Understanding Short-Range Electron-Transfer Dynamics in Proteins

Short-range electron-transfer (ET) reactions in biological systems are usually ultrafast, having transfer rates comparable to or even faster than corresponding environmental fluctuations, and often display nonexponential behaviors. To understand these nonequilibrium ET dynamics, we carried out detailed theoretical analyses based on the Sumi-Marcus model. It is shown that the ET dynamics is largely determined by the relative time scales of the ET reaction and its surrounding motions. Significantly, different environmental fluctuations can produce a variety of apparent ET dynamics even with the same driving force, Δ G^o, and reorganization energy, λ. We applied our analyses to an ultrafast ET process in DNA repair by (6-4) photolyase and directly obtained the inner and outer reorganization energies (λ_i and λ_o) as well as the free energy Δ G^o of various mutants, providing mechanical insight into ultrafast short-range ET reactions in proteins.