Chemical Aspects of the Visual Process

Abstract

THE chemistry of photo-reception remains obscure in spite of a vast literature, particularly rich in biophysical data; indeed it is doubtful whether on the chemical side present theories are more than the merest scaffolding. Recent statements by well-known workers display a confidence about the nature of visual purple (rhodopsin) which it is not easy to share. It is said to be "safe to assume that visual purple is a conjugated carotenoid-protein" (Hecht¹); rhodopsin "is a conjugated protein with a carotenoid prosthetic group called retinene; it is estimated that one molecule contains 10 prosthetic groups and that each such group contains one molecule of vitamin A" (Mitchell²). "Rhodopsin is a rose-coloured carotenoid-protein, in aqueous solution its absorption spectrum consists of a single broad band maximal at 500 mµ. In light it bleaches in a succession of photochemical and thermal reactions to orange and yellow products, liberating in the process the carotenoid retinene" (Wald³). "No doubt, therefore, the A vitamins are of importance for the formation of the ehromophoric group of the visual purple molecule, even though it may not be possible at present to describe the nature of this relationship" (Granit⁴).