Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems

• Published in: Nature Vol. 446; no. 7137; pp. 782 - 786
• Main Authors: Fleming, Graham R, Engel, Gregory S, Calhoun, Tessa R, Read, Elizabeth L, Ahn, Tae-Kyu, Man al, Tomáš, Cheng, Yuan-Chung, Blankenship, Robert E
• Format: Journal Article
• Language: English
• Published: London Nature Publishing 12.04.2007; Nature Publishing Group
• Subjects: Bacterial Proteins - metabolism; Biological and medical sciences; Biology; Cell structures and functions; Chlorobi - metabolism; Chlorobi - radiation effects; Chlorobium - metabolism; Chlorobium - radiation effects; Chlorobium tepidum; Chloroplast, photosynthetic membrane and photosynthesis; Electron Transport - radiation effects; Electrons; Energy storage; Energy transfer; Fourier transforms; Fundamental and applied biological sciences. Psychology; Light-Harvesting Protein Complexes - metabolism; Molecular and cellular biology; Photosynthesis; Photosynthesis - radiation effects; Physics; Quantum theory; Spectroscopy; Spectrum Analysis; Chlorobiaceae; Light harvesting system; Reaction center; Quantum coherence; Photosynthetic system; Spectrometry; Bacteriochlorophyll; Coherence transfer; Pigment protein complex; Bacteria; Models; Chlorobiales; Energy transfer
• ISSN: 0028-0836; 1476-4687; 1476-4679
• DOI: 10.1038/nature05678

Photosynthetic complexes are exquisitely tuned to capture solar light efficiently, and then transmit the excitation energy to reaction centres, where long term energy storage is initiated. The energy transfer mechanism is often described by semiclassical models that invoke 'hopping' of excited-state populations along discrete energy levels. Two-dimensional Fourier transform electronic spectroscopy has mapped these energy levels and their coupling in the Fenna-Matthews-Olson (FMO) bacteriochlorophyll complex, which is found in green sulphur bacteria and acts as an energy 'wire' connecting a large peripheral light-harvesting antenna, the chlorosome, to the reaction centre. The spectroscopic data clearly document the dependence of the dominant energy transport pathways on the spatial properties of the excited-state wavefunctions of the whole bacteriochlorophyll complex. But the intricate dynamics of quantum coherence, which has no classical analogue, was largely neglected in the analyses-even though electronic energy transfer involving oscillatory populations of donors and acceptors was first discussed more than 70 years ago, and electronic quantum beats arising from quantum coherence in photosynthetic complexes have been predicted and indirectly observed. Here we extend previous two-dimensional electronic spectroscopy investigations of the FMO bacteriochlorophyll complex, and obtain direct evidence for remarkably long-lived electronic quantum coherence playing an important part in energy transfer processes within this system. The quantum coherence manifests itself in characteristic, directly observable quantum beating signals among the excitons within the Chlorobium tepidum FMO complex at 77 K. This wavelike characteristic of the energy transfer within the photosynthetic complex can explain its extreme efficiency, in that it allows the complexes to sample vast areas of phase space to find the most efficient path.