Substituents at the C13 Position of Retinal and Their Influence on the Function of Bacteriorhodopsin

Retinal analogues in which the 13-methyl group is replaced by H, C₂H₅, CF₃, and OCH₃ residues are studied by means of quantumchemical modified neglect of diatomic overlap-correlated version (MNDOC) calculations. The analogues are suitable to test the stereochemical mechanism of proton pumping in bacteriorhodopsin. The results explain the proton-pumping activities of bacterio-opsin reconstituted with these analogues and elucidate the decisive role of retinal's ground-state intramolecular properties in the pump cycle of bacteriorhodopsin.