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PHOTOCHEMICAL BEHAVIOR OF BACTERIORHODOPSIN IMMOBILIZED IN NaCl PELLETS

Abstract— Some photochemical reactions of bacteriorhodopsin (BR) embedded in NaCl pellets (BR‐NaCI) in the visible region are described here. BR in these preparations is a mixture of two classes of species: a drastically blue‐shifted form and the unchanged purple pigment. Depending on the illumination history of the BR before being immobilized, both kinds of BR could be demonstrated in light‐adapted (LA) and dark‐adapted (DA) forms, but light adaptation was not possible once the pellets were made. Analogously to BR suspensions, the light‐adapted blue‐shifted BRexhibited an a/l‐trans type photocycle, but the thermal steps were greatly slowed down (time constants 1 to 5 min). The parent species absorb at 506 nm. The DA blue‐shifted BR exhibited absorption changes resolved into two photoreactions, one all‐(rans‐ like (as in LA‐BR) and another, 13‐cw like, whose decay rate is also greatly slowed down (recovery time several hours). The parent species of the 13‐cis like cycle absorb at 480 nm. That pigment fraction in the pellets whose absorption was not blue‐shifted, also exhibited similar photoreactions to BR in suspension, but with an overall turnover rate only one order of magnitude slower. From a previous report (Lazarev and Terpugov, Biochim. Biophys. Acta590,324–338,1980) and this one, it appears that the very slow photocycles in NaCl‐BR of low moisture content originate from blue‐shifted chromophores rather than from unchanged BR.