Early photolysis intermediates of the artificial visual pigment 13-demethylrhodopsin

Nanosecond time-resolved absorption measurements are reported for the room temperature photolysis of a modified rhodopsin pigment, 13-demethylrhodopsin, which contains the chromophore 13-demethylretinal. The measurements are consistent with the formation of an equilibrium between a BA-THO-13-demethylrhodopsin species and a blue-shifted species (relative to the parent pigment), BSI-13-demethylrhodopsin. The results are compared to those acquired after photolysis of native bovine rhodopsin [Hug, S. J., Lewis, J. W., Einterz, C. M., Thorgeirsson, T. E., & Kliger, D. S. (1990) Biochemistry (preceding paper in this issue)] and to results obtained after photolysis of several modified isorhodopsin pigments in which the BSI species was first observed. It is concluded that in all of the pigments the results are consistent with the formation of an equilibrium between BATHO and BSI, which subsequently decays on a nanosecond time scale at room temperature to a lumirhodopsin intermediate.