Wavelength regulation in rhodopsin: effects of dipoles and amino acid side chains

The effects of dipoles and aromatic amino acid side-chain models on the absorption and optical activity of the rhodopsin chromophore were calculated by using perturbation theory, and the results were compared with those of a Pariser-Parr-Pople calculation for the unperturbed system. The interaction was assumed to result from purely electrostatic interactions. It was concluded that the side chains of phenylalanine and tryptophan should have no important effects. However, the charge separation in tyrosine is sufficient to cause substantial electrostatic perturbation; in fact, the effect of tyrosine is large enough to approximately many of the spectral properties of rhodopsin quantitatively. This is encouraging because the use of aromatic amino acid side-chain analogs probably provides a better physical model than the use of isolated full charges, except in the case of the counterion to the protonated Schiff base.