Time-resolved infrared spectral analysis of the KL-to-L conversion in the photocycle of bacteriorhodopsin.

Time-resolved infrared spectra of the hydrated film of light-adapted bacteriorhodopsin were recorded from earlier than 200 ns to 450 microseconds after light excitation in the 1800-900-cm-1 region on a newly designed dispersive-type infrared spectrometer [Iwata & Hamaguchi (1989) Appl. Spectrosc. 44, 1431-1437]. Both the KL-to-L and L-to-M conversions were detected in this time range. The spectral shape of KL is similar to K measured at 77 K except for the intense hydrogen out-of-plane vibrational band at 984 cm-1. The kinetics of this band are different from those of the other KL-specific bands at 1510, 1296, and 956 cm-1. Since the hydrogen out-of-plane vibrational bands are intensified by twists of the polyene chain, a change in the twist of the chromophore is suggested within the lifetime of KL. During the decaying process of L, the KL-specific vibrational bands are observed in parallel with L, indicating that KL and L are in equilibrium.