FEMTOSECOND STUDIES OF PRIMARY PHOTOPROCESSES IN OCTOPUS RHODOPSIN

Femtosecond spectroscopy of octopus rhodopsin in H2O and D2O was performed over a very wide spectral region of 400-1000 nm. Transient gain and absorption from the excited state were observed for the first time around 650 and 700 nm, respectively, just after 300 fs pulse excitation. Bathorhodopsin was formed within 400 fs from the excited state; therefore, the cis-trans isomerization completes within 400 fs. The first intermediate ''primerhodopsin'' found in our previous paper is most likely ''quasi-thermal'' bathorhodopsin, in which the local thermalization of the chromophore is achieved. Then cooling down of the chromophore to the surrounding protein temperature takes place with 20 +/- 10 ps along with blue-shifting of a spectrum of 10 +/- 5 nm. In addition to these observations, a prominent gain in the region of > 850 nm was observed and decayed with 2-3 ps in H2O. A similar time constant was estimated for a partial decay of an induced absorption around 600 nm. This process may be related with two forms of bathorhodopsin reported previously. In this scheme, two forms of bathorhodopsin are formed with time constants of about 400 fs and 2 ps. In the sample in D2O, time constant of 3-4 ps was obtained for the slower process.