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

TL;DR

This study models light harvesting complex II (LHC-II) to demonstrate that at low temperatures, excitons are localized on small chlorophyll groups, with coherence diminishing at higher temperatures, highlighting localized excitonic behavior in photosynthetic complexes.

Contribution

The paper provides numerical evidence showing exciton localization on small chlorophyll subsystems in LHC-II at low temperatures, contrasting with more delocalized states in bacterial complexes.

Findings

Excitons are localized on 2-3 chlorophyll sites at low temperatures.

Short-range coherence persists among specific chlorophyll pairs and triplets.

Exciton localization increases with temperature, indicating temperature-dependent coherence.

Abstract

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.