THE INFLUENCE OF STRUCTURAL-DYNAMIC ORGANIZATION OF RC FROM PURPLE BACTERIUM RHODOBACTER-SPHAEROIDES ON PICOSECOND STAGES OF PHOTOINDUCED REACTIONS

Effects of the hydrogen bond network on the rate constants of energy m igration (k(m)), charge separation (k(e)), electron transfer to Q(A) ( k(Q)) and P+I- recombination in RC of Rhodobacter sphaeroides were ana lysed in control and modified RC preparations at different temperature s. Modification of RC were made by the addition of 40% v/v DMSO. The r ate constants k(m) k(e), k(Q) were evaluated from pump-and-probe measu rements of the absorption difference kinetics at 665 nm corresponding to BPhL- formation and subsequent electron transfer to Q(A). For the i nvestigation of P+I- recombination a primary quinone acceptor was pre- reduced in the dark by adding of 1 mg/ml of dithionite and 1 mM sodium ascorbate. Recombination kinetics were measured at 665 and 870 nm. Th e numerical analysis of the temperature dependence of k(e) and k(Q) Wa s performed on the basis of the model proposed by Kakitani and Kakitan i (T. Kakitani and Il. Kakitani (1981), Biochim. Biophys. Acta, 635, 4 98-514). It was found that: (a) in control samples the molecular rate constants k(m), k(e) and k(Q) were about (3.4 ps)(-1), (4.5 ps)(-1) an d(200 ps)(-1), respectively; (b) under modification by DMSO these rate s decrease up to (5.3 ps)(-1), (10.3 ps)(-1) and(500 ps)(-1), respecti vely; (c) as the temperature drops from 300 K to 77 K the rate constan t k(m) decreases by 1.8 times in control and by 3.2 times in modified samples. In contrast to the observed k(m) changes the increase in k(e) and k(Q) values by 2 and more times under cooling was found in contro l and modified RC; (d) in control preparations with Q(A) acceptor pre- reduced in the dark the lowering of the temperature caused the increas e in the time of P+I- recombination from 10 to 20 ns. After DMSO modif ication the kinetics of charge recombination in RC was biexponential a t room temperature with tau=10 ns and tau(1)=0.8 ns, and at 77 K with tau=20 ns and tau(1)=0.6 ns, correspondingly. The results obtained rev eal that in RC isolated from Rb. sphaeroides the processes of energy m igration, charge separation, electron transfer to Q(A) and ion-radical pair P+I- recombination depend on the state of hydrogen bonds of wate r-protein structure. Fast relaxation processes in RC structure includi ng polarization of H-containing, molecules in the surrounding of elect ron carriers can accept electron energy dissipated at the initial step s of energy and electron transfer. (C) 1998 Elsevier Science B.V. All rights reserved.