Directly Measuring the Connectivity between Isoenergetic Light-Harvesting Antennas in Plant Photosystem II at Physiological Temperature

The plant photosystem II (PSII) supercomplex features a relatively "flat" energy landscape in the excitonic energy transfer (EET) network between the light-harvesting antenna and core subunits. The resulting intersubunit EET proceeds predominantly between isoenergetic excitonic states. Visualizing these EET dynamics is difficult due to the heavy spectral overlaps between the components, particularly at physiological temperature. We employ polarization-resolved two-dimensional electronic spectroscopy to measure and compare the anisotropy kinetics between the LHCII-CP29-CP24 complex and some of its constituents. Using the orientational differences between the subunits, together with the EET time scales estimated from energy transfer theory and phenomenological kinetic modeling, we identify the contribution of intra- and intersubunit EET processes to the observed anisotropy decay components. The results suggest that EET rates between the antenna subunits are not homogeneous and can be sensitive to the interprotein arrangement. The approach provides an effective method for studying EET in large multichromophoric systems.