The protonation of a retinyl Schiff base: a study by FTIR spectroscopy at low temperature in solution.

The hydrogen bonding-protonation equilibrium for retinyl Schiff base/propionic acid or 3-chloropropionic acid systems was examined by Fourier transform infrared spectroscopy in non polar solutions at temperatures ranging from 25 degrees C to about -150 degrees C. The spectra give evidence for the gradual increase in the degree of protonation as temperature is lowered. The bearing of this on applying low temperature spectroscopic results to physiological conditions in rhodopsin research is discussed.

First step in vision: proton transfer or isomerization?

Recent views on the photochemistry of (vertebrate) vision are examined. Problems related to the structure of the proton bridge at the Schiff base chromophore are considered, in particular the possible shapes of the section of the potential surface governing the motion of the proton in the bridge. This leads to the question as to whether proton translocation occurs in the initial step of visual transduction and if it precedes or follows cis-trans isomerisation. The related controversy could be solved through the assumption of the presence of water molecules that stabilize the ions in the proton bridge. The causes of the instability of bathorhodopsin are discussed and the importance of additional perturbations by polar groups is stressed.

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

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.