Robust and fragile quantum effects in the transfer kinetics of delocalized excitons between B850 units of LH2 complexes

TL;DR

This study uses a generalized master equation to analyze exciton transfer between LH2 complexes, revealing quantum effects that are both robust and fragile under different conditions, with implications for understanding photosynthetic energy transfer.

Contribution

It provides a quantitative analysis of how quantum delocalization influences exciton transfer rates and their dependence on structural parameters in LH2 complexes.

Findings

Quantum delocalization enhances transfer rates significantly.

Transfer rates are robust against disorder in distance and angle.

Resonance peaks in transfer rates are fragile under disorder.

Abstract

Aggregates of light harvesting 2 (LH2) complexes form the major exciton-relaying domain in the photosynthetic unit of purple bacteria. Application of a generalized master equation to pairs of the B850 units of LH2 complexes, where excitons predominantly reside, provide quantitative information on how the inter-LH2 exciton transfer depends on the distance, relative rotational angle, and the relative energies of the two LH2s. The distance dependence demonstrates significant enhancement of the rate due to quantum delocalization of excitons, the qualitative nature of which remains robust against the disorder. The angle dependence reflects isotropic nature of exciton transfer, which remains similar for the ensemble of disorder. The variation of the rate on relative excitation energies of LH2 exhibits resonance peaks, which however is fragile as the disorder becomes significant. Overall, the average transfer times between two LH2s are estimated to be in the range of 4 - 25 ps for physically plausible inter-LH2 distances.