Lasers probe photosynthetic proteins for renewable fuels

Ashley N. Paddock, ashley.paddock@photonics.com

Unlocking the workings of sunlight-to-fuel conversion could yield safer sustainable fuels, and a new technique looks at wavelength shifts to bring scientists closer to that goal.

Plants and other photosynthetic organisms grow through a complex process of harvesting the sun’s energy and storing it in chemical bonds. Antenna proteins, made up of light-absorbing pigments, are essential to this process because they capture sunlight and transfer the energy to a reaction center, which kick-starts the building of sugars. Probing energy levels and pigment couplings in photosynthetic systems is key to understanding, modeling and testing their function.

To better understand the elementary steps of energy transport in photosynthetic systems, scientists have designed an experiment with three spectrally tunable laser pulses to identify interactions between excitations. Courtesy of Jahan Dawlaty, UC Berkeley.

Researchers at the University of California have found that, when a pigment molecule absorbs light, one of its electrons is boosted into a higher energy state. When multiple pigments absorb light simultaneously, their excitation states may overlap and become linked, affecting the path of the energy transfer.

“Understanding the spatial overlap between localized excitation is the most important factor in determining energy transport,” team member Jahan Dawlaty said.

The researchers tested the mechanism by exciting a well-studied photosynthetic antenna protein called Fenna-Matthews-Olson, using two different frequencies of laser light. They found that, when a third laser pulse was used to prompt the protein to release energy, it emitted frequencies different from those it had received, signaling that the two excitation states had linked. While alternative methods for observing overlapping excitations have been proposed before, this new technique could be easier to implement because it relies only on frequency shifts, not on timed pulses. The findings appeared in the American Institute of Physics’ Journal of Chemical Physics (doi: 10.1063/1.3607236).

“This technique will provide new insight into the role of interacting excitations in complicated systems such as molecular aggregates, photosynthetic complexes and semiconductors, among others,” Dawlaty said.