Efficient Intrinsic Photoprotection in Strongly Coupled (Bacterio) Chloropyll Complexes

Abstract

The earliest steps in bacterial photosynthesis require that an antenna system efficiently capture incident photons and shuttle the excitation energy to the ''special pair'' bacteriochlorophylls within the membrane-bound reaction center where charge separation occurs. Previous work has shown coherent energy transfer-a wavelike transfer process-among peripheral chromophores, bacteriopheophytins and accessory bacteriochlorophylls, at cryogenic temperatures. Whether or not this coherent transfer extends to the special pair, however, has remained elusive at any temperature. Here we report direct evidence that the special pair is coherently coupled to the accessory bacteriochlophylls and that this coherence dephases only upon transfer to the special pair-the maximal amount of coherence physically possible. We employ Gradient Assisted Photon Echo Spectroscopy to simultaneously excite the bacteriopheophytins, accessory bacteriochlorophylls and the special pair in the reaction center from Rhodobacter sphaeroides. These results suggest the bacteria exploits coherent energy transfer at room temperature.