Photochemical cis–trans isomerisation of bovine rhodopsin at liquid helium temperatures

Abstract

RHODOPSIN is the visual pigment in the disk membranes of vertebrate rod photoreceptors. It consists of a chromophore, 11-cis-retinal, covalently bound in the form of a protonated Schiff base to an ɛ-amino group of a lysine in its apoprotein opsin. The primary photochemical event in visual excitation involves the formation of a species known as bathorhodopsin¹ which has been detected at room temperature² and at 4 K (ref. 3). The rhodopsin–bathorhodopsin transformation has been thought to involve a photochemical 11-cis → all-trans isomerisation; however, this conclusion has recently been questioned^2–5 primarily because bathorhodopsin is formed within a few picoseconds at room temperature² and in 36 ps at 4 K (ref. 3), and because of the large isotope effect involved in its formation³. In this letter, we show on the basis of resonance Raman experiments that isorhodopsin (the artificial pigment with 9-cis-retinal as its chromophore) is formed photochemically from rhodopsin at liquid helium temperatures. All models for the primary event other than isomerisation about the 11–12 double bond are found to be inconsistent with this result. We present a specific model (see also ref. 6) for the formation of bathorhodopsin which accounts for all the available data.