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Azide as a probe of proton transfer reactions in photosynthetic oxygen evolution.

ABSTRACT: In oxygenic photosynthesis, photosystem II (PSII) is the multisubunit membrane protein responsible for the oxidation of water to O2 and the reduction of plastoquinone to plastoquinol. One electron charge separation in the PSII reaction center is coupled to sequential oxidation reactions at the oxygen-evolving complex (OEC), which is composed of four manganese ions and one calcium ion. The sequentially oxidized forms of the OEC are referred to as the S(n) states. S(1) is the dark-adapted state of the OEC. Flash-induced oxygen production oscillates with period four and occurs during the S(3) to S(0) transition. Chloride plays an important, but poorly understood role in photosynthetic water oxidation. Chloride removal is known to block manganese oxidation during the S(2) to S(3) transition. In this work, we have used azide as a probe of proton transfer reactions in PSII. PSII was sulfate-treated to deplete chloride and then treated with azide. Steady state oxygen evolution measurements demonstrate that azide inhibits oxygen evolution in a chloride-dependent manner and that azide is a mixed or noncompetitive inhibitor. This result is consistent with two azide binding sites, one at which azide competes with chloride and one at which azide and chloride do not compete. At pH 7.5, the K(i) for the competing site was estimated as 1 mM, and the K(i)' for the uncompetitive site was estimated as 8 mM. Vibrational spectroscopy was then used to monitor perturbations in the frequency and amplitude of the azide antisymmetric stretching band. These changes were induced by laser-induced charge separation in the PSII reaction center. The results suggest that azide is involved in proton transfer reactions, which occur before manganese oxidation, on the donor side of chloride-depleted PSII.

SUBMITTER: Cooper IB

PROVIDER: S-EPMC2599857 | biostudies-literature | 2008 Dec

REPOSITORIES: biostudies-literature

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Azide as a probe of proton transfer reactions in photosynthetic oxygen evolution.

Cooper Ian B IB Barry Bridgette A BA

Biophysical journal 20080919 12

In oxygenic photosynthesis, photosystem II (PSII) is the multisubunit membrane protein responsible for the oxidation of water to O2 and the reduction of plastoquinone to plastoquinol. One electron charge separation in the PSII reaction center is coupled to sequential oxidation reactions at the oxygen-evolving complex (OEC), which is composed of four manganese ions and one calcium ion. The sequentially oxidized forms of the OEC are referred to as the S(n) states. S(1) is the dark-adapted state of ...[more]

PMID: 18805932

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Project description:The azide radical (N3●) is one of the most important one-electron oxidants used extensively in radiation chemistry studies involving molecules of biological significance. Generally, it was assumed that N3● reacts in aqueous solutions only by electron transfer. However, there were several reports indicating the possibility of N3● addition in aqueous solutions to organic compounds containing double bonds. The main purpose of this study was to find an experimental approach that allows a clear assignment of the nature of obtained products either to its one-electron oxidation or its addition products. Radiolysis of water provides a convenient source of one-electron oxidizing radicals characterized by a very broad range of reduction potentials. Two inorganic radicals (SO4●-, CO3●-) and Tl2+ ions with the reduction potentials higher, and one radical (SCN)2●- with the reduction potential slightly lower than the reduction potential of N3● were selected as dominant electron-acceptors. Transient absorption spectra formed in their reactions with a series of quinoxalin-2-one derivatives were confronted with absorption spectra formed from reactions of N3● with the same series of compounds. Cases, in which the absorption spectra formed in reactions involving N3● differ from the absorption spectra formed in the reactions involving other one-electron oxidants, strongly indicate that N3● is involved in the other reaction channel such as addition to double bonds. Moreover, it was shown that high-rate constants of reactions of N3● with quinoxalin-2-ones do not ultimately prove that they are electron transfer reactions. The optimized structures of the radical cations (7-R-3-MeQ)●+, radicals (7-R-3-MeQ)● and N3● adducts at the C2 carbon atom in pyrazine moiety and their absorption spectra are reasonably well reproduced by density functional theory quantum mechanics calculations employing the ωB97XD functional combined with the Dunning's aug-cc-pVTZ correlation-consistent polarized basis sets augmented with diffuse functions.

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Azide as a probe of proton transfer reactions in photosynthetic oxygen evolution.

Publications

Azide as a probe of proton transfer reactions in photosynthetic oxygen evolution.

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