ULTRAFAST CAROTENOID BAND SHIFTS PROBE STRUCTURE AND DYNAMICS IN PHOTOSYNTHETIC ANTENNA COMPLEXES

We report observations of ultrafast carotenoid band shifts correlated with energy transfer dynamics between bacteriochlorophyll (BChl) molec ules within the peripheral light-harvesting complex (LH2) from the pho tosynthetic bacterium Rhodobacter sphaeroides. Direct excitation of th e bacteriochlorophyll Q(y) bands yielded distinct changes in the carot enoid S-2 absorption from 430 to 530 nm. Transient absorption spectra and kinetics were measured in a femtosecond pump-probe experiment, rev ealing the ultrafast carotenoid response to excited BChl pigments. The se data are an indication of a new property of carotenoids that is man ifested as a unique ability to detect and report changes in their imme diate environment, thereby serving as sensitive probes of local struct ure and dynamics.