Primary reactions of sensory rhodopsins

The first steps in the photocycles of the archaeal photoreceptor proteins s ensory rhodopsin (SR) I and II from Halobacterium salinarum and SRII from N atronobacterium pharaonis have been studied by ultrafast pump/probe spectro scopy 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 300-4 00 fs and a transition between two red-shifted product states in 4-5 ps. Th us 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 electronic state occurring on the picosecond time scale (5 ps and 33 ps) is observed. The results suggest that the primary reactions a re controlled by the charge distribution in the vicinity of the Schiff base and demonstrate that there is no direct connection between absorption prop erties and reaction dynamics for the retinal protein family.