| Abstract
| - Continuous illumination at temperatures above 250 K ofphotosystem II samples which havebeen depleted of calcium or chloride or treated with fluoride, acetate,or ammonia results in productionof a broad radical EPR signal centered at g = 2.0.This EPR signal, called the S3 EPR signal, has beenattributed to an organic radical interacting with the S2state of the oxygen-evolving complex to give thespecies S2X+ (X+ = organicradical). A tyrosine radical has been proposed as the speciesresponsible forthe S3 EPR signal. On the basis of experiments demonstrating thatnitric oxide binds reversibly to thetyrosyl radical in ribonucleotide reductase, nitric oxide has been usedto probe the S3 EPR signal inacetate-treated photosystem II. In experiments usingmanganese-depleted photosystem II, nitric oxidewas found to bind reversibly to both redox-active tyrosines,YD• and YZ•, toform EPR-silent adducts.Next, acetate-treated photosystem II was illuminated to form theS3 EPR signal in the presence of nitricoxide to test whether the S3 EPR signal behaves likeYZ•. Under conditions that produce themaximumyield of the S3 EPR signal in acetate-treated photosystem II, no S3 EPRsignal was observed in thepresence of nitric oxide. Upon removal of nitric oxide, the S3 EPRsignal could be induced. Quenchingof the S3 EPR signal by nitric oxide yielded an S2-statemultiline EPR signal. Its amplitude was 45% ofthat found for uninhibited photosystem II illuminated at 200 K; thisyield is the same as the yield of theS3 EPR signal under equivalent conditions but without nitric oxide.These results suggest that the S3EPR signal is due to the configurationS2YZ• in which the S2state of the oxygen-evolving complex givesa broadened multiline EPR signal as a result of exchange and dipolarinteractions with YZ•. Thebindingof nitric oxide to YZ• to form a diamagneticYZ−NO species uncouples the S2 state fromYZ•, yielding anoninteracting S2-state multiline EPR signalspecies.
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