Spectroscopic study of the batho-to-lumi transition during the photobleaching of rhodopsin using ring-modified retinal analogues.

Photochemical and subsequent thermal reactions of rhodopsin containing 9-cis-retinal [Rh(9)] or one of four analogues with 9-cis geometries formed from ring-modified retinals, alpha-retinal [alpha Rh(9)], acyclic retinal [AcRh(9)], acyclic alpha-retinal [Ac alpha Rh(9)], and 5-isopropyl-alpha-retinal [P alpha Rh(9)] were investigated by low-temperature spectrophotometry and nanosecond laser photolysis. Irradiation of each pigment at -180 degrees C produced a photosteady-state mixture containing the original 9-cis pigment, its 11-cis pigment, and a photoproduct, indicating that the primary process of each pigment is a photoisomerization of its chromophore. The photoproduct produced by the irradiation of AcRh(9) had an absorption spectrum red shifted from the original AcRh(9) and was identified as the batho intermediate of AcRh(9). It was converted to the lumi intermediate through a metastable species, the BL intermediate, which has never been detected in Rh(9) at low temperature and whose absorption maximum was at shorter wavelengths than that of the batho intermediate. In contrast, the absorption maxima of the photoproducts produced from the other analogue pigments were at shorter wavelengths than those of the original pigments. They were identified as BL intermediates on the basis of their absorption maxima and thermal stabilities. The formation time constant of the lumi intermediate at room temperature was found to be dependent on the extent of modification of the ring portion of the chromophore, decreasing with the complete truncation of the cyclohexenyl ring [Ac alpha Rh(9)] and increasing with the attachment of the isopropyl group to the ring [P alpha Rh(9)].(ABSTRACT TRUNCATED AT 250 WORDS)