Dependency of photon density on primary process of cattle rhodopsin

The primary photochemical reactions of cattle rhodopsin suspended in H2O or D2O were compared between excitation with both a weak and an intense picosecond laser pulse (wavelength, 532 nm; duration, 25 ps) at room temperature. The time-dependent change of absorbance at about 575 nm demonstrated that photohodopsin, a precursor of bathorhodopsin, was produced immediately after the excitation with a weak picosecond laser pulse. It decayed to bathorhodopsin with a time constant of 45 ps which is close to the value reported previously [Shichida et al., (1984) Photobiochem. Photobiophys., 7, 221-228]. No deuterium effect was observed in this process. Excitation with an intense laser pulse induced instantaneous increase of the absorbance at about 575 nm and remained at almost constant level on the picosecond time scale, which was in good agreement with the pioneering work [Busch et al., (1972) Proc. Natl. Acad. Sci., USA, 69, 2802-2806]. No deuterium effect was observed in this photochemical process.