The light-harvesting complexes (LHCs) of plants can regulate the energy flux to the reaction centers in response to fluctuating light by virtue of their vast conformational landscape. They do so by switching from a light-harvesting state to a quenched state, dissipating the excess absorbed energy as heat. However, isolated LHCs are prevalently in their light-harvesting state, which makes the identification of their photoprotective mechanism extremely challenging. Here, ensemble time-resolved fluorescence experiments show that monomeric CP29, a minor LHC of plants, exists in various emissive states with identical spectra but different lifetimes. The photoprotective mechanism active in a subpopulation of strongly quenched complexes is further investigated via ultrafast transient absorption spectroscopy, kinetic modeling, and mutational analysis. We demonstrate that the observed quenching is due to excitation energy transfer from chlorophylls to a dark state of the centrally bound lutein.

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The subtle details of the mechanism of energy flow from carotenoids to chlorophylls in biological light-harvesting complexes are still not fully understood, especially in the ultrafast regime. Here we focus on the antenna complex peridinin–chlorophyll a–protein (PCP), known for its remarkable efficiency of excitation energy transfer from carotenoids—peridinins—to chlorophylls. PCP solutions are studied by means of 2D electronic spectroscopy in different experimental conditions. Together with a global kinetic analysis and multiscale quantum chemical calculations, these data allow us to comprehensively address the contribution of the potential pathways of energy flow in PCP. These data support dominant energy transfer from peridinin S2 to chlorophyll Qy state via an ultrafast coherent mechanism. The coherent superposition of the two states is functional to drive population to the final acceptor state, adding an important piece of information in the quest for connections between coherent phenomena and biological functions.

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