Laser pulses shed light on energy transfer process in photosynthetic proteins

Seeking to understand the inner workings of photosynthesis, researchers from the University of California, Berkeley have developed an experimental technique that involves exciting a photosynthetic antenna protein called Fenna-Matthews-Olson (FMO) with two different frequencies of laser light.

Plants and other photosynthetic organisms grow by harvesting the sun's energy and storing it in chemical bonds. Antenna proteins, which are made up of multiple light-absorbing pigments, capture sunlight over a large surface area and then transfer the energy through a series of molecules to a reaction center, where it kickstarts the process of building sugars. Photosynthetic processes take place is spaces so tightly packed with pigment molecules that strange quantum mechanical effects can come into play. When a pigment molecule absorbs light, one of its electrons is boosted into an "excited" higher energy state. If multiple pigments in a protein absorb light close to simultaneously, their wave-like excitation states may overlap and become linked to one another, affecting the path of the energy transfer.

Upon exciting the FMO with the two laser light frequencies, the researchers, led by Graham Fleming, used a third laser pulse to prompt the protein to release energy. They found it emitted different frequencies than those it had received, a sign that the two excitation states had linked. Alternative methods for observing overlapping excitations had been proposed before, but the new technique may be easier to implement since it relies only on color shifts, and not on precisely timed pulses.

Team member Jahan Dawlaty notes that separating colors from one another is a relatively simple task, and that the evidence of overlap was not hidden among other optical effects, as it might be when using a different technique. The new technique could be used to catalog the various excitation states in FMO and their potential combinations, according to the team.

The technique could apply to larger complexes and reaction centers, says Shaul Mukamel, a chemist at University of California, Irvine, who was not part of the research team. Probing energy levels and pigment couplings in photosynthetic systems is essential to understanding, modeling and testing the function of these systems, he says.

The research team's findings are published in the Journal of Chemical Physics.

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Posted by Lee Mather

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