New Technique Enables Scientists to Track Molecular Energy Transfer in Photosynthesis

Scientists have been able to follow the flow of excitation energy in both time and space in a molecular complex using a new technique called two-dimensional electronic spectroscopy. While holding great promise for a broad range of applications, this technique has already been used to make a surprise finding about the process of photosynthesis. The technique was developed by a team of researchers with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley.

Graham Fleming, Deputy Director of Berkeley Lab, led the development of new technique, called two-dimensional electronic spectroscopy, that enables scientists to map the flow of excitation energy through space with nanometer spatial resolution and femtosecond temporal resolution.“I think this will prove to be a revolutionary method for studying energy flow in complex systems where multiple molecules interact strongly,” said Graham Fleming, Deputy Director of Berkeley Lab, and an internationally acclaimed leader in spectroscopic studies of the photosynthetic process. “Using two-dimensional electronic spectroscopy, we can map the flow of excitation energy through space with nanometer spatial resolution and femtosecond temporal resolution.”

Fleming, also a professor of chemistry with UC Berkeley, is the principal investigator of this research, and co-author of a paper which appears in the March 31, 2005 issue of the journal Nature, entitled “Two-Dimensional Spectroscopy of Electronic Couplings in Photosynthesis.” Co-authoring the paper with Fleming were Tobias Brixner, Jens Stenger, Harsha Vaswani, Minhaeng Cho and Robert Blankenship.

Two-dimensional electronic spectroscopy involves sequentially flashing a sample with light from three laser beams, delivered in pulses only 50 femtoseconds (50 millionths of a billionth of a second) in length, while a a fourth beam is used as a local oscillator to amplify and phase-match the resulting spectroscopic signals. Fleming likens the technique to that of the early super-heterodyne radios, in which an incoming high frequency radio signal was converted by an oscillator to a lower frequency for more controllable amplification and better reception. In the case of 2-D electronic spectroscopy, scientists can track the transfer of energy between molecules that are coupled (connected) through their electronic and vibrational states in any photoactive system, macromolecular assembly or nanostructure.

“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.

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