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 ε-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 questioned2-5 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.
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