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 inter-subunit 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. In this study, we employ polarization-resolved two-dimensional electronic spectroscopy (2DES) to measure the anisotropy decay of the excitations in the light-harvesting antennas of PSII. We measure and compare the anisotropy kinetics between the LHCII(M)-CP29-CP24 (termed M-CP_2), trimeric LHCII, and monomeric LHCII complexes. Together with the EET timescales estimated from structure-based energy transfer theory and kinetic modeling, we assign the contributions of intra- and inter-subunit EET processes to the observed anisotropy decay lifetimes. The sub-picosecond (ps) decay processes are similar in the three complexes and can be attributed to intra-subunit origins, while the slower processes are mainly driven by inter-subunit EET and vary between the complexes. The M-CP_2 complex particularly exhibits a long-lived, 24-ps anisotropy decay component, which we assign as representing the EET processes between LHCII(M) and CP24. The results also suggest that EET rates between the subunits are not homogeneous, and can be influenced by the relative arrangement between the proteins. It is shown that EET processes between the isoenergetic antenna subunits can be obtained by leveraging on their orientations, which can be a basis for studying exciton dynamics in large multi-chromophoric systems.