title: Fourier transform infrared evidence for Schiff base alteration in the first step of the bacteriorhodopsin photocycle.
abstract: The first step of the bacteriorhodopsin (bR) photocycle involves the formation of a red-shifted product, K. Fourier transform infrared difference spectra of the bR570 to K630 transition at 81 K has been measured for bR containing different isotopic substitutions at the retinal Schiff base. In the case of bacteriorhodopsin containing a deuterium substitution at the Schiff base nitrogen, carbon 15, or both, we find spectral changes in the 1600-1610- and 1570-1580-cm-1 region consistent with the hypothesis that the K630 C=N stretching mode of a protonated Schiff base is located near 1609 cm-1. A similar set of Schiff base deuterium substitutions for retinal containing a 13C at the carbon 10 position strongly supports this conclusion. This assignment of the K630 C=N stretching vibration provides evidence that the bR Schiff base proton undergoes a substantial environmental change most likely due to separation from a counterion. In addition, a correlation is found between the C=N stretching frequency and the maximum wavelength of visible absorption, suggesting that movement of a counterion relative to the Schiff base proton is the main source of absorption changes in the early stages of the photocycle. Such a movement is a key prediction of several models of proton transport and energy transduction. Evidence is also presented that one or more COOH groups are involved in the formation of the K intermediate.
mesh_terms: D001436:Bacteriorhodopsins; D002247:Carbon Isotopes; D002338:Carotenoids; D003903:Deuterium; D005583:Fourier Analysis; D006217:Halobacterium / Q000032:analysis; D007553:Isotope Labeling; D010777:Photochemistry; D012172:Retinaldehyde; D012545:Schiff Bases; D013055:Spectrophotometry, Infrared
chemical_list: D002247:Carbon Isotopes; D012545:Schiff Bases; D002338:Carotenoids; D001436:Bacteriorhodopsins; D003903:Deuterium; D012172:Retinaldehyde