Excited-state reaction dynamics of bacteriorhodopsin 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 (λ >
770 nm) the previously reported formation of the J intermediate
(with a time constant of 500±100 fs) was confirmed. With probing
pulses around 700 nm, a faster process with a relaxation time of
200±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 S1 potential energy surface
reached in 200 fs lies approximately 13300 cm−1 above the ground
state of bacteriorhodopsin and from this minimum the intermediate J
is formed with a time constant of 500 fs.