RECOMBINATION DYNAMICS AND EPR-SPECTRA OF THE PRIMARY RADICAL PAIR INBACTERILA PHOTOSYNTHETIC REACTION CENTERS WITH BLOCKED ELECTRON-TRANSFER TO THE PRIMARY ACCEPTOR

Recent electron paramagnetic resonance (EPR) experiments (Proskuryakov , I. I.; Klenina, I. B.; Hore, P. J.; Bosch, M. K.; Gast, P.; Hoff, A. J. Chem. Phys. Lett. 1996, 257, 333) on the primary radical pair in b acterial reaction centers are interpreted in detail. The spin-correlat ed radical pair model is extended to include the time dependence of th e EPR intensities, differential lifetime broadening of the basic four- lint: spectrum, and averaging over the inhomogeneous distributions of hyperfine interactions in the two radicals. Values of the singlet and tripler recombination rate constants and the magnitude and sign of the exchange interaction of the radical pair are obtained from the shape and linewidth of the spin-polarized EPR spectra and the kinetics of fo rmation of the triplet state of the primary electron donor. For Rhodob acter sphaeroides at 70 K, k(s) = (1.2 +/- 0.3) x 10(7) s(-1), k(T) = (5.7 - 8.8) x 10(8) s(-1), -J = 8 - 6 G; and for Rhodopseudomonas viri dis at 190 K, k(s) = (8 +/- 2) x 10(7) s(-1), k(T) = (6.5 - 10.0) x 10 (8) s(-1), -J = 17 - 16 G. The reliability of these values is discusse d in the light of the energetic heterogeneity of the radical pair ener gies (Ogrodnik, A.; Keupp, W.; Volk, M.; Aumeier, G.; Michel-Beyerle, M. E. J. Phys. Chem. 1994, 98, 3432-3439) and of the sensitivity and s electivity of the EPR measurements toward the three parameters.