The oxidation of carotenoid upon illumination at low temperature has been studied in Mn-depleted photosystem II (PSII) using EPR and electronic absorption spectroscopy. Illumination of PSII at 20 K results in carotenoid cation radical (Car+*) formation in essentially all of the centers. When a sample which was preilluminated at 20 K was warmed in darkness to 120 K, Car+* was replaced by a chlorophyll cation radical. This suggests that carotenoid functions as an electron carrier between P680, the photooxidizable chlorophyll in PSII, and ChlZ, the monomeric chlorophyll which acts as a secondary electron donor under some conditions. By correlating with the absorption spectra at different temperatures, specific EPR signals from Car+* and ChlZ+* are distinguished in terms of their g-values and widths. When cytochrome b559 (Cyt b559) is prereduced, illumination at 20 K results in the oxidation of Cyt b559 without the prior formation of a stable Car+*. Although these results can be reconciled with a linear pathway, they are more straightforwardly explained in terms of a branched electron-transfer pathway, where Car is a direct electron donor to P680(+), while Cyt b559 and ChlZ are both capable of donating electrons to Car+*, and where the ChlZ donates electrons when Cyt b559 is oxidized prior to illumination. These results have significant repercussions on the current thinking concerning the protective role of the Cyt b559/ChlZ electron-transfer pathways and on structural models of PSII.

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Gallic acid (GA) and its derivatives are natural polyphenolic substances widely used as antioxidants in nutrients, medicine and polymers. Here, nanoantioxidant materials are engineered by covalently grafting GA on SiO(2) nanoparticles (NPs). A proof-of-concept is provided herein, using four types of well-characterized SiO(2) NPs of specific surface area (SSA) 96-352 m(2)/g. All such hybrid SiO(2)-GA NPs had the same surface density of GA molecules (~1 GA per nm(2)). The radical-scavenging capacity (RSC) of the SiO(2)-GA NPs was quantified in comparison with pure GA based on the 2,2-diphenyl-1-picrylhydrazyl (DPPH(•)) radical method, using electron paramagnetic resonance (EPR) and UV-vis spectroscopy. The scavenging of DPPH radicals by these nanoantioxidant SiO(2)-GA NPs showed mixed-phase kinetics: An initial fast-phase (t(1/2) <1 min) corresponding to a H-Atom Transfer (HAT) mechanism, followed by a slow-phase attributed to secondary radical-radical reactions. The slow-reactions resulted in radical-induced NP agglomeration, that was more prominent for high-SSA NPs. After their interaction with DPPH radicals, the nanoantioxidant particles can be reused by simple washing with no impairment of their RSC.

The semiquinone radical QA - has been studied by Electron Spin−Echo Envelope Modulation (ESEEM) spectroscopy in Photosystem II membranes at various pH values. The observed nuclear modulations have been assigned by the use of two-dimensional Hyperfine Sublevel Correlation Spectroscopy (HYSCORE) and numerical simulations. Two protein 14N nuclei (NI and NII) were found to be magnetically coupled with the QA - spin, and on the basis of 14N-NQR and 14N-ESEEM data from the literature, NI is assigned to an amide nitrogen from the protein backbone while NII is assigned to the amino nitrogen, Nδ, of an imidazole. A physical explanation for such couplings is suggested where the coupling occurs through H-bonds from the protein to the carbonyls of the semiquinone. In PSII membranes treated with CN-, only the NI coupling is present above pH 8.5 while both NI and NII couplings are present at lower pH values. In samples treated at high pH to remove the iron, both NI and NII couplings are present throughout the pH range studied but at pH <6 these couplings strengthen. These results are interpreted in terms of a model based on the structure of the bacterial reaction center and involving two determining factors. (1) The nonheme iron, when present, is liganded to the imidazole that H-bonds to one of the QA - carbonyls. This physical attachment of the imidazole to the iron limits the strength of the H-bond to QA -. (2) A pH-dependent group on the protein controls the strength of the H-bonds to QA -. The pK a of this group is influenced by the biochemical treatment used to uncouple the iron, being around pH 7.5 in CN--treated PSII but around pH 6 in high pH-treated PSII. It is proposed that such a pH effect on the H-bond strength exists in untreated PSII and that earlier observations of pH-induced changes in the EPR signal from the semiquinone iron may reflect this change.

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(7) was prepared by the reaction of [VOCl 2 (thf) 2 ] with phen in a methanolic solution. The X-ray structure of 3 shows that the vanadium(iv) atom is ligated to a tridentate mpg 3À ligand at the S thiolato , N peptide and O carboxylato atoms. The X-ray structure of 7 is also reported. The optical, infrared, magnetic, electron paramagnetic resonance, and electrochemical properties of compounds 1 ± 5Ć H 3 OH and 7 were studied. Combination of the correlation plots of the EPR parameters g z versus A z , or the groundstate orbital population (b*) 2 versus A z , together with the additivity relationship, A z,calcd Sn i A zi /4, were shown to provide a powerful tool for probing the equatorial donor atoms in an oxovanadium(iv) compound and consequently in biomolecules. Thus, these methods provide valuable evidence for the assignment of the equatorial donor atoms for the V IV O 2 center of the V IV O 2 ± glutathione system at various pH values. Model NMR studies (interaction of vanadium(v) with H 3 mpg) showed that there is a possibility of vanadium(v) ligation to glutathione. The contribution of a deprotonated peptide(amide) nitrogen to A z is not a fixed quantity (it varies from 29 to 43 Â 10 À4 cm À1), but is influenced by the presence of the three other donor atoms in the equatorial plane and, in particular, their charge.

Humic acids have stable radicals that are indigenous to their structure. Hydroxybenzoic acid derivatives such as gallic acid (GA) and protocatechuic acid are appropriate models for the radical properties of humic acids. Here we show that the adsorption or intercalation of gallic acid in Laponite clay results in a significant thermodynamic stabilization of gallic acid radicals. Moreover, the formed organoclay shows enhanced stability against acid dissolution. The structural details of the association of gallic acid with Laponite depend on the GA/Laponite loading. At low GA/Laponite ratios (approximately 10(-6) M of gallic acid per gram of clay), gallic acid is adsorbed at the variable charge sites of Laponite. This adsorption can be adequately described by surface complexation modeling. At higher GA/Laponite ratios (approximately 10(-3) M of gallic acid per gram of clay), X-ray diffraction data show that gallic acid is intercalated at the interlamellar sites of Laponite. In the presence of Pb2+ ions, the formed GA/Pb complex is associated with Laponite in an analogous structural manner, that is, adsorption at variable charge sites or intercalation at the interlamellar sites of Laponite, depending on the loading. Laponite stabilizes the GA/Pb radicals. At prolonged exposure to ambient O2, Laponite promotes the formation of stable oligomeric GA/Pb radical species, which are intercalated into interlamellar sites.

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