Electron spin echo studies on chloroplasts. Spectral characteristics of electron transport components and light-induced transients

Nishi, N.; Hoff, A.J.; Waals, J.H. van der

doi:10.1016/0005-2728(80)90147-4

Cited by 9 publications

(1 citation statement)

References 22 publications

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“…Similarly, the g values for a variety of model quinone cation radicals are in the range 2.0034-2.0038 (25), significantly lower than the 2.0046 value measured for Z+ and Dt. Finally, the lineshape of the D+ radical is independent of temperature over the range 1.5-300 K (26). If the partially resolved structure in the Dt spectrum Abbreviations: PSII, photosystem II; FAB, fast-atom bombardment.…”

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Tyrosine radicals are involved in the photosynthetic oxygen-evolving system.

Barry

Babcock

1987

Proc. Natl. Acad. Sci. U.S.A.

418

312

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In addition to the reaction-center chlorophyll, at least two other organic cofactors are involved in the photosynthetic oxygen-evolution process. One of these cofac.tors, called "Z," transfers electrons from the site of water oxidation to the reaction center of photosystem II. The other species, "D," has an uncertain function but gives rise to the stable EPR signal known as signal II. ZV and D+ have identical EPR spectra and are generally assumed to arise from species with the same chemical structure. Results from a variety of experiments have suggested that Z and D are plastoquinones or plastoquinone derivatives. In general, however, the evidence to support this assignment is indirect. To address this situation, we have developed more direct methods to assign the structure of the ZV/Dt radicals. By selective in vivo deuteration of the methyl groups of plastoquinone in cyanobacteria, we show that hyperfine couplings from the methyl protons cannot be responsible for the partially resolved structure seen in the Dt EPR spectrum. That is, we verify by extraction and mass spectrometry that quinones are labeled in algae fed deuterated methionine, but no change is observed in the line shape of signal II.Considering the spectral properties ofthe Dt radical, a tyrosine origin is a reasonable alternative. In a second series of experiments, we have found that deuteration of tyrosine does indeed narrow the Dt signal. Extraction and mass spectral analysis of the quinones in these cultures show that they are not labeled by tyrosine. These results eliminate a plastoquinone origin for Dt; we conclude instead that Dt, and most likely Zt, are tyrosine radicals.In plant and algal photosynthesis, photosystem II (PSII) catalyzes the light-induced oxidation of water and reduction of plastoquinone. The smallest purified unit that is capable of carrying out this reaction consists of seven polypeptides and contains chlorophyll, plastoquinone, manganese, and several other bound cofactors (for review, see refs. 1 and 2). Water oxidation occurs at a site that contains a cluster of four Mn atoms. This center is interfaced to the photochemically active PSII reaction-center chlorophyll, P680, by at least one intermediate electron carrier, which is usually designated The oxidized form of this cofactor, Zt, has a light-induced EPR signal that identifies it as an organic radical (3-6). A variety of kinetic data indicates that Z reduces P680+ directly and that the resulting Zt species is in turn reduced by the manganese ensemble (refs. 1 and 2, but see refs. 7 and 8). In addition to the EPR signal from Z+, PSII preparations also show a stable EPR signal (signal II) with the same lineshape as Zt (9). The radical giving rise to this spectrum is now usually referred to as "D+". Recent data reported by Styring and Rutherford suggest that D may be involved in maintaining the integrity of the manganese complex (10).Because the Zt and Dt EPR spectra are identical, it is generally assumed that Z and D are species that have the same chemical str...

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confidence: 99%