Femtosecond studies of tryptophan fluorescence dynamics in proteins: local solvation and electronic quenching

We report our systematic examination of tryptophan fluorescence dynamics in proteins with femtosecond resolution. Distinct patterns of femtosecond-resolved fluorescence transients from the blue to the red side of emission have been characterized to distinguish local ultrafast solvation and electronic quenching. It is shown that tryptophan is an ideal local optical probe for hydration dynamics and protein-water interactions as well as an excellent local molecular reporter for ultrafast electron transfer in proteins, as demonstrated by a series of biological systems, here in melittin, human serum albumin, and human thioredoxin, and at lipid interfaces. These studies clarify the assignments in the literature about the ultrafast solvation or quenching dynamics of tryptophan in proteins. We also report a new observation of solvation dynamics at far red-side emission when the relaxation of the local environment is slower than 1 ps. These results provide a molecular basis for using tryptophan as a local molecular probe for ultrafast protein dynamics in general.