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. spha eroides 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 distance s and driving forces of the ET reactions are also employed in the calc ulations. The only extra kinetic parameter required by the models is t he coupling between reactive and non-reactive modes, which is assumed to be constant and is taken from earlier studies on similar intramolec ular ETs. No additional parameters are fitted in the calculation of ET s from the excited special pair to an accessory bacteriochlorophyll (B -L), from B-L(-) to bacteriopheophytin (H-L), from H-L(.-) to the prim ary quinone acceptor (Q(A)), from Q(A)(.-) to the secondary quinone ac ceptor (Q(B)), and in the charge recombinations between each of the re duced cofactors and the photo-oxidized special pair (P.+); the reducti on of P.+ by a cytochrome c in Rb. sphaeroides and in Chromatium is al so studied. The calculated free-energy and temperature dependences are within one order of magnitude of the rates measured in mutant and cof actor-substituted RCs, except for the temperature dependence of the ch arge shift from Q(A)(.-) to Q(B). It is suggested that the rate measur ed for this process does not reflect an elementary ET reaction. (C) 19 97 Elsevier Science S.A.