Time-resolved tryptophan phosphorescence spectroscopy: a sensitive probe of protein folding and structure

Room-temperature tryptophan phosphorescence from proteins is a sensitive probe of the local structural environment of the luminescent amino acid. Tryptophan phosphorescence lifetimes can vary over 3-4 orders of magnitude depending on the structural rigidity of the emitter environment and the proximity of quenching interactions, and this sensitivity can be used to characterize the local structural environment of the chromophore. Tryptophan phosphorescence lifetime can be easily monitored as a function of time, and thus is capable of producing effectively real-time information on dynamic processes in proteins. We discuss the origins and characteristics of tryptophan phosphorescence, and its application to monitoring folding processes in proteins and to studying protein conformation and flexibility. We also present the extension of phosphorescence techniques to study, for the first time, triplet emission at room temperature from tryptophan residues engineered into specific positions as reporters of protein structure.