OPTICALLY DETECTED MAGNETIC-RESONANCE STUDY ON THE ORIGIN OF THE PHEOPHYTIN TRIPLET-STATE IN D1/D2 CYTOCHROME B-559 COMPLEXES

In order to determine the origin of the photoexcited pheophytin triplet state ((3)Phe) in plant Photosystem II preparations (D1/D2-cyt b-559 complexes) an absorption and fluorescence detected magnetic resonance (ADMR, FDMR) study was conducted in zero magnetic field at low temperatures. The ADMR signal intensity dependence on excitation light flux was linear showing that (3)Phe is formed by a 1-photon process. Upon successive exposure of the sample to strong white light it was shown that (3)Phe is not directly correlated with the primary donor triplet state (3)P680. This is consistent with the absence of a double resonance signal connecting (3)Phe with (3)P680 down to an instrumental sensitivity of Delta I/I (Delta transmission/transmission) below 10(-6). An intermediate state of photoinhibition between intact and fully degraded reaction centres is responsible for the formation of (3)Phe. It may be a conformationally changed state of the reaction centre protein, most likely having a larger distance between P680 and the pheophytin whose triplet state is observed. Factor analysis of the absorption spectra of a series of gradually degraded samples yielded two spectral components which were interpreted as the spectra of fully intact and completely degraded reaction centres.