Dynamics of low-barrier proton transfer in polar solvents and protein media

Abstract

Abstract Proton transfer in chemical, physical, and biological systems frequently involves low and shallow barriers. This is caused by strong donor-acceptor interaction which also imposes fully adiabatic character on the process. Low or shallow barriers are not covered straightforwardly by tunnel views otherwise central in proton transfer theory. We introduce a formalism focused on a shallow enough proton transfer mode that the proton dynamics is indistinguishable from classical behaviour, strongly coupled to stochastic environmental nuclear motion. We derive rate constants for three cases, namely fast solvent relaxation, fast proton equilibration, and proton transfer followed by fast decay of the donor-acceptor complex. These cases may constitute useful descriptions of several kinds of processes including fast intramolecular proton transfer, hydrolytic and redox enzyme catalysis, and physical or biological proton conductivity.