Photosynthetic complexes use several mechanisms to improve the transfer of excitation energy from the peripheral antennas to the central reaction center. In particular, a downward energy funnel can direct excitons in the right direction, while coherent excitonic delocalization can enhance the transfer rate through the cooperative phenomenon of supertransfer. Both processes are present in the light-harvesting complexes of purple bacteria, which feature delocalized excitonic states due to a tight packing of bacteriochlorophylls. <br/><br/>However, isolating the role of purely coherent effects is difficult because any change to the delocalization also changes the energy landscape. To determine the relative importance of the two processes, we compared models of the light-harvesting apparatus with thousands of counterfactual situations, covering all possible combinations of delocalization and energy offsets. Although supertransfer did enhance the rates in some cases, the energetic funnelling played the decisive role. Because delocalization has a minor role (and is sometimes detrimental), it is most likely not adaptive, being a side-effect of the dense chlorophyll packing that evolved to increase light absorption per reaction center.<br/><br/>Preprint: arXiv:1509.01537.

Light harvesting in purple bacteria benefits more from a favorable energy landscape than from coherent delocalization

251st National Meeting (2016)

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