Structure and efficiency in bacterial photosynthetic light-harvesting

TL;DR

This paper models bacterial light-harvesting complexes, showing high efficiency despite lack of long-range quantum coherence, and suggests structure plays a minor role in energy transfer efficiency.

Contribution

The study introduces a detailed, dynamic model of bacterial light-harvesting that accounts for vibrational effects and demonstrates high efficiency with various chromophore arrangements.

Findings

High energy transfer efficiency without long-range coherence

Efficiency remains high in randomly arranged chromophore models

Vibrational dynamics significantly influence energy fluctuations

Abstract

Photosynthetic organisms use networks of chromophores to absorb sunlight and deliver the energy to reaction centres, where charge separation triggers a cascade of chemical steps to store the energy. We present a detailed model of the light-harvesting complexes in purple bacteria, including explicit interaction with sunlight; energy loss through radiative and non-radiative processes; and dephasing and thermalizing effects of coupling to a vibrational bath. An important feature of the model is that we capture the effect of slow vibrational modes by introducing time-dependent disorder. Our model describes the experimentally observed high efficiency of light harvesting, despite the absence of long-range quantum coherence. The one-exciton part of the quantum state fluctuates due to slow vibrational changes, but remains highly mixed at all times. This lack of long-range coherence suggests a relatively minor role for structure in determining the efficiency of bacterial light harvesting. To investigate this we built hypothetical models with randomly arranged chromophores, but still observed high efficiency when typical nearest-neighbour distances are comparable with those found in nature. This helps to explain the efficiency of energy transport in organisms whose chromophore networks differ widely in structure, while also suggesting new design criteria for efficient artificial light-harvesting devices.