Theoretical Analyses of Electron-Transfer Reactions

Reorganization Energy of the Initial Electron-Transfer Step in Photosynthetic Bacterial Reaction Centers

“…Conformational relaxations that occur on time scales of more than 10 ps will have little bearing on the kinetics. As long as the electronic coupling between P*B and P ϩ B Ϫ is sufficiently weak so that reversibility can be neglected (i.e., as long as the reaction is nonadiabatic), the kinetics can probably be simulated best by using the semiclassical trajectory method to evaluate the rate of surface hopping (Creighton et al, 1988;Warshel and Parson, 1991;Parson and Warshel, 1995). The situation may be different, however, in the second step of charge separation, where the coupling between P ϩ B Ϫ H and P ϩ BH Ϫ may be stronger.…”

“…For a recent discussion of the identity of the dispersed-polaron and spinboson models, see Hwang and Warshel (1997). Path integral methods Mak, 1993, 1994;Makri et al, 1996) can be used to study the quantum mechanical time evolution of rapid electron transfer in both the diabatic and adiabatic limits, as can density matrix methods, once the spectral distribution of vibrations coupled to the reaction has been obtained by dispersed-polaron simulations (Warshel and Parson, 1991;Parson and Warshel, 1995). Here we use the semiclassical approach as a starting point.…”

“…The rate constant for an electron transfer process is determined by the probability per unit time that a trajectory in the reactant state will reach a point where the potential energy surfaces of the reactant and product states intersect (Warshel and Parson, 1991;Parson and Warshel, 1995). In the FEP approach, this probability is calculated by using umbrella sampling to sample the entire configuration space of the system (Hwang and Warshel, 1987;Kuharski et al, 1988;King and Warshel, 1990;Warshel and Parson, 1991;Kollman, 1993).…”

Effects of Temperature and ΔG° on Electron Transfer from Cytochrome c2 to the Photosynthetic Reaction Center of the Purple Bacterium Rhodobacter sphaeroides

“…where ប is Planck's constant divided by 2, H DA is the electronic tunneling matrix element between donor and acceptor, and (FC) is the nuclear Franck-Condon factor that describes the nuclear motions associated with electron transfer (DeVault, 1984;Marcus and Sutin, 1985;Bertrand, 1991;Parson and Warshel, 1995). The tunneling matrix element H DA depends upon donoracceptor separation and orientation and upon the nature of the intervening medium.…”

“…The reorganization energy includes contributions from changes in bond lengths, bond angles, distances, and orientations involving the donor, the acceptor, and the medium. The Franck-Condon factor can be evaluated by using different approximations at various levels of sophistication (DeVault, 1984;Parson and Warshel, 1995). A classical treatment of nuclear motion in a harmonic approximation leads to the simplest and most widely used expression, derived by Marcus (Marcus and Sutin, 1985), which predicts a Gaussian dependence of the rate constant k on ⌬G°:…”