Photosynthetic organisms construct light-harvesting structures consisting of closely spaced arrays of electronic chromophores, which promote delocalization of the excitation energy captured from solar photons. An important example of a system that employs quantum coherence in a strategy to optimize excitation energy transfer efficiency is the peridinin–chlorophyll protein (PCP) from marine dinoflagellates. We have used broadband two-dimensional electronic spectroscopy (2DES) to determine how replacement of the native chlorophyll (Chl) a acceptor with Chl b affects the rate and efficiency of excitation transfer from the carotenoid peridinin, which serves as the principal antenna chromophore in this system. Here we report that the quantum entanglement that arises from delocalization of excitation in PCP controls the rate of energy transfer. The presence of the Chl b acceptor chromophores markedly slows the rate of energy transfer from the carotenoid peridinin despite narrowing the donor–acceptor energy gap and despite improving the resonance with chromophore vibrations. This change in rate accompanies an increase in the rate of quantum decoherence due to the presence of the formyl group on the Chl b macrocycle, which strengthens the coupling to electrostatic noise in the surroundings. These findings establish that the advantage a light-harvesting protein obtains from quantum coherence is critically determined by how well the structure correlates the donor and acceptor energy levels.

Structural tuning of quantum entanglement in the peridinin–chlorophyll protein

256th National Meeting (2018)

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