Evidence for a 13,14-cis cycle in bacteriorhodopsin

The authors discuss to what extent the vibrational spectra of bacteriorhodopsin are in agreement with a photo-cycle of bacteriorhodopsin that involves the sequence BR, IO(all-trans) → K(13,14-cis) → L(13,14-cis) → M(13-cis) → N(13-cis) → O(all-trans). The discussion is based on a quantumchemical modified neglect of diatomic overlap [MNDO] calculation of the vibrational spectra of the relevant isomers of the protonated retinal Schiff base. In particular, they investigated in these calculations the effects of different charge environments on the frequencies of the relevant C-C single bond stretching vibrations of these isomers