Two-dimensional spectroscopy of electronic couplings in photosynthesis

• Published in: Nature 2005-03, Vol.434 (7033), p.625-628
• Main Authors: Fleming, Graham R, Stenger, Jens, Blankenship, Robert E, Brixner, Tobias, Cho, Minhaeng, Vaswani, Harsha M
• Format: Article
• Language: English
• Subjects: Bacteria; Bacterial Proteins - chemistry; Bacterial Proteins - metabolism; Biological and medical sciences; Chlorobium - chemistry; Chlorobium - metabolism; Electron Transport; Fundamental and applied biological sciences. Psychology; Humanities and Social Sciences; Infrared spectroscopy; letter; Light; Light-Harvesting Protein Complexes - chemistry; Light-Harvesting Protein Complexes - metabolism; Mechanisms. Catalysis. Electron transfer. Models; Models, Chemical; Models, Molecular; Molecular biophysics; multidisciplinary; Photosynthesis; Photosynthesis - radiation effects; Physical chemistry in biology; Protein Conformation; Proteins; Protons; Science; Science (multidisciplinary); Spectrum analysis; Spectrum Analysis - methods; Time Factors; Vibration
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature03429

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.