Reorganization energies and rate constants for electron reactions in glass-forming media and proteins

We discuss situations occurring in intramolecular electron transfer in glassy solvents and in proteins which themselves undergo glassing transitions. In these circumstances slow solvent relaxation causes the reaction pathway to vary substantially from the equilibrium reaction path, resulting in increased effective barriers and reduced effective outer-sphere reorganization energies compared to the transition-state (equilibrium) result. This effect is modelled simply by introducing a temperature dependent reorganization energy. Through inclusion of these non-equilibrium effects, sharp falloffs of electron transfer rate with decreasing temperature can be modelled with reasonable values of the equilibrium reorganization energy, rather than the inappropriately large values needed otherwise.