Optical picosecond studies of bacteriorhodopsin containing a sterically fixed retinal

The photochemical behaviour of an analogous bacteriorhodopsin
(9,12-Ph-BR) which contains the sterically fixed 9,12-phenylretinal
has been investigated with picosecond spectroscopy. The following
results have been obtained. No ground-state intermediate
photoproduct is found in agreement with the previous observation
that 9,12-Ph-BR does not exhibit proton pumping under illumination.
The excited singlet state has a lifetime of τS = 10 ± 2 ps. This
lifetime agrees favourably with the value calculated from the
radiative lifetime τrad = 6.2 ns and the fluorescence quantum
efficiency of 1.2·10−3. Excited-state absorption occurs which
results in fluorescence in the ultraviolet region. These various
observations differ drastically from the corresponding findings on
bacteriorhodopsin. Most important for an understanding of the
differences is the fact that 9,12-phenylretinal does not isomerize
in the protein's binding site in contrast to retinal. Our data
therefore suggest that the formation of the intermediate K observed
in bacteriorhodopsin is accompanied by the all-trans to 13-cis
isomerization.