Photo-induced electron transfer between tryptophan and cysteine: Temperature dependence

Photo-induced electron transfer (PET) is used in experimental techniques to measure intra-molecular contact formation rates in intrinsically disordered proteins (IDPs) and polymers. A nanosecond UV excitation is used to excite W to its triplet state, which is quenched by cysteine (C) (or cystine (C_C)) via electron transfer upon contact, due to stochastic collisions. When a W and C (or C_C) are introduced in the IDP sequence at given positions, the rate of quenching via PET reports on the rate of contact formation between those two positions in the IDP. Our group has been using this technique to reveal important information about the conformational ensembles and dynamics of IDPs (see poster on NTAIL by L. Otteson et al.). While in the past we have been able to collect a large amount of temperature-dependent PET data in IDPs, their quantitative interpretation is complicated by the intrinsic temperature-dependence of the quenching process itself. Accurate measurements of the temperature dependence of the electron transfer rate (kET), between W and C in solution, are missing. Similar to our previous study, where we used C_C as a quencher (Kodis et al. 2021), here we present an experimental study of bimolecular quenching rates between W and C, measured in aqueous solution as a function of viscosity and temperature. We obtain reaction-limited (kET) and diffusion-limited (kD) contributions to the quenching rate at different temperatures. From these, we obtain the activation energy for the electron transfer rate (kET) between W and C and compare it to the value we previously found using C_C as a quencher. We discuss the implications of our findings for temperature dependent PET measurements in IDPs.