  Diffusional dynamics of the donor-acceptor distance in electron-transfer
reactions are responsible for the appearance of a new time scale of diffusion
over the distance of falloff of electronic tunneling. The distance dynamics
compete with the medium polarization dynamics in the solvent-controlled
electron transfer kinetics. A new solution incorporating the medium and
donor-acceptor dynamics into the electron-transfer rate constant is proposed.
The pre-exponential factor of the rate constant switches between a
distance-independent solvent-controlled regime and exponential distance decay.
The crossover between two regimes is controlled by an effective relaxation time
slowed down by a factor exponentially depending on the variance of the
donor-acceptor displacement. Flexible donor-acceptor complexes must show a
greater tendency for solvent-controlled electron transfer. Electron-transfer
energy chains are best designed by placing the redox cofactors near the
crossover distance.
