"This technique should also be useful in studies aimed at improving the efficiency of molecular solar cells," Fleming said. In the Nature paper, he and his colleagues describe how they successfully used 2-D electronic spectroscopy to record the first direct measurement of electronic couplings in the Fenna-Matthews-Olson (FMO) photosynthetic light-harvesting protein, a molecular complex in green sulphur bacteria that absorbs photons and directs the excitation energy to a reaction center where it can be converted to chemical energy.
"FMO is a model system for studying energy transfer in the photosynthetic process because it is relatively simple (consisting of only seven pigment molecules) and its chemistry has been well characterized," Fleming said.
"As in all photosynthetic systems, the conversion of light into chemical energy is driven by electronic couplings between molecules and we monitored the process as a function of time and frequency."
Fleming and his colleagues expected to find that the excitation energy from harvested photons in the light-capturing pigment molecules was transported to the FMO reaction center molecules step-by-step down the energy ladder. Instead, they discovered distinct energy pathways, based on the spatial arrangements of the molecules, whereby some of the intermediate steps in the energy ladder are skipped.
"Excitation energy moved through the FMO complex in a smaller number of steps but larger energy increments than was previously supposed," said Fleming. "What we're seeing is that Nature exploits quantum mechanical effects by de-localizing excitation energy over two or more molecules in a system."
Photosynthesis should make any short-list
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Contact: Lynn Yarris
lcyarris@lbl.gov
510-486-5375
DOE/Lawrence Berkeley National Laboratory
31-Mar-2005