Intramolecular electron transfer in cyclopeptide involving tryptophan and tyrosine and corrected two-sphere model for solvent reorganization energy

Evidence for concerted electron proton transfer in charge recombination between FADH- and 306Trp• in Escherichia coli photolyase

Proton-coupled electron-transfer (PCET) is a mechanism of great importance in protein electron transfer and enzyme catalysis, and the involvement of aromatic amino acids in this process is of much interest. The DNA repair enzyme photolyase provides a natural system that allows for the study of PCET using a neutral radical tryptophan (Trp(•)). In Escherichia coli photolyase, photoreduction of the flavin adenine dinucleotide (FAD) cofactor in its neutral radical semiquinone form (FADH(•)) results in the formation of FADH(-) and (306)Trp(•). Charge recombination between these two intermediates requires the uptake of a proton by (306)Trp(•). The rate constant of charge recombination has been measured as a function of temperature in the pH range from 5.5 to 10.0, and the data are analyzed with both classical Marcus and semi-classical Hopfield electron transfer theory. The reorganization energy associated with the charge recombination process shows a pH dependence ranging from 2.3 eV at pH ≤ 7 and 1.2 eV at pH(D) 10.0. These findings indicate that at least two mechanisms are involved in the charge recombination reaction. Global analysis of the data supports the hypothesis that PCET during charge recombination can follow two different mechanisms with an apparent switch around pH 6.5. At lower pH, concerted electron proton transfer (CEPT) is the favorable mechanism with a reorganization energy of 2.1-2.3 eV. At higher pH, a sequential mechanism becomes dominant with rate-limiting electron-transfer followed by proton uptake which has a reorganization energy of 1.0-1.3 eV. The observed 'inverse' deuterium isotope effect at pH < 8 can be explained by a solvent isotope effect that affects the free energy change of the reaction and masks the normal, mass-related kinetic isotope effect that is expected for a CEPT mechanism. To the best of our knowledge, this is the first time that a switch in PCET mechanism has been observed in a protein.

Continuous medium theory for nonequilibrium solvation: IV. Solvent reorganization energy of electron transfer based on conductor-like screening model

In this work the authors present some evidences of defects in the popular continuous medium theories for nonequilibrium solvation. Particular attention has been paid to the incorrect reversible work approach. After convincing reasoning, the nonequilibrium free energy has been formulated to an expression different from the traditional ones. In a series of recent works by the authors, new formulations and some analytical application models for ultrafast processes were developed. Here, the authors extend the new theory to the cases of discrete bound charge distributions and present the correct form of the nonequilibrium solvation energy in such cases. A numerical solution method is applied to the evaluation of solvent reorganization energy of electron transfer. The test calculation for biphenyl-cyclohexane-naphthalene anion system achieves excellent agreement with the experimental fitting. The central importance presented in this work is the very simple and a consistent form of nonequilibrium free energy for both continuous and discrete charge distributions, based on which the new models can be established.