Hypsorhodopsin: the first intermediate of the photochemical process in vision

The primary processes in visual excitation are initiated by a photochemical event, the absorption of a photon by the photoreceptor, rhodopsin, resulting in successive thermal reaction forming several intermediates. Spectral changes due to the formation of these intermediates after excitation of the rhodopsin occur very rapidly at physiological temperature. The spectral changes have been observed by freezing samples in glass-forming solvents at low temperatures where interconversion rates are so slow that ordinary absorption spectroscopy suffices. By the low temperature method, bathorhodopsin (formerly called prelumirhodopsin) [ 1,2] and hypsorhodopsin [2 ] were observed by irradiating bovine rhodopsin at 77 K and 4 K, respectively. Since the first observation of hypsorhodopsin, it has been a very important question whether hypsorhodopsin or bathorhodopsin is the first intermediate after the absorption of the photon by the photoreceptor, rhodopsin. The room temperature kinetics of formation and decay of an intermediate species absorbing at 560 nm were investigated [3] using picosecond spectroscopy. Using the second harmonic of a Nd/glass laser (fwhm 6 ps, 530 nm) as a picosecond excitation light source, it was concluded that after picosecond excitation of bovine rhodopsin solubilized in LDAO (Ammonyx LO) detergent, bathorhodopsin was formed within 6 ps [4]. Using a 20 ps step echelon, hypsorhodopsin was not found to be formed by 530 nm excitation of bovine rhodopsin in LDAO [S]. Squid hypsorhodopsin has been observed at room temperature with the use of picosecond ruby laser