A NEW FACET IN RHODOPSIN PHOTOCHEMISTRY

Abstract— A structure is proposed for the prosthetic group of the visual pigments rhodopsin, prelumirhodopsin (bathorhodopsin) and lumirhodopsin. The intrinsic photochemical step in this model is tautomerization of the prosthetic group of rhodopsin to a hexaeneamine retrotautomer with an exomethylene group for prelumirhodopsin. Based on the proposed structures, molecular orbital calculations were carried out; the absorption maxima calculated snowed the same trends as the Λ_max values observed. An exact fit was not obtained because many interactions had to be neglected. Essential information of the laser Raman resonance spectrum of prelumirhodopsin can be interpreted based on the structures proposed by our model.
The model elucidates why some retinal derivates can and others cannot form visual pigments with opsin and visual pigments having vastly differing absorption maxima yet employ the same mechanism for their photoreaction.