The first steps in the photocycles of the archaeal photoreceptor proteins sensory rhodopsin (SR) Ⅰ and Ⅱ from Halobacterium sali- narum and SRⅡ from Natronobacterium pharaonis have been studied by ultra fast pump/probe spectroscopy and steady-state fluorescence spectroscopy. The data for both species of the blue- light receptor SRⅡ suggests that their primary reactions are nearly analogous with a fast decay of the excited electronic state in 300--400 fs and a transition between two red-shifted product states in 4--5 ps. Thus SRⅡ behaves similarly to bacteriorhodopsin. In contrast for SRI at pH 6.0, which absorbs in the orange part of the spectrum, a strongly increased fluorescence quantum yield and a drastically slower and biexponential decay of the excited elec- tronic state occurring on the picosecond time scale (5 ps and 33 ps) is observed. The results suggest that the primary reactions are controlled by the charge distribution in the vicinity of the Schiff base and demonstrate that there is no direct connection between absorption properties and reaction dynamics for the retinal protein family.
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