Solvent effects on the spectra of retinal Schiff bases--I. models for the bathochromic shift of the chromophore spectrum in visual pigments.

Abstract

Abstract— Retinylidine Schiff base spectra have been investigated in a variety of solvents that were chosen primarily to mimic possible environments of the chromophore in visual pigments. These studies have identified particular amino acid side chains which can have important influences on visual pigment spectra. Most amino acid side chain analogues or solvents which mimic the peptide bond environment have little effect on retinylidine Schiff base spectra. Tyrosine, tryptophan, and cysteine analogues and ionized aspartic and glutamic acid analogues produce pronounced red shifts in the spectra. As is explained in more detail in the following paper the presence of these amino acids in the chromophore “pocket” of the protein is sufficient to account for the spectra of many visual pigments. Calculations of the spectra of protonated Schiff bases under a variety of conditions have been considered in conjunction with the spectral data to provide a model for the nature of the chromophore interactions with its environment.In solution and in rhodopsin the protonated Schiff base linkage is likely to be associated with an anion. The presence of certain amino acid side chains along the remainder of the retinal chain in the protein can modify the spectrum by altering the polarizability and polarity of the environment. The polarity in the region of the ionone ring appears to be particularly important for shifting the spectrum. In solution the nucleophilicity of the solvent also has an important influence on the anion‐protonated Schiff base region of the chromophore.