FEMTOSECOND DYNAMICS OF PHOTOSYNTHETIC LIGHT HARVESTING

摘要

Understanding the mechanisms underlying the process of light harvesting is a major goal in photosynthesis research, and relies on information from a variety of techniques that probe the structural, spectroscopic, and ultrafast temporal dynamics of the relevant pigment molecules. In general, the excitation dynamics in a photosynthetic system can be described as a sequence of ultrafast transfer steps between species with distinct spectral bands; a downhill energy gradient dictates the flow between chromophores. Recent determinations of the structures of light-harvesting complexes in purple bacteria have created a means for direct correlation of function and design within a relatively simple photosynthetic system.Here we report observations of ultrafast carotenoid band shifts correlated with energy transfer dynamics between bacteriochlorophyll (BChl) molecules within purple photosynthetic bacterial antenna complexes. Direct excitation of the BChl Q_y bands yielded distinct changes in the carotenoid S_2 absorption. Transient absorption spectra and kinetics were measured in a femtosecond pump-probe experiment at both room and low temperatures, revealing the ultrafast carotenoid response to excited BChl molecules. Our results indicate a new property of carotenoids manifested as a unique ability to detect and report changes in their immediate environment, thereby serving as sensitive probes of local structure and dynamics.