The energy in sunlight is converted into food by plants and some bacteria. According to recent work, the process may involve a quantum mechanical coupling between electrons that helps channel energy through photosynthetic proteins. A new technique allows this effect to be seen directly in a two-dimensional spectrum of a protein sample. The spectrum cleanly distinguishes “random hopping” of energy within the protein from more coordinated movement involving “coherently coupled” electrons. The results–presented in the 6 February Physical Review Letters–provide new insights into the various electron excitations and the order in which they occur. These added details open the door to more rigorous investigation of why energy transfers so efficiently over these biological wires.
Photosynthetic organisms have large protein complexes that absorb light and convert it to usable energy, such as “fuel” molecules for use elsewhere in the cell. Each complex has a large, dense scaffold of protein atoms, with ring-shaped, organic molecules such as chlorophyll and similar molecules distributed throughout the structure. In the initial steps of photosynthesis, the energy from a solar photon is transferred through the protein by sequentially exciting electrons in the organic molecules and is eventually delivered where it’s needed. Biologists have long thought that the energy moved like a hot potato: an excited electron in one molecule passes its energy to an electron in another, nearby molecule, and so on.
However, laser-based experiments have suggested other possibilities. In 2007 a team hitting a photosynthetic protein with laser pulses and measuring the time variation in the output light found strong evidence that some of the electrons were coherently coupled . The quantum wave nature of the electrons seemed to be connecting some of the chlorophyll-like molecules and helping energy flow through the protein like a wave on a string.
One of the disadvantages of the 2007 experiment was that it required multiple measurements lasting minutes to hours, which can degrade the sample, and a good deal of computer analysis to unpack the result. Now Ian Mercer of University College Dublin and his colleagues at Imperial College London have developed a different technique, which gives an instantaneous snapshot of the electrons’ interactions and the types of coupling. They combined three laser pulses as before but used “broadband” light containing many wavelengths and arranged for the protein’s emitted light to spread out in both the horizontal and vertical directions, rather than being detected at a single point. Peaks in this two-dimensional light pattern correspond to emission from specific molecules within the protein. Unlike an ordinary one-dimensional spectrum, this map reveals how the electron responsible for a peak became excited–whether it involved coherent coupling or not–and in what combination with other mechanisms.
The sample consisted of photosynthetic proteins from purple algae. The team controlled the angles at which the broadband pulses arrived and inserted a delay of zero to three picoseconds between the first two pulses and the third. From the two-dimensional pattern of light emitted by the protein, the team could take any bright spot and essentially read out the wavelengths of the three incoming photons that led to the outgoing photon, thanks to a phenomenon called four-wave mixing. Conservation of momentum enforces a strict relationship among the directions and wavelengths of the four photons involved. So even though the three input photons have fixed directions, the output photon’s direction is not fixed but depends upon the wavelengths selected by the protein from the broadband input pulses.
These wavelength “selections,” it turns out, can be related directly to the manner in which the energy traveled before being emitted. If it involved coherently coupled electrons, the wavelengths of the input photons were different than if it hopped between uncoupled electrons.
The team confirmed previous evidence of a strong coherent coupling in the protein. But their technique also revealed new kinds of information, such as the order in which some of the “selected wavelength” photons interact with the protein. Mercer says data like these will better constrain computer simulations that are trying to reproduce the way cells harvest sunshine in hopes of mimicking it in future solar cells.
“This is an unusual and useful application of an optical phenomenon which researchers usually try to eliminate,” says chemist David Klug of Imperial College London. “It will be interesting to see [this technique] applied to other problems.”
- G. S. Engel et al., “Evidence for Wavelike Energy Transfer through Quantum Coherence in Photosynthetic Systems,” Nature (London) 446, 782 (2007).