Inter-protein energy transfer dynamics in the PSII antenna

Efficient long-distance energy transport is a cornerstone of photosynthetic light harvesting, enabling excitation energy to traverse multiple antenna proteins to reach the reaction center (RC), where it drives photochemistry. While extensive studies on energy transfer dynamics within individual light-harvesting complexes (LHCs) have been conducted, the inter-protein transfers crucial for understanding the overall efficiency of these systems have remained experimentally elusive. This arises mainly because the spectral signatures of the subunits are often remarkably similar, complicating the identification of energy transfer pathways among them. This study bridges this gap by utilizing ultrafast transient absorption spectroscopy, under conditions with and without singlet-singlet annihilation, on the photosystem II (PSII) LHCII-CP24-CP29 subcomplex and on its constituents. Our findings reveal rapid equilibration within monomeric complexes, contrasted by six-times slower equilibration in the LHCII trimer and eight-times slower equilibration in the LHCII-CP24-CP29 subcomplex, highlighting the inter-complex energy transfer as the rate-limiting step in excitation delivery to the RC.