title: Are C14-C15 single bond isomerizations of the retinal chromophore involved in the proton-pumping mechanism of bacteriorhodopsin?
abstract: Resonance Raman spectroscopy is used to examine the possibility that C14-C15 single bond isomerizations of the retinal prosthetic group are involved in the photochemical reactions of bacteriorhodopsin. Normal mode calculations show that the vibration that contains predominantly C14-C15 stretch character is approximately equal to 70 cm-1 lower in frequency in the 14-s-cis conformer than in the s-trans case. This geometric effect is insensitive to out-of-plane twists and should be observed in the sterically hindered 13-cis, 14-s-cis retinal protonated Schiff base, which has been proposed as the chromophore in the K and L intermediates of bacteriorhodopsin. Resonance Raman spectra were obtained of K625 by using the low temperature (77 K) spinning-cell technique. Isotopic substitutions with 13C and 2H show that significant C14-C15 stretch character is observed in normal modes at approximately equal to 1185-1195 cm-1. The relatively high frequency of the C14-C15 stretch argues that K625 contains a 13-cis, 14-s-trans chromophore. Similarly, isotopic derivatives show that L550 has a localized C14-C15 stretch at 1172 cm-1, consistent with a 14-s-trans chromophore. These results argue that the primary step in bacteriorhodopsin is a C13=C14 trans----cis photoisomerization that does not involve C14-C15 s-cis structures.
mesh_terms: D001436:Bacteriorhodopsins / Q000502:physiology*; D002338:Carotenoids / Q000502:physiology*; D011487:Protein Conformation; D011522:Protons; D012172:Retinaldehyde / Q000502:physiology*; D012176:Retinoids / Q000502:physiology*; D012545:Schiff Bases / Q000502:physiology*; D013059:Spectrum Analysis, Raman
chemical_list: D011522:Protons; D012176:Retinoids; D012545:Schiff Bases; D002338:Carotenoids; D001436:Bacteriorhodopsins; D012172:Retinaldehyde