MAGNETIC FIELD-SENSITIVE STAGES OF ELECTRON-TRANSFER IN ISOLATED REACTION CENTERS OF THE HIGHER-PLANT PHOTOSYSTEM-II

The mechanisms of the magnetic field effect on the fluorescence yield of the chlorophyll, DELTAF(B)/F, and the optical absorbance-difference T-S spectra of the primary electron donor are studied in D1/D2/cytochrome b-559 photosystem II complexes (isolated PS II RC) in the temperature range 9-295 K. It is demonstrated that in the temperature range 40-295 K DELTAF(B)/F is due to superposition of two magnetic field-sensitive fluorescence components which are related to charge recombination of RP (P680+I-). Occurence of these magnetic field-sensitive fluorescence components is explained by the formation of relaxed and nonrelaxed states of RP (P680+I-) and/or heterogeneity of RCs preparation. It is proposed that effect of the magnetic field on fluorescence yield in the temperature range 9-35 K is due to mixing of the eigenfunctions (mixing of the decay rate constants and population probabilities) of the triplet sublevels of P680 and charge recombination of RP (P680+I-) to the ground state of P680. The investigation of absorbance-difference T-S spectra of P680 indicates that it is a dimer of chlorophyll molecules with sufficient interactions between the two monomers in the singlet state and minor interactions in the triplet state. It is suggested that absorbance-difference T-S spectra of P680 reflect the heterogeneity of the RCs. Energy transfer from P680 triplet state to beta-carotene in the studied D1/D2/cytochrome b-559 complexes is not observed.