On modeling the vibrational spectra of 14- s-cis retinal conformers in bacteriorhodopsin

Abstract

The vibrational properties of 13- cis,14- s-trans and 13- cis,14- s-cis protonated retinal Schiff base model compounds are explored with MNDO calculations. In particular, the effect of isomerization about the C 14C 15 single bond on the vibrational properties of the deuterium in-plane rocking vibrations has been examined. Our MNDO calculations, using a variety of lysine models, lysine conformations and Schiff base charge environments, demonstrate that the C 14-D and C 15-D in-plane rocking vibrations in the 14,15-dideuterio retinal protonated Schiff base are strongly coupled in 13- cis,14- s-cis molecules producing a splitting of ca. 80 cm −1 between the symmetric and antisymmetric rocking mode combinations but that these modes are only weakly coupled in 14- s-trans molecules. This analysis demonstrates that the 14,15-dideuterio labeling method developed earlier for determining C 14C 15 conformation (S.P.A. Fodor, W.T. Pollard, R. Gebhard, E.M.M. van den Berg, J. Lugtenburg and R.A. Mathies, Proc. Natl. Acad. Sci. USA, 85 (1988) 2156–2160) is valid, and hence that the structure of the retinal chromophore in bacteriorhodopsin's L 550 intermediate is 13- cis,14- s-trans. The reasons for the misleading conclusions derived from MNDO calculations performed earlier by Schulten and Tavan are discussed.