Distorted Intermediate SX (1Bu–) State in Xanthophylls Drives Efficient Energy Transfer in Light-Harvesting Complex II

Understanding energy transfer in photosynthetic light-harvesting systems, especially the role of carotenoids and their structural dynamics, remains challenging. Despite their critical function in light harvesting, the mechanisms by which carotenoids transfer excitation energy to chlorophylls have not been fully understood. This study used temperature-dependent femtosecond stimulated Raman spectroscopy, quantum chemical calculations, and molecular dynamics simulations to investigate ultrafast conformational changes in free xanthophylls and those bound in light-harvesting complex II (LHCII). Our findings reveal a small-scale distortion of xanthophyll in the S 2 (1 B u + ) state, forming a distorted intermediate S X (1 B u – ) state, which facilitates an efficient energy transfer pathway from xanthophylls to chlorophylls in LHCII. Structural analysis shows slight twisting along the polyene backbone, enhancing electronic coupling with nearby chlorophylls by 30–50% without altering intermolecular distances. These results provide direct evidence of xanthophyll conformational dynamics driving energy transfer and highlight the crucial role of the S X state in efficient light harvesting.