Excited-state dynamics of pharaonis phoborhodopsin probed by femtosecond fluorescence spectroscopy

Femtosecond time-resolved fluorescence spectroscopy has been used to study the excited-state dynamics of pharaonis phoborhodopsin (ppR) from Natronobacterium pharaonis. Upon excitation of ppR with a femtosecond pulse (444 nm), fluorescence decay kinetics are measured in the wavelengths between 488 and 678 nm. The obtained kinetics are strongly dependent on the probing wavelengths, and the decay components are approximately classified into the femtosecond (80-250 fs) and early picosecond (1.7-3.0 ps) components. The lifetimes are similar to those for the chromophore (a protonated Schiff base of all-trans retinal; AT-PSB) in methanol solution (90-800 fs and 2.5-3.7 ps) [Kandori, H.; Sasabe, H. Chem. Phys. Lett. 1993, 216, 126-132], implying that rapid deactivation from the excited state in rhodopsins is essentially, part of the nature of the chromophore itself. In contrast, the femtosecond component is more predominant in the protein environment of ppR than in solution. Together with their quantum yields of photoisomerization (0.5 for ppR and 0.15 for AT-PSB in methanol) and product formation time in the recent pump-probe measurement of ppR [Lutz et al. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 962-967], we concluded that the femtosecond components correspond to highly efficient photoisomerization through barrierless transition in the excitedstate whose quantum yield is close to unity. On the other hand, picosecond components are correlated with deactivation processes back to the ground state of ppR, whose isomerization quantum yield is low. Highly organized protein matrix could concentrate the excited molecules into the reaction coordinate for photoisomerization.