Theory of electron transfer reactions in photosynthetic bacteria reaction centers

The intersecting-state model and an electron tunneling model are used to calculate the rates of electron transfer (ET) reactions in Rb. sphaeroides reactions centers (RCs). The models characterize the reactants by their bond lengths, force constants and bond orders, and the RC by its refractive index and thermal expansion. The experimental distances and driving forces of the ET reactions are also employed in the calculations. The only extra kinetic parameter required by the models is the coupling between reactive and non-reactive modes, which is assumed to be constant and is taken from earlier studies on similar intramolecular ETs. No additional parameters are fitted in the calculation of ETs from the excited special pair to an accessory bacteriochlorophyll (B_L), from B_L^- to bacteriopheophytin (H_L), from H_L^•- to the primary quinone acceptor (Q_A), from Q_A^•- to the secondary quinone acceptor (Q_B), and in the charge recombinations between each of the reduced cofactors and the photo-oxidized special pair (P^•+); the reduction of P^•+ by a cytochrome c in Rb, sphaeroides and in Chromatium is also studied. The calculated free-energy and temperature dependences are within one order of magnitude of the rates measured in mutant and cofactor-substituted RCs, except for the temperature dependence of the charge shift from Q_A^•- to Q_B. It is suggested that the rate measured for this process does not reflect an elementary ET reaction.