Excitonic energy transfer in light-harvesting complexes in purple bacteria

TL;DR

This study investigates how long-range dipolar interactions influence excitonic energy transfer efficiency in purple bacteria light-harvesting complexes using two theoretical models, revealing the environment's role in quantum coherence and energy transfer optimization.

Contribution

It demonstrates the significant impact of dipolar interactions on energy transfer efficiency and symmetry breaking in light-harvesting complexes, advancing understanding of natural photosynthesis mechanisms.

Findings

Dipolar interactions increase intra- and inter-ring transfer efficiency.

Symmetry breaking affects dependence on trapping positions.

Environment influences quantum coherence and energy transfer.

Abstract

Two distinct approaches, the Frenkel-Dirac time-dependent variation and the Haken-Strobl model, are adopted to study energy transfer dynamics in single-ring and double-ring light-harvesting systems in purple bacteria. It is found that inclusion of long-range dipolar interactions in the two methods results in significant increases in intra- or inter-ring exciton transfer efficiency. The dependence of exciton transfer efficiency on trapping positions on single rings of LH2 (B850) and LH1 is similar to that in toy models with nearest-neighbor coupling only. However, owing to the symmetry breaking caused by the dimerization of BChls and dipolar couplings, such dependence has been largely suppressed. In the studies of coupled-ring systems, both methods reveal interesting role of dipolar interaction in increasing energy transfer efficiency by introducing multiple intra/inter-ring transfer paths. Importantly, the time scale (~4ps) of inter-ring exciton transfer obtained from polaron dynamics is in good agreement with previous studies. In a double-ring LH2 system, dipole-induced symmetry breaking leads to global minima and local minima of the average trapping time when there is a finite value of non-zero dephasing rate, suggesting that environment plays a role in preserving quantum coherent energy transfer. In contrast, dephasing comes into play only when the perfect cylindrical symmetry in the hypothetic system is broken. This study has revealed that dipolar interaction between chromophores may play an important part in the high energy transfer efficiency in the LH2 system and many other natural photosynthetic systems.