W
Abstract: In previous studies of the agostic bonding in Tp(Me2)NbCl(R'CCR' ')(R), we have made use of a hybrid QM/MM protocol (B3LYP:UFF) where the QM partition ([Nb(Cl)(iPr)(HCCH)(NHCH2)3](+)) was rather small, but the optimized structures were nevertheless in apparently good agreement with experiment. In attempting to improve this model by expanding the size of the QM region, we were surprised to discover that a full QM treatment of the whole molecule using the B3LYP functional failed to locate an agostic structure of any kind. A systematic assessment of density functionals reveals that the poor performance of B3LYP in these systems is typical of all DFT methods that do not obey the uniform electron gas (UEG) correlation limit. Those that do obey the UEG limit, in contrast, provide an excellent description of the agostic structure when the complete ligand system is treated at the QM level. The apparently good performance of our original (B3LYP:UFF) hybrid method can be traced to a cancellation of errors: the B3LYP functional underestimates the intrinsic strength of the agostic interaction relative to competing Nb-Cl π bonding, but this is offset by an additional but unphysical electrostatic component to the agostic bond introduced by the presence of a positive charge in the QM region.
Publisher: American Chemical Society (ACS)
Date: 21-09-2010
DOI: 10.1021/JA1061505
Abstract: We report the synthesis of a niobium cyclopropyl complex, Tp(Me2)NbMe(c-C(3)H(5))(MeCCMe), and show that thermal loss of methane from this compound generates an intermediate that is capable of activating both aliphatic and aromatic C-H bonds. Isotopic labeling, trapping studies, a detailed kinetic analysis, and density functional theory all suggest that the active intermediate is an η(2)-cyclopropene complex formed via β-hydrogen abstraction rather than an isomeric cyclopropylidene species. C-H activation chemistry of this type represents a rather unusual reactivity pattern for η(2)-alkene complexes but is favored in this case by the strain in the C(3) ring which prevents the decomposition of the key intermediate via loss of cyclopropene.
Publisher: Wiley
Date: 02-12-2022
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.
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.
Publisher: Wiley
Date: 21-08-2012
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).
Publisher: American Association for the Advancement of Science (AAAS)
Date: 15-08-2014
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
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.
Publisher: Royal Society of Chemistry (RSC)
Date: 2004
DOI: 10.1039/B405609D
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.
Publisher: American Chemical Society (ACS)
Date: 15-11-2006
DOI: 10.1021/OM060712J
Publisher: Wiley
Date: 17-08-2007
Abstract: Abstraction of a Cl(-) ion from the P-chlorophospholes, R4C4PCl (R=Me, Et), produced the P--P bonded cations [R4C4P--P(Cl)C4R4]+, which reacted with PPh3 to afford X-ray crystallographically characterised phosphine-phosphenium cations [R4C4P(PPh3)]+ (R=Me, Et). Examination of the 31P-{1H} NMR spectrum of a solution (CH2Cl(2)) of [Et4C4P-(PPh3)]+ and PPh3 revealed broadening of the resonances due to both free and coordinated PPh3, and importantly it proved possible to measure the rate of exchange between PPh3 and [Et4C4P-(PPh3)]+ by line shape analysis (gNMR programmes). The results established second-order kinetics with DeltaS( not equal)=(-106.3+/-6.7) J mol(-1) K(-1), DeltaH( not equal)=(14.9+/-1.6) kJ mol(-1) and DeltaG( not equal) (298.15 K)=(46.6+/-2.6) kJ mol(-1), values consistent with a SN2-type pathway for the exchange process. This result contrasts with the dominant dissociative (S(N)1-type) pathway reported for the analogous exchange reactions of the [ArNCH2CH2N(Ar)P(PMe3)]+ ion, and to understand in more detail the factors controlling these two different reaction pathways, we have analysed the potential energy surfaces using density functional theory (DFT). The calculations reveal that, whilst phosphine exchange in [Et4C4P(PPh3)]+ and [ArNCH2CH2N(Ar)P(PMe3)](+) is superficially similar, the two cations differ significantly in both their electronic and steric requirements. The high electrophilicity of the phosphorus center in [Et4C4P]+, combined with strong pi-pi interactions between the ring and the incoming and outgoing phenyl groups of PPh3, favours the SN2-type over the SN1-type pathway in [Et4C4P(PPh3)]+. Effective pi-donation from the amide groups reduces the intrinsic electrophilicity of [ArNCH2CH2N(Ar)P]+, which, when combined with the steric bulk of the aryl groups, shifts the mechanism in favour of a dissociative SN1-type pathway.
Publisher: Wiley
Date: 23-03-2022
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.
Publisher: Wiley
Date: 13-10-2006
Publisher: Proceedings of the National Academy of Sciences
Date: 10-09-2013
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.
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.
Publisher: American Chemical Society (ACS)
Date: 03-2006
DOI: 10.1021/JA0581402
Abstract: The remarkable polymorphism exhibited by the linear tricobalt compounds, Co3(mu3-dpa)4Cl2 and Co3(mu3-dpa)4Br2, can be explained using a model involving three distinct electronic states. At high temperatures, symmetric and unsymmetric forms arise from the population of doublet (2A) and quartet (4B) states, respectively, the latter containing a localized high-spin Co(II) center. In the unsymmetric form, a reduction in temperature leads to a spin-crossover to a second quite distinct doublet state, 2B, where, uniquely, the d(x2-y2) character on the localized Co(II) center is distributed between the occupied and vacant manifolds. The variable population of the Co d(x2-y2) orbital gives rise to the continuous change in Co-Co and Co-N bond lengths as the temperature is decreased.
