Effects of solvents on a model of rhodopsin consisting of a conjugated imine and substituted acetic acids

A model system of rhodopsin consisting of a dienylidene Schiff base and different halogeno acids has been studied in two solvents, chloroform and methanol, by uv and 400-MHz nmr spectroscopies. The uv results indicate that in chloroform, a nonpolar solvent, monohalogenoacetic acids can protonate the Schiff base only partially, while nmr data show that the aldimine hydrogen is slightly affected by acids and that the acidic proton is moving rapidly between the donor and the acceptor. In methanol, data indicate a leveling effect. In relation with rhodopsin, our results imply that, depending on the environment surrounding the chromophore, the state of protonation of the Schiff base could be strongly affected. Finally, our model shows experimental evidence that Nakanishi's external point charge could be operating in visual pigments.