Numerical Evidence of Small Coherent Subsystems at Low Temperatures in Light Harvesting Complex II

The extent of exciton coherence in protein-pigment complexes has significant implications for the initial light harvesting step in photosynthetic organisms. In this work we model the main antenna protein of photosystem II, namely light harvesting complex II (LHC-II), with a single-exciton Hamiltonian with sites coupled via dipole-dipole interaction, with linear coupling to a dissipative phonon bath. With appropriate parameters, Monte Carlo path integral (MCPI) results of the exciton coherence length from 1 K to 500 K show that at thermodynamic equilibrium, an exciton in LHC-II is localized mostly on 2 single chlorophyll pigment sites, with persistent short-range coherence over the A2-B2 pair, A3-B3 pair and B1-B5-B6 triplet. Quasi-adiabatic path integral (QUAPI) calculations of the subsystems mentioned above show a smooth, incoherent relaxation towards thermodynamic equilibrium. The results obtained imply that with the exception of small coherent subsystems at cryogenic temperatures, excitons in LHC-II are more localized than in the analogous light harvesting complex II (LH-II) of the purple bacterium Rs. molischianum, which may be expected from the lower symmetry of the former.