Resonance Raman spectroscopy of squid and bovine visual pigments: the primary photochemistry in visual transduction

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Abstract
Resonance Raman spectra of squid [Loligo peali] rhodopsin were obtained under a variety of temperature and illumination conditions. The data were characterized in terms of spectral contributions from squid rhodopsin, isorhodopsin, bathorhodopsin, lumirhodopsin, mesorhodopsin, P-465 and acid metarhodopsin. The results were compared with the spectral features obtained from bovine rhodopsin, isorhodopsin and bathorhodopsin. The data supported a proposed structure for the chromophore in bathorhodopsin which is not all trans, 11-cis, or 9-cis. This structure could be generated from either rhodopsin or isorhodopsin by a similar motion (simultaneously rotating chromophore C atoms 10 and 11 out-of-plane). The same distinct bathorhodopsin vibrational modes were detected when rhodopsin was illuminated between 4-100.degree. K. This demonstrated that under steady-state illumination the light-induced chromophore structural alterations occurring at 4.degree. K were very similar to those occurring at higher temperatures. The data indicated that bathorhodopsin is generated not only by structural transitions in the chromophore but also alterations in the opsin conformation, as recently proposed.

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