Molecular dynamics simulation of proton-coupled electron transfer in solution

A new approach for the molecular dynamics simulation of proton-coupled electron transfer reactions in solution is presented. The solute is represented by a four-state valence bond model, and the solvent is described by explicit solvent molecules. The nuclear quantum effects of the transferring hydrogen are incorporated with a procedure based on a series of purely classical molecular dynamics simulations. The resulting mixed electronic/vibrational free energy surfaces depend on two solvent reaction coordinates corresponding to electron and proton transfer. This approach is shown to be equivalent to adiabatic mixed quantum/classical molecular dynamics, in which the nuclear quantum effects are included during the simulation, under well-defined, physically reasonable conditions. The results of the application of this approach to a model system are compared to those from a previous study based on a dielectric continuum treatment of the solvent. In addition, specific molecular motions of the solvent associated with proton-coupled electron transfer are identified, and solvent configurations that couple the proton and electron transfer reactions are characterized. This methodology may be implemented using commercial molecular dynamics software packages with little or no modification to the existing programs.