PRIMARY PHOTOCHEMICAL EVENTS IN HALORHODOPSIN STUDIED BY SUBPICOSECOND TIME-RESOLVED SPECTROSCOPY

Primary photochemical events of the light-driven chloride-pump halorhodopsin (hR) are studied at room temperature by subpicosecond transient absorption measurements- On excitation of hR with a 600-nm, 0.6-ps pulse, excited-state absorption and stimulated emission appear immediately in the 420-530-nm and 650-770-nm wavelength regions, respectively, and both decay with a time constant of 2.3 ps. The calculated absorption spectrum of the excited state of hR (hR*) has a peak at 516 nm and a shoulder at about 460 nm. Accompanied by the decay of hR*, the primary ground-state product appears at around 645 nm. The quantum yield of the product formation is determined to be 0.27. Thc detailed analysis of the kinetics at 645 nm provided the faster rise time of the product (less-than-or-equal-to 1.0 ps) than the decay of the excited state (2.3 ps), as well as the possible presence of the J-intermediate (hR(J)). Instead of the simple sequential kinetic model considering three states of hR*, hR(J), and hR, which has been applied to the primary process of bacteriorhodopsin, a parallel channel model is suggested for the primary process of hR. After Franck-Condon excitation, the cis-trans isomerization to hR(J) and the relaxation to hR* take place simultaneously. The latter decays only to hR via radiative and nonradiative processes. The present results suggest that the excited state having a reaction channel to cis-trans isomerization is not located at the potential minimum of hR* and that the relaxation process in the excited state is a process in competition with isomerization.