Carbon-13 NMR studies of model compounds for bacteriorhodopsin: factors affecting the retinal chromophore chemical shifts and absorption maximum

Absorption maxima and 13 C NMR chemical shifts were measured for retinal iminium salts. A good correlation was round between the absorption maxima and the 13 C chemical shifts or the retinal polyene carbons within the same solvent system. The chemical shifts or the odd-membered carbons or the polyene are affected much more than the even carbons by π-electron delocalization caused by perturbations in the Schiff base linkage vicinity