ИСТИНА

This communication presents an overview of EPR applications to the study of photosynthetic processes in chloroplasts. A brief consideration of structural and functional organization of photosynthetic apparatus is focused on the analysis of the EPR spectra of electron carriers in photosynthetic electron transport chain. Kinetics of the light-induced redox transients of P700 (primary electron donor in photosystem I) has been investigated to elucidate the nature of the rate-limiting step of electron transfer between photosystems II and I, and to study the mechanisms of electron transport control in chloroplasts in situ . It was demonstrated that plastoquinone (PQ) turnover between the PSII and Cyt b 6 f complexes is the rate-limiting step in the intersystem chain of electron transport. Within a wide range of experimental conditions (upon varying pH, temperature, and ionic strength), the reduction of PQ to plastoquinol (PQH2), dissociation of PQH2 from PSII, and its diffusion occur much faster than the oxidation of PQH2 bound to the Cyt b 6 f complex. The quantum chemical mechanisms of PQH2 oxidation by the Cyt b 6 f complex are considered in the context of photosynthetic control in chloroplasts. Structure-function relationships in chloroplasts were studied using the spin-labeling technique. It was found that the rates of the intersystem electron transport and ATP synthesis in chloroplasts correlated with the temperature-induced changes in the physical state of the thylakoid membrane. Finally, the EPR applications to measuring the trans-thylakoid pH difference (∆pH) in chloroplasts will be considered. ∆pH values were estimated by two approaches: (i) the use of pH-sensitive spin-probes (imidazolidine and imidazoline derivatives of nitroxide radicals), and (ii) the analysis of pH-dependent post-illumination reduction of oxidized centers P700+. In the state of photosynthetic control, ∆pH ~ 2.3-2.5 (pHout 7.8-8.0). Under the ATP synthesis conditions, when the proton drain from the lumen via active ATP synthases occurs, there is a moderate acidification of the thylakoid lumen (∆pH ~ 1-2, pHout 7.8-8.0). The data obtained are discussed in the context of the problem of energy coupling in laterally heterogeneous lamellar system of chloroplasts.