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

13C NMR and absorption measurements are performed on retinal protonated Schiff bases in solution and solid state to analyze the effects of Schiff base environment changes, ring-chain s-trans planarity and nonconjugated positive and negative charges. Results show that the Schiff base environment affects the C5 chemical shift of the retinal protonated Schiff base. The nonconjugated charge effect is controlled by the spatial arrangement between the chemical shift and the double bond. Moreover, the absorption maximum of bacteriorhodopsin is controlled by ring-chain s-trans planarity.