Femtosecond studies of protein–ligand hydrophobic binding and dynamics: Human serum albumin

In this contribution, we report studies of the nature of the dynamics and hydrophobic binding in protein–ligand complexes of human serum albumin with 2-(2′-hydroxyphenyl)-4-methyloxazole. With femtosecond time resolution, we examined the orientational motion of the ligand, its intrinsic nuclear motions, and the lifetime changes in the hydrophobic phase. For comparisons, with similar but chemical nanocavities, we also studied the same ligand in micelles and cyclodextrins. The hydrophobic interactions in the binding crevice are much stronger than those observed in cyclodextrins and micelles. The confined geometry restrains the nonradiative decay and significantly lengthens the excited-state lifetime. The observed dynamics over the femtosecond-to-nanosecond time scale indicate that the binding structure is rigid and the local motions of the ligand are nearly “frozen” in the protein. Another major finding is the elucidation of the directed dynamics by the protein. Proton transfer and intramolecular twisting of 2-(2′-hydroxyphenyl)-4-methyloxazole were observed to evolve along two routes: one involves the direct stretching motion in the molecular plane (≈200 fs) and is not sensitive to the environment; the second, less dominant, is related to the twisting motion (≈3 ps) of the two heterocyclic rings and drastically slows down in the protein hydrophobic pocket.