¹³C NMR Studies of Model Compounds for Bacteriorhodopsin: Factors Affecting the Retinal Chromophore Chemical Shifts and Absorption Maximum

Absorption maxima and ¹³C NMR chemical shifts were measured for retinal iminium salts. A good correlation was found between the absorption maxima and the ¹³C chemical shifts of the retinal polyene carbons within the same solvent system. The chemical shifts of the odd-membered carbons of the polyene are affected much more than the even carbons by π-electron delocalization caused by perturbations in the Schiff base linkage vicinity. The absorption maxima of bacteriorhodopsin (bR) (568 nm) is closely mimicked by protonated Schiff base chromophores bearing ring-chain s-trans planarity and weak hydrogen bonding between their positively charged Schiff base linkage and its counteranion. These chromophores exhibit a C₅ chemical shift similar to the unusual one found in bacteriorhodopsin. These results indicate that it is possible to closely mimic the absorption maximum of bR and its C₅ chemical shift without requiring a nonconjugated negative charge in the vicinity of the retinal ring. The effect of nonconjugated positive and negative charges on the ¹³C chemical shifts of the retinal polyene is evaluated using synthetic retinal chromophores. The charges affect the chemical shift in an alternating fashion (namely, upfield and downfield shifts) and mainly affect the double bond located in the immediate vicinity of the charge. The influence of the charge is diminished as its distance is increased. The spatial arrangement of the charge, relative to the polyene, is crucial for its effect. A symmetric C=C/charge arrangement causes only a minor change in the chemical shift, still, however, affecting the absorption maximum of the retinal protonated Schiff base. The implications of these measurements for bacteriorhodopsin are discussed.