Structural Tuning of Quantum Decoherence and Coherent Energy Transfer in Photosynthetic Light Harvesting

Photosynthetic organisms capture energy from solar photons by constructing light-harvesting proteins containing arrays of electronic chromophores. Collective excitations (excitons) arise when energy transfer between chromophores is coherent, or wavelike, in character. Here we demonstrate experimentally that coherent energy transfer to the lowest-energy excitons is principally controlled in a light-harvesting protein by the temporal persistence of quantum coherence rather than by the strength of vibronic coupling. In the peridinin-chlorophyll protein from marine dinoflagellates, broad-band two-dimensional electronic spectroscopy reveals that replacing the native chlorophyll a acceptor chromophores with chlorophyll b slows energy transfer from the carotenoid peridinin to chlorophyll despite narrowing the donor-acceptor energy gap. The formyl substituent on the chlorophyll b macrocycle hastens decoherence by sensing the surrounding electrostatic noise. These findings demonstrate how quantum coherence enhances the efficiency of energy transfer despite being very short lived in light-harvesting proteins at physiological temperatures.