The Schiff base bond configuration in bacteriorhodopsin and in model compounds.
The Schiff base linkage bond configuration of bacteriorhodopsin was studied using model compounds consisting of all-trans- and 13-cis-retinal-protonated Schiff bases bearing C = N anti and syn bond configurations. The C = N configuration was analyzed using a combination of Fourier transform infrared spectroscopy and isotopically labeled chromophores. It was found that, in the model compounds, the coupling between the C14--C15 stretching frequency and the N--H rock is weak in the all-trans-retinal-protonated Schiff base in both the anti and syn C = N configurations. However, this coupling is relatively strong in the 13-cis-retinal-protonated Schiff base in both the anti and syn C = N configurations. Thus, it is concluded that, in model compounds, the C14--C15 mode can serve as a marker for the C13 = C14 bond configuration but not for the C = N. A different situation may prevail in bacteriorhodopsin due to different conformations of the retinal chromophore in the protein binding site and in solution. This difference suggests that the C14--C15/NH coupling in retinal-protonated Schiff bases is affected by the retinal conformation.
Livnah
N
N
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, Israel.
Sheves
M
M
D001436:Bacteriorhodopsins / Q000737:chemistry*; D005583:Fourier Analysis; D009682:Magnetic Resonance Spectroscopy; D012172:Retinaldehyde / Q000737:chemistry; D012545:Schiff Bases / Q000737:chemistry*; D013055:Spectrophotometry, Infrared; D013059:Spectrum Analysis, Raman
D012545:Schiff Bases; D001436:Bacteriorhodopsins; D012172:Retinaldehyde