Publisher: Royal Society of Chemistry (RSC)
Date: 2008
DOI: 10.1039/B717204D
Abstract: The electronic structure of the 1,2,3-triphosphaindenyl ligand suggests that it should exhibit enhanced pi-acceptor properties when compared to the eta(5)-indenyl system this insight encouraged us to develop a simple synthetic pathway from 1,2-diphosphinobenzene to the 1,2,3-C(6)H(4)P(3) and 2-As-1,3-C(6)H(4)P(2) anions, both of which have been structurally characterised by X-ray crystallography as a bonus from these studies we also obtained the first structurally characterised organo derivative of the P(8) unit present in Hittorf's phosphorus.
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.
Publisher: American Chemical Society (ACS)
Date: 26-08-2015
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.
Publisher: American Chemical Society (ACS)
Date: 14-03-2017
DOI: 10.1021/ACS.INORGCHEM.6B02777
Abstract: A recently reported synthetic complex with a Mn
Publisher: The Royal Society
Date: 06-06-2015
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.
Publisher: Royal Society of Chemistry (RSC)
Date: 2008
DOI: 10.1039/B804401E
Abstract: A high yield of the tetraphosphaladderene, anti-tetraphosphatricyclo[4.2.0.0(2,5)]octa-3,7-diene, is obtained from reaction of the zirconocene 1,3-diphosphabicyclo[1.1.0]butane with Ph(2)SbCl(3) in THF or CH(2)Cl(2). Exploration of the reaction pathway using density functional theory suggests that an envelope-type adduct of Ph(2)SbCl and 1,3-diphosphabicyclo[1.1.0]butane plays a pivotal role in the reaction. The zwitterionic character of this intermediate species allows it to act simultaneously as both an ene and an eneophile, and a symmetry-allowed bimolecular reaction leads to the tetraphosphaladderene species via a spirocyclic intermediate.
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.
Publisher: Wiley
Date: 23-03-2022
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.
Publisher: American Chemical Society (ACS)
Date: 07-11-2003
DOI: 10.1021/IC034867K
Abstract: A survey of the potential energy surface for a 1:1 copper dioxygen complex, (C(3)N(2)H(5))CuO(2), reveals two distinct states in the valence region, a singlet ((1)A(1)) and a triplet ((3)B(1)). The former spans a continuum from Cu(III)-O(2)(2-) to Cu(I)-O(2)((1)Delta(g)), while the latter spans Cu(II)-O(2)(1-) to Cu(I)-O(2)((3)Sigma(g)(-)). The point at which the potential energy curves for the two states cross marks an abrupt discontinuity in electron distribution, where the system shifts from dominant Cu(III)-O(2)(2-) character to Cu(II)-O(2)(1-). On this basis, we argue that there is no continuum between Cu(III)-peroxide and Cu(II)-superoxide: the two are represented by distinct states that differ both in symmetry and multiplicity.
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.
Publisher: Royal Society of Chemistry (RSC)
Date: 2008
DOI: 10.1039/B715021K
Abstract: Density functional theory is used to explore the electronic states involved in the remarkable two-step spin crossover (S = 0 --> S = 1 --> S = 2) in the cationic extended metal atom chain [Co(3)(dpa)(4)Cl(2)](+) (dpa = the anion of 2-dipyridylamine) (R. Clérac, F. A. Cotton, K. R. Dunbar, T. Lu, C. A. Murillo and X. Wang, J. Am. Chem. Soc., 2000, 122, 2272). The calculations are consistent with a model in which all three spin states share one common feature-a vacancy in the d(xy) orbital on the central cobalt atom which is stabilised by pi donation from four amide groups. As a result, all three can be considered to contain a Co(2+)-Co(3+)-Co(2+) chain. The singlet and triplet states arise from antiferromagnetic and ferromagnetic coupling, respectively, between the unpaired electron in this d(xy) orbital and another localised entirely on the terminal cobalt centres (the antisymmetric combination of Co d(z(2))). The singlet-triplet transition does not, therefore, populate any additional antibonding orbitals, and as a result the structure is almost invariant around the characteristic temperature of the singlet-triplet transition. In the most stable quintet, in contrast, the symmetry of the Co-Co-Co chain is broken, giving rise to a localised high-spin Co(II) centre (S = 3/2), ferromagnetically coupled to a Co(III)-Co(II) dimer (S = 1/2). The structural changes associated with this transition are apparent in the X-ray data in subtle changes in both Co-N and Co-Cl bond lengths, although their magnitude is d ed by the relatively low population (18%) of the quintet even at 300 K.
Publisher: Royal Society of Chemistry (RSC)
Date: 2006
DOI: 10.1039/B517710C
Abstract: The cationic cages nido-[C2Bu(t)2P2E]+ (E = As, Sb), which are isolobal to the cyclopentadienyl cation, adopt square based pyramidal structures with the heavy pnictogen atom at the apex NMR and computational methods have been used to probe the dynamic behaviour of the complexes.
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.
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.
Location: United Kingdom of Great Britain and Northern Ireland
Location: Germany
Start Date: 06-2020
End Date: 03-2024
Amount: $390,000.00
Funder: Australian Research Council
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