Energy Transfer Pathways in Plant Photosystem I from First-Principles Modeling

The photosynthetic supercomplex Photosystem I-light harvesting complex I (PSI-LHCI) of plants is a molecular machine involved in energy conversion. Concentration of energy in the PSI reaction center is strikingly efficient, despite the unusual presence of low-energy, trapping states (red forms) spatially far from the reaction center. The main energy transfer processes occurring in PSI-LHCI are still unclear, as the size and complexity of the system prevent immediate interpretation of spectroscopic signals and challenges traditional modeling approaches. Here we present a multiscale quantum chemical model able to characterize the exciton structure of the full Chlorophyll aggregate in plant PSI-LHCI, including the red forms. The calculation of transfer rates between all pigments allows us to simulate population decay upon different initial conditions and to ultimately identify the most relevant, disorder-robust pathways at molecular resolution. The time scales associated with such pathways closely match the ones extracted from experiments, demonstrating the effectiveness of a quantum mechanical description. The inclusion of charge-transfer states in the red sites of the LHCI antennas allows us to characterize the role of red forms in the dynamics and quantify their effect on the overall efficiency.