Excited-state Reaction Dynamics of Bacteriobhodopsin Studied by Femtosecond Spectroscopy

The photodynamics of bacteriorhodopsin were studied by transient absorption and gain measurements after excitation with femtosecond pulses at 620 nm. With probing pulses at longer wavelengths (A> 770 nm) the previously reported formation of the J intermediate (with a time constant of 500f 100 fs) was confirmed. With probing pulses around 700 nm, a faster process with a relaxation time of 200 rt 70 fs was observed. The data analysis strongly suggests that this kinetic constant describes the reactive motion of the polyatomic molecule on its excited-state potential energy surface, i.e. one observes directly the incipient isomerization of the retinal molecule. The minimum of the S, potential energy surface reached in 200 fs lies approximately 13300 cm’ above the ground state of bacteriorhodopsin and from this minimum the intermediate J is formed with a time constant of 500 fs.