ORCID Profile
0000-0003-3791-129X
Current Organisation
Consejo Superior de Investigaciones Cientificas
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Publisher: Oxford University Press (OUP)
Date: 08-2012
DOI: 10.1093/JXB/ERS174
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.
Publisher: American Chemical Society (ACS)
Date: 16-09-2006
DOI: 10.1021/BI0614683
Abstract: We have employed absorption, circular dichroism (CD), and persistent spectral hole-burning measurements at 1.7 K to study the photoconversion properties and exciton coupling of low-energy chlorophylls (Chls) in the CP43 proximal antenna light-harvesting subunit of photosystem II (PSII) isolated from spinach. These approximately 683 nm states act as traps for excitation energy in isolated CP43. They "bleach" at 683 nm upon illumination and photoconvert to a form absorbing in the range approximately 660-680 nm. We present new data that show the changes in the CD spectrum due to the photoconversion process. These changes occur in parallel with those in absorption, providing evidence that the feature undergoing the apparent bleach is a component of a weakly exciton-coupled system. From our photoconversion difference spectra, we assign four states in the Chl long-wavelength region of CP43, two of which are the known trap states and are both highly localized on single Chls. The other two states are associated with weak exciton coupling (maximally approximately 50 cm(-)(1)) to one of these traps. We propose a mechanism for photoconversion that involves Chl-protein hydrogen bonding. New hole-burning data are presented that indicate this mechanism is distinct to that for narrow-band spectral hole burning in CP43. We discuss the photophysical behavior of the Chl trap states in isolated CP43 compared to their behavior in intact PSII preparations. The latter represent a more intact, physiological complex, and we find no clear evidence that they exhibit the photoconversion process reported here.
Publisher: Elsevier BV
Date: 09-2016
DOI: 10.1016/J.BBABIO.2016.06.007
Abstract: The identification of low-energy chlorophyll pigments in photosystem II (PSII) is critical to our understanding of the kinetics and mechanism of this important enzyme. We report parallel circular dichroism (CD) and circularly polarized luminescence (CPL) measurements at liquid helium temperatures of the proximal antenna protein CP47. This assembly hosts the lowest-energy chlorophylls in PSII, responsible for the well-known "F695" fluorescence band of thylakoids and PSII core complexes. Our new spectra enable a clear identification of the lowest-energy exciton state of CP47. This state exhibits a small but measurable excitonic delocalization, as predicated by its CD and CPL. Using structure-based simulations incorporating the new spectra, we propose a revised set of site energies for the 16 chlorophylls of CP47. The significant difference from previous analyses is that the lowest-energy pigment is assigned as Chl 612 (alternately numbered Chl 11). The new assignment is readily reconciled with the large number of experimental observations in the literature, while the most common previous assignment for the lowest energy pigment, Chl 627(29), is shown to be inconsistent with CD and CPL results. Chl 612(11) is near the peripheral light-harvesting system in higher plants, in a lumen-exposed region of the thylakoid membrane. The low-energy pigment is also near a recently proposed binding site of the PsbS protein. This result consequently has significant implications for our understanding of the kinetics and regulation of energy transfer in PSII.
Publisher: Elsevier BV
Date: 2016
DOI: 10.1016/J.BBABIO.2015.09.012
Abstract: Circularly polarized luminescence (CPL) spectroscopy is an established but relatively little-used technique that monitors the chirality of an emission. When applied to photosynthetic pigment assemblies, we find that CPL provides sensitive and detailed information on low-energy exciton states, reflecting the interactions, site energies and geometries of interacting pigments. CPL is the emission analog of circular dichroism (CD) and thus spectra explore the optical activity only of fluorescent states of the pigment-protein complex and consequently the nature of the lowest-energy excited states (trap states), whose study is a critical area of photosynthesis research. In this work, we develop the new approach of temperature-dependent CPL spectroscopy, over the 2-120 K temperature range, and apply it to the CP43 proximal antenna protein of photosystem II. Our results confirm strong excitonic interactions for at least one of the two well-established emitting states of CP43 named "A" and "B". Previous structure-based models of CP43 spectra are evaluated in the light of the new CPL data. Our analysis supports the assignments of Shibata et al. [Shibata et al. J. Am. Chem. Soc. 135 (2013) 6903-6914], particularly for the highly-delocalized B-state. This state dominates CPL spectra and is attributed predominantly to chlorophyll a's labeled Chl 634 and Chl 636 (alternatively labeled Chl 43 and 45 by Shibata et al.). The absence of any CPL intensity in intramolecular vibrational sidebands associated with the delocalized "B" excited state is attributed to the dynamic localization of intramolecular vibronic transitions.
Publisher: Elsevier BV
Date: 11-2013
DOI: 10.1016/J.PHYTOCHEM.2013.07.012
Abstract: We analyzed the molecular mechanism controlling ω-3 fatty acid desaturases during seed germination and leaf development in soybean. During germination, soybean seeds were characterized by a high 18:2(Δ9,12) level (more than 50%) and reduced 18:3(Δ9,12,15) content (10%). Interestingly, transcripts from all endoplasmic reticulum (GmFAD3A and GmFAD3B) and plastidial (GmFAD7-1/GmFAD7-2 or GmFAD8-1/GmFAD8-2) desaturase genes were detected during seed germination. Upon germination, soybean trifoliate leaf development was accompanied by an increase in linolenic acid (18:3(Δ9,12,15)). Our data showed that transcripts corresponding to the endoplasmic reticulum ω-3 desaturases GmFAD3A and GmFAD3B decreased with leaf development. No changes in the expression profile of the plastidial ω-3 desaturases GmFAD7-1 and GmFAD7-2 genes were detected. On the contrary, GmFAD8-2 transcript levels increased while GmFAD8-1 transcripts decreased during leaf development. Given this expression profile, our data suggested the existence of a temporal regulatory mechanism controlling ω-3 desaturases during leaf development in which the endoplasmic reticulum ω-3 desaturases would be more important in young leaves while plastidial ω-3 desaturases might contribute to 18:3(Δ9,12,15) production in mature leaves. Photosynthetic cell cultures showed 18:3(Δ9,12,15) levels similar to those from leaves. No changes in the 18:3(Δ9,12,15) content or expression of the ω-3 desaturase genes were detected along the cell culture cycle. A comparison of our data with those available in Arabidopsis or wheat suggested that the regulatory mechanism controlling the expression and activity of both endoplasmic reticulum and plastidial desaturases during leaf development might differ among plant species.
Location: United States of America
No related grants have been discovered for Rafael Picorel.