of the bimolecular quenching rates between the tryptophan triplet state and cysteine, in aqueous solution, is so far lacking. This limits the application of PET. We present a careful experimental and theoretical study of bimolecular quenching rates, measured in aqueous solution, as a function of viscosity and temperature, via time-resolved transient absorption. We obtain both the reaction-limited (k ET) and diffusion-limited (k D) contributions to the quenching rate at different temperatures. The resulting activation energy for k ET is found to be 4.5Kcal/mol. When comparing to theory, we find that the measured rates cannot be described by nonadiabatic ET. A quantitative description of the rates is achieved when longitudinal dielectric relaxation time is used in the rate expression, accounting for solvent dynamical control. Our data clearly show that solvent reconfiguration dynamics contributes significantly to the ET process in solution, and cannot be ignored.
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