Factors affecting the absorption maxima of polyene iminium systems a model study for rhod opsin and bacterio rhod opsin
Sheves, M.; Nakanishi, K.
Journal of the American Chemical Society 105(12): 4033-4039
External point-charge models were proposed to explain the variance in absorption maxima of cattle-rhodopsin and the purpose color of bacteriorhodopsin. In the former visual pigment model, a negative counteranion resides .apprx. 3 .ANG. from the protonated Schiff base (SBH+) linkage and another point charge is located .apprx. 3 .ANG. from C-12 and C-14 of the 11-cis-retinal chromophore. In the bacteriorhodopsin model a negative counteranion is located .apprx. 3 .ANG. from SBH+ and another point charge is present near the ionone ring. A total of 15 nonsteroidal and steroidal di-, tetra-, and hexaene iminium compounds with and without COOH/COO- groups near the polyene chain were synthesized in order to structurally mimic the charge distributions within the binding sites of the 2 pigments. The electronic spectra of these polyene iminium compounds were measured in different solvents with and without addition of base. The interplay of the following 3 factors determines the absorption maxima of these compounds, i.e., the additional negative charge, the distance between the iminium N and its counteranion, and the solvent polarity. In the case of the natural pigments, the evidence obtained so far shows that the external point charges are important spectroscopic determinants.