Gated Electron Transfer: When Are Observed Rates Controlled by Conformational Interconversion?

We explore the consequences that arise when long-range intramolecular electron transfer (ET) occurs in a system that exhibits several stable conformational states that can interconvert at rates competitive with electron transfer. Section I recalls the kinetic scheme appropriate for conventional intramolecular ET where reactant and product each exhibit a single stable conformation. Section II discusses the role of conformational dynamics and then proceeds to develop the kinetic scheme that applies to a two-state system in which electron transfer is energetically and kinetically coupled to the conversion between two stable conformations. Section III presents the solution of the corresponding kinetic equations and analyzes their most interesting limiting forms. We find that changes in conformation can entirely alter both the rate and the mechanism of electron-transfer processes in these systems, and thus interpretations of kinetic measurements that ignore such phenomena can be dramatically in error. These results are applied to the task of disentangling electron-transfer and conformational influences on systems of current interest.