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 pm (115 cm^-1) in bacteriorhodopsin, a transmembrane protein. The data show that these modes have extremely long lifetime of vibrational excitations, over 500 picoseconds, accommodating 1500 vibrations. We suggest that there is a connection between this relatively slow anharmonic relaxation rate of approximately 10 g sec^-1 and the similar observed rate of conformational transitions in proteins, which require require multi-level vibrational excitations and energy exchanges with other vibrational modes and collisional motions of solvent molecules.