The Electronic Structure of Lutein 2 Is Optimized for Light Harvesting in Plants

In green plants, carotenoids (Cars) are bound within chlorophyll-containing proteins to serve light-harvesting, photoprotective, and structural roles. The major light-harvesting complex of green plants (LHCII) contains four Cars, each within its own protein pocket with distinct structure and energetics. The photophysics of the Cars, including the balance of their diverse roles, remain unresolved because of the complexity of their electronic structure. We performed ultrabroadband two-dimensional electronic spectroscopy on LHCII with the spectral bandwidth and temporal resolution to map the energy transfer dynamics and electronic structure of the Cars. We found that one Car, lutein 2, provides a nexus for light harvesting, collecting energy from higher-lying states and funneling it downhill, partially via a debated dark state observed exclusively on lutein 2. These results show that the protein pocket can tune Car electronic structure via tuning the geometry, a mechanism by which plants control the photophysics of solar energy capture.