Evidence for a common BATHO-intermediate in the bleaching of rhodopsin and isorhodopsin

Nanosecond transient spectroscopic measurements of the BATHO products formed from photolysis of bovine rhodopsin (RHO) and isorhodopsin (ISO) are discussed. BATHO absorption spectra of RHO and ISO differ slightly, but both pigments exhibit wavelength maxima near 560 nm. An additional transient absorption at 440 nm is observed immediately following excitation of RHO but not ISO. The decay times and Arrhenius activation energies of the two 560 nm absorbing transients are the same. In addition to spectral differences in the photolysis of RHO and ISO, the bleaching yield as a function of 532 nm laser power is different, with the yield in RHO saturating at lower laser power than ISO. The bleaching yield of the two pigments has been modeled using the known extinction coefficients and quantum yields for the interconversion of RHO, ISO, and a single BATHO species. Agreement between experiment and the model is found if the effects of the laser polarization are considered. The data are consistent with a common BATHO in the photolysis of RHO and ISO.