SPECTROSCOPY, IMAGING, OTHER TECHNIQUES |
Visualization of Excitonic Structure in the Fenna-Matthews-Olson Photosynthetic Complex by Polarization-Dependent Two-Dimensional Electronic Spectroscopy
Elizabeth L. Read 1, Gabriela S. Schlau-Cohen 1, Gregory S. Engel 2, Jianzhong Wen 3, Robert E. Blankenship 4 and Graham R. Fleming 5*
1 UC Berkeley
2 University of Chicago
3 Washington University St. Louis
4 Washington University, St. Louis
5 Univ. of Calif., Berkeley; Berkeley Lab
* To whom correspondence should be addressed. E-mail: grfleming{at}lbl.gov.
Submitted on December 23, 2007
Revised on February 2, 2008
Accepted on 21 February 2008
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Abstract |
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Photosynthetic light-harvesting proceeds by the collection and highly efficient transfer of energy through a network of pigment-protein complexes. Inter-chromophore electronic couplings and interactions between pigments and the surrounding protein determine energy levels of excitonic states and dictate the mechanism of energy flow. The excitonic structure (orientation of excitonic transition dipoles) of pigment-protein complexes is generally deduced indirectly from x-ray crystallography in combination with predictions of chromophore transition energies and couplings. Here, we demonstrate that coarse-grained excitonic structural information in the form of projection angles between transition dipole moments can be obtained from polarization-dependent two-dimensional electronic spectroscopy of an isotropic sample, particularly when the nonrephasing or free polarization decay signal rather than the photon echo signal is considered. The method provides an experimental link between atomic and electronic structure with femtosecond time resolution. In an investigation of the Fenna-Matthews-Olson complex from green sulfur bacteria, energy transfer connecting two exciton states is isolated as being the primary contributor to a cross peak in the nonrephasing 2D spectrum at 400 fs under a particular sequence of polarized excitation pulses. The results suggest the possibility of designing experiments using combinations of tailored polarization sequences to separate and monitor individual relaxation pathways.
Key Words:
Energy Transfer, Femtosecond Dynamics, Light-Harvesting, Nonlinear Optical Spectroscopy, Photosynthesis
