Structure of the chromophoric group in bathorhodopsin

IN the photolysis of the visual pigment rhodopsin the intermediate first observed is bathorhodopsin1,2(formerly called prelumirhodopsin). It is generally held that isomerisation of 11-cis to all-trans retinal occurs during this initial step. On theoretical grounds, however, an hexaene–amine structure (Fig. 1) has been proposed as an alternative for the chromophore of bathorodopsin (ref. 3 and unpublished results of K. Van der Meer, J. J. C. Mulder and J.L.). This chromophore has an exomethylene double bond between the atoms C5 and C18 and the nitrogen of its ene–amine moiety derives from the ɛ-amino group of a lysine residue of opsin. It is a retrotautomer of the chromophoric group in native rhodopsin which is a protonated Schiff base of 11-cis retinal. We now present two lines of evidence which support the structure depicted in Fig. 1: first, the interpretation of a recently published laser resonance Raman spectrum of bathorhodopsin4, and second, experiments which establish hydrogen (deuterium) exchange during photolysis of rhodopsin.