Justin Jankunas Dissertation Award in Chemical Physics: Carotenoid-Mediated Light Harvesting in Plants Uncovered with Ultrabroadband Two-Dimensional Electronic Spectroscopy

Plants absorb across the visible solar spectrum and rapidly funnel the energy downhill to power growth. In excess sunlight, they dissipate harmful energy as heat to protect against photodamage. Previous measurements have been limited to the two lowest-energy, exclusively chlorophyll transitions of their light-harvesting machinery, leaving the carotenoid-mediated pathways unexplored. We report the development of an ultrabroadband two-dimensional (2D) electronic spectrometer that enables us to map energy flow in the major light-harvesting protein of plants, LHCII, across the visible range. Using this apparatus, we uncover two previously inaccessible pathways of light harvesting as well as dissipation in LHCII, both mediated by carotenoids. By analyzing the vibrational wavepackets in the 2D spectra, we identify a debated dark state (S_X) specific to a single carotenoid, lutein 2, that serves as a key mediator for efficient light harvesting. On a second carotenoid, lutein 1, we resolve a dissipative energy transfer from the chlorophyll to its dark S₁ state. The distinct photophysics revealed for these two chemically identical pigments highlight the capability of the protein binding pocket to control the electronic structure, and in turn, function of carotenoids in photosynthesis.