Chemical Physics Theory at the University of Sydney

Exciton and charge transport in photosynthesis

Image: Joanne Kim / freeimages

Photosynthetic organisms harvest light using large antenna complexes containing many chlorophyll molecules. The energy harvested by the antennas is then transported to the reaction centres, where it drives the first chemical steps of photosynthesis. Recent experiments have suggested that, surprisingly, the excitation energy can be transported in a partially coherent, wavelike manner. It was previously doubted that quantum effects could survive for so long in biological systems at room temperature.

We have shown that many of the quantum effects observed in photosynthetic complexes are artefacts of the ultrafast laser excitation and are not relevant for biological function in incoherent sunlight. As a consequence, the complete description of energy transport in incoherent light is dramatically simplified, allowing us to rapidly screen hypothetical scenarios to determine whether natural light-harvesting architectures are already optimal or whether they could be improved.

Through the screening studies, we identified several quantum effects that are important even in incoherent light. In particular, we reported the most statistically significant quantum enhancement in a photosynthetic complex, showing that the light-harvesting apparatus of purple bacteria is more than five standard deviations more efficient than would be expected by chance alone, and that the enhancement is largely due to supertransfer, a cooperative enhancement of energy transfer due to coherent delocalisation.

We have also analysed the performance of reaction centres to understand why their dimeric structures—and, especially, the strongly coupled special pair—have been conserved for several billion years. We found that the most probable explanation for the dimerism is that it arose because it deepened the excitonic trap, possibly leading to considerable improvements in reaction-centre efficiency.

Selected papers