Excited-State Lifetimes of Far-Infrared Collective Modes in Proteins

Vibrational excitations of low frequency collective modes are essential for functionally important conformational transitions in proteins. Here we report the first direct measurement on the lifetime of vibrational excitations of the collective modes at 87 [Formula presented] [Formula presented] in bacteriorhodopsin, a transmembrane protein. The data show that these modes have extremely long lifetime of vibrational excitations, over 500 ps, accommodating 1500 vibrations. We suggest that there is a connection between this relatively slow anharmonic relaxation rate of approximately [Formula presented] and the similar observed rate of conformational transitions in proteins, which require multilevel vibrational excitations.