title: Two-dimensional spectroscopy of electronic couplings in photosynthesis.
abstract: Time-resolved optical spectroscopy is widely used to study vibrational and electronic dynamics by monitoring transient changes in excited state populations on a femtosecond timescale. Yet the fundamental cause of electronic and vibrational dynamics--the coupling between the different energy levels involved--is usually inferred only indirectly. Two-dimensional femtosecond infrared spectroscopy based on the heterodyne detection of three-pulse photon echoes has recently allowed the direct mapping of vibrational couplings, yielding transient structural information. Here we extend the approach to the visible range and directly measure electronic couplings in a molecular complex, the Fenna-Matthews-Olson photosynthetic light-harvesting protein. As in all photosynthetic systems, the conversion of light into chemical energy is driven by electronic couplings that ensure the efficient transport of energy from light-capturing antenna pigments to the reaction centre. We monitor this process as a function of time and frequency and show that excitation energy does not simply cascade stepwise down the energy ladder. We find instead distinct energy transport pathways that depend sensitively on the detailed spatial properties of the delocalized excited-state wavefunctions of the whole pigment-protein complex.
authors.affiliation: Department of Chemistry, and the Institute for Quantitative Biomedical Research (QB3), University of California, Berkeley, and Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
authors.forename: Tobias
authors.initials: T
authors.lastname: Brixner
authors.forename: Jens
authors.initials: J
authors.lastname: Stenger
authors.forename: Harsha M
authors.initials: HM
authors.lastname: Vaswani
authors.forename: Minhaeng
authors.initials: M
authors.lastname: Cho
authors.forename: Robert E
authors.initials: RE
authors.lastname: Blankenship
authors.forename: Graham R
authors.initials: GR
authors.lastname: Fleming
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism; D041883:Chlorobium / Q000737:chemistry / Q000378:metabolism; D004579:Electron Transport; D008027:Light; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry / Q000378:metabolism; D008956:Models, Chemical; D008958:Models, Molecular; D010788:Photosynthesis / Q000528:radiation effects; D011487:Protein Conformation; D011522:Protons; D013057:Spectrum Analysis / Q000379:methods*; D013997:Time Factors; D014732:Vibration
chemical_list: D001426:Bacterial Proteins; D045342:Light-Harvesting Protein Complexes; D011522:Protons; C026083:bacteriochlorophyll A proteins, Bacteria