Photochemical studies of 7-cis-rhodopsin at low temperatures. Nature and properties of the bathointermediate.

The photoreaction of 7-cis-rhodopsin derived from 7-cis-retinal and cattle opsin was studied by low-temperature spectrophotometry. Upon irradiation of 7-cis-rhodopsin at liquid nitrogen temperature (-190 degrees C) with blue light, its spectrum shifted to the longer wavelengths, indicating the formation of a bathoproduct. The bathoproduct thus formed was found to be identical with bathorhodopsin formed from rhodopsin in their spectroscopic, photochemical, and thermal properties. Therefore, we believe that the bathoproduct is, in fact, bathorhodopsin. The fact that 7-cis-rhodopsin can be readily converted to rhodopsin and to 9-cis-rhodopsin shows that the identical retinal binding site of opsin is involved in the three isomeric rhodopsins. These results appear to be consistent with the notion that the chromophore of bathorhodopsin is a twisted all-trans isomer, which is readily obtainable from the 7-cis, 9-cis, and 11-cis isomers.