Dynamic Equilibrium between the Fluorescent State of Tryptophan and Its Cation-Electron Ion Pair Governs Triplet State Population.

Tryptophan is the most efficient fluorophore of the naturally occurring amino acids and is widely used as a fluorescence probe of protein structure and function. As a result of its importance, there have been numerous studies of the ultrafast photochemical dynamics of tryptophan. Nonetheless, these studies have not identified the pathway to the triplet state, which competes with fluorescence emission. Here, we combine femtosecond-to-microsecond time-resolved transient absorption spectroscopy and time-resolved infrared spectroscopy to explore the photochemical pathway from the UV excitation of tryptophan in aqueous solution to the population of the triplet state and its subsequent relaxation. We observe prompt formation of cations and solvated electrons consistent with autoionization to form a cation-electron ion pair. We find that the cation-electron ion pair subsequently decays with time scales that match the fluorescence lifetime of tryptophan in aqueous solution, indicative of a dynamic equilibrium between the fluorescent state and the cation-electron ion pair. We also find that population of the triplet state occurs on the same time scale as the decay of the cation-electron ion pair and fluorescence, indicating that the triplet state is populated either by recombination of a separated cation and electron after a spin flip or by intersystem crossing from the fluorescent state. Regardless of which mechanism dominates, population of the triplet state of tryptophan is governed by the dynamic equilibrium between the fluorescent state and the cation-electron ion pair.