Photochemistry of visual pigments: an interpretation of spectral changes in terms of molecular associations and isomerization

Abstract

A unified view of the photochemical part of the visual process is presented. It is proposed that both conformational changes and changes in intermolecular interactions in the sequence that leads from rhodopsin through batho-lumi- and meta-I to meta-II- rhodopsin have to be considered in order to elucidate the mechanism of the visual process. The main intermolocular associations are assumed to be the hydrogen bond involving the nitrogen atom of the Schiff base and the interaction between a negative group and the beta-ionone ring. The two together can be used to explain the absorption wavelength of rhodopsin without actual protonation. The main line of thought is as follows: when light is absorbed the basicity of the Schiff base increases significantly. This triggers proton transfer in the H-bond. At the same time cis-trans isomerization begins but it only reaches the coplanar all-trans stage at metarhodopsin-II. Lumi-, meta-I and meta-II are way stations in the stepwise isomerization whereby the energy of the photon is used together with thermal energy. Batho- is probably still close to 11-cis which then becomes successively strained 13-cis and 15-cis. In vertebrate rhodopsins at the meta-II stage both the H-bond and the beta-ionone interaction are severed and meta-II becomes exposed to attack by water molecules. The importance of syn-anti isomerization on the C=N bond is emphasized. The irreversibility necessary for the production of a signal requires that the proton does not return to its original donor. The possible identity of the donor is discussed: it might be an amino acid or the polar part of a lipid. Relevant observations made on bacteriorhodopsin, squid rhodopsin and chicken iodopsin are discussed.