Solvent Effects on the Spectra of Retinal Schiff Bases—11. Models for Convergence and Clustering of Visual Pigment Spectra*

Electronic absorption spectra of retinal (A,) and 3‐dehydroretinal (A 2 ) Schiff bases have been studied under a variety of solvent conditions. The spectra of the two species have different solvent dependencies. Variation in solvent conditions have a larger effect in the A 2 spectra, although the wavelength maxima of A, and A, Schiff bases smoothly converge in solvents that shift them both to shorter wavelengths. Convergence curves for the A, and A 2 Schiff bases can thus be generated in solution which resemble the convergence curves observed in visual pigments. It is also shown that the spectra of visual pigments made by combining A, and A 2 with opsins from various fish species can be reproduced theoretically by changing the number of tryptophan or tyrosine side chains which are assumed to line the chromophore pocket. Discrete changes in the number of these side chains can exactly reproduce the clustering effect observed for a large variety of fish visual pigments. Variations in solvent characteristics which are analogous to possible chromophore environments in visual pigments have thus been shown to effect Schiff base spectra in such a way as to mimic the bathochromic shift, spectral convergence, and spectral clustering phenomena exhibited by visual pigments.