By Jessica M. Anna, Gregory D. Scholes, and Rienk van Grondelle
Photosynthesis is responsible for life on our planet, from supplying the oxygen we breathe to the food that we eat. The process of photosynthesis is complex, involving many protein complexes and enzymes that work together in a concerted effort to convert solar energy to chemical bonds. Oxygenic photosynthesis is catalyzed via two light drive electron transfer reactions. The first step in this process is the absorption of a photon by light-harvesting complexes which then funnel this excitation energy to the reaction center (Figure 1). Given the complexity of photosynthesis, it can be hard to imagine the role that quantum mechanics, i.e. coherence, could play in this process.
Recent experiments employing ultrafast laser pulses have investigated light-harvesting complexes to gain insight into the light initiated processes occurring on the femtosecond (1×10-15 s) timescale. These experiments have found that concepts from quantum mechanics are required to sufficiently describe the observations.
In our research, we explored how the experimental technique of two dimensional electronic spectroscopy gives a rather direct measurement of these quantum mechanical effects, i.e. coherences. In these measurements a series of ultrafast laser pulses excite light-absorbing pigment molecules embedded in the protein matrix of a light-harvesting complex (Figure 2). Then the laser pulses record how this absorbed energy flows among the pigment molecules, allowing for scientists to directly track pathways of energy flow. Results of these measurements indicate that upon excitation, individual light-harvesting molecules act collectively to absorb and transfer energy, as opposed to the light-harvesting molecules acting individually. To describe this cooperative action requires the incorporation of concepts from quantum mechanics.
This quantum mechanical aspect of the shared excitation leads to wave-like properties that can interfere when troughs and peaks of different waves overlap. In this sense, one could imagine that this interference could act to provide efficient passage through the array of pigment molecules. However, there is now ample evidence which suggests that this is not necessarily the case and the role coherence plays in photosynthesis is still elusive.
For further research we may ask how coherence in light-harvesting complexes couples to incoherent phenomena, perhaps facilitating some other aspect of the cell. A deeper understanding of these processes could help scientists to incorporate these concepts into artificial photosynthetic devices and bio-inspired solar cells.
Jessica M. Anna, Gregory D. Scholes, and Rienk van Grondelle are the authors of “A Little Coherence in Photosynthetic Light Harvesting” (available to read for free for a limited time) in BioScience. Jessica M. Anna is a Postdoctoral Fellow at the University of Toronto. Gregory D. Scholes is the D.J. LeRoy Distinguished Professor of Chemistry at the University of Toronto. Rienk van Grondelle is a Professor of Biophysics at Vrije Universiteit in Amsterdam.
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