Direct Observation of the Femtosecond Excited-State CIS-TRANS Isomerization in Bacteriohodopsin.

Femtosecond optical measurement techniques have been used to study the primary photoprocesses in the light-driven transmembrane proton pump, bacteriorhodopsin. Light-adapted bacteriorhodopsin is excited with a 60 fs pump pulse at 618 nm and the transient absorption spectra over a 150 nm range are recorded from - 50 to 1000 fs using 6 fs probe pulses. By 60 fs, a broad transient hole appears in the absorption spectrum whose amplitude remains constant for about 200 fs. Excited state emission in the 660 to 710 nm region and excited state absorption in the 560 to 580 nm region appear promptly and then shift and decay from 0 to ~150 fs. These spectral features provide a direct observation of the 13-trans -> 13-cis torsional isomerization of the retinal chromophore on the excited state potential surface. Absorption due to the primary ground state photoproduct J appears with a tau sub (1/2) of ~500 fs.