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Abstract
Plants prevent photodamage under high light by dissipating excess energy as heat. Conformational changes of the photosynthetic antenna complexes activate dissipation by leveraging the sensitivity of the photophysics to the protein structure. The mechanisms of dissipation remain debated, largely due to two challenges. First, because of the ultrafast timescales and large energy gaps involved, measurements lacked the temporal or spectral requirements. Second, experiments have been performed in detergent, which can induce non-native conformations, or in vivo, where contributions from homologous antenna complexes cannot be disentangled. Here, we overcome both challenges by applying ultrabroadband two-dimensional electronic spectroscopy to the principal antenna complex, LHCII, in a near-native membrane. Our data provide evidence that the membrane enhances two dissipative pathways, one of which is a previously uncharacterized chlorophyll-to-carotenoid energy transfer. Our results highlight the sensitivity of the photophysics to local environment, which may control the balance between light harvesting and dissipation in vivo.
Highlights
Plants prevent photodamage under high light by dissipating excess energy as heat
While the results provided some insight into dissipative pathways, the multiplicity of environments is a contributor to the multiplicity of proposed conformational and photophysical mechanisms of photoprotection
Circular dichroism (CD) spectra of lightharvesting complex II (LHCII) in detergent and in discs provide a sensitive measure of the spatial configuration of pigments bound to the complex, because CD peak shape and intensity are directly related to the mutual orientation of the transition dipoles and the strength of their interactions[39,40]
Summary
Plants prevent photodamage under high light by dissipating excess energy as heat. Conformational changes of the photosynthetic antenna complexes activate dissipation by leveraging the sensitivity of the photophysics to the protein structure. The states with charge transfer character are thought to appear as redshifted fluorescence peaks[15,16], yet recent results indicate that the redshifted and the quenched species are distinct[21] This series of observations and their associated limitations highlights the challenges in understanding the photophysics in green plants. Previous investigations on LHCII suggested that a conformational change of the antenna complexes is an important trigger for the transition into the dissipative state[10,22,23,24,25] This transition is thought to leverage the sensitivity of the electronic interactions to the relative orientation and distance between the Chls and Cars, and so various conformational changes of the Cars have been proposed[10,22,23]
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