What are the benefits of bound (protonation) states for the electron-transfer kinetics?

TL;DR

This paper presents a model showing how local binding and protonation states influence electron transfer kinetics, predicting non-Gaussian fluctuations and a maximum in charge-transfer bandwidth with varying binding particle concentration.

Contribution

It introduces a non-Gaussian, non-parabolic free energy surface model for electron transfer affected by binding, explaining pH-dependent rate changes.

Findings

Charge-transfer bandwidth peaks with binding particle concentration.

Electron transfer rates are modulated by protonation states.

Non-Gaussian energy gap fluctuations influence kinetics.

Abstract

We describe a model of electron transfer reactions affected by local binding to the donor or acceptor sites of a particle in equilibrium with the solution. The statistics of fluctuations of the donor-acceptor energy gap caused by binding/unbinding events are non-Gaussian, and the resulting free energy surfaces of electron transfer are non-parabolic. The band-width of the charge-transfer optical transition is predicted to pass through a maximum as a function of the concentration of binding particles in the solution. The model is used to rationalize recent observations of pH-dependence of electron transfer rates involving changes in the protonation state of the donor-acceptor complex.