ИСТИНА

EPR played a great role in deciphering the structural and functional organization of photosynthetic apparatus. This communication starts with a brief historical overview of EPR applications to the study of photosynthetic electron transport chain (ETC) in chloroplasts in situ and in vitro. A consideration of photosynthetic ETC is focused on the analysis of the EPR spectra of electron carriers involved into photosynthetic electron transport: P700 and P680 (the primary electron donor in photosystems I and II, PSI and PSII), plastocyanin, the low-potential iron-sulphur redox centers of PSI and the high-potential Rieske protein of the Cyt b6f complex. Kinetics of the light-induced redox transients of P700 has been investigated to elucidate the mechanisms of the light-induced regulation of electron transport in chloroplasts in situ. Monitoring the redox state of P700 by the EPR method, we have demonstrated that there are two basic mechanisms of electron transport control in chloroplasts: (i) pH-dependent retardation to plastoquinone turnover between the PSII and Cyt b6f complexes, and (ii) acceleration of electron outflow from PSI due to activation of the Calvin-Benson cycle. Using the spin-labeling technique, we demonstrated a correlation between the rates of the intersystem electron transport and ATP synthesis, on the one hand, and the temperature-induced changes in the physical state of the thylakoid membrane, on the other hand. The EPR applications to measuring the trans-thylakoid pH difference (pH) are considered. pH values were estimated by two approaches: (i) the use of pH-sensitive spin-probes, and (ii) the analysis of pH-dependent post-illumination reduction of oxidized centers P700+. The data obtained are discussed in the context of the problem of coupling electron and proton transport to ATP synthesis in chloroplasts.