EFFECTS OF TEMPERATURE AND DELTA-G-DEGREES ON ELECTRON-TRANSFER FROM CYTOCHROME C(2) TO THE PHOTOSYNTHETIC REACTION-CENTER OF THE PURPLE BACTERIUM RHODOBACTER-SPHAEROIDES

The kinetics of electron transfer from cytochrome c(2) to the primary donor (P) of the reaction center from the photosynthetic purple bacter ium Rhodobacter sphaeroides have been investigated by time-resolved ab sorption spectroscopy. Rereduction of P+ induced by a laser pulse has been measured at temperatures from 300 K to 220 K in a series of speci fically mutated reaction centers characterized by altered midpoint red ox potentials of P+/P varying from 410 mV to 765 mV (as compared to 50 5 mV for wild type). Rate constants for first-order electron donation within preformed reaction center-cytochrome c(2) complexes and for the bimolecular oxidation of free cytochrome c, have been obtained by mul tiexponential deconvolution of the kinetics. At all temperatures the r ate of the fastest intracomplex electron transfer increases by more th an two orders of magnitude as the driving force -Delta G degrees is va ried over a range of 350 meV. The temperature and Delta G degrees depe ndences of the rate constant fit the Marcus equation well. Global anal ysis yields a reorganization energy lambda = 0.96 +/- 0.07 eV and a se t of electronic matrix elements, specific for each mutant, ranging fro m 1.2 10(-4) eV to 2.5 10(-4) eV. Analysis in terms of the Jortner equ ation indicates that the best fit is obtained in the classical limit a nd restricts the range of coupled vibrational modes to frequencies low er than similar to 200 cm(-1). An additional slower kinetic component of P+ reduction, attributed to electron transfer from cyt c(2) docked in a nonoptimal configuration of the complex, displays a Marcus type d ependence of the rate constant upon Delta G degrees, characterized by a similar value of lambda (0.8 +/- 0.1 eV) and by an average electroni c matrix element smaller by more than one order of magnitude. In all o f the mutants, as the temperature is decreased below 260 K, both intra complex reactions are abruptly inhibited, their rate being negligible at 220 K. The free energy dependence of the second-order rate constant for oxidation of cyt c(2) in solution suggests that the collisional r eaction is partially diffusion controlled, reaching the diffusion limi t at exothermicities between 150 and 250 meV over the temperature rang e investigated.