The B850/B875 Photosynthetic Complex Ground and Excited State are Both Coherent

TL;DR

This paper proposes that both ground and excited states of the B850/B875 photosynthetic complexes exhibit quantum coherence, involving charge density waves and exciton polaritons, which may enhance energy transfer efficiency.

Contribution

It introduces a novel model where ground states are pseudo one-dimensional metals with charge density waves, explaining coherence and energy transfer mechanisms in these complexes.

Findings

Charge density waves create energy gaps in the complexes.

Coherent phonons span the entire rings, supporting quantum coherence.

Exciton polaritons may facilitate uphill energy transfer.

Abstract

A bacterial photosynthetic light harvesting complex PLHC absorbs a photon and transfers this energy almost perfectly at room temperature RT to a Reaction Center RC, where charge separation occurs. While there are a number of possible light absorbers involved in this process, our focus is the B850 and B875 complexes. We propose that the dominant feature of the ground states in the B850 ring and the B875 open chain are pseudo one dimensional metals due to each bacteriochlorophyll a BChl containing a coordinated magnesium ion Mg2+. The Mg ion structure undergoes a static Peierls distortion that results in symmetry breaking that changes the even spacing of the Mg/BChl molecules comprising the chains to the experimentally observed Mg/BChl dimers. The results are charge density waves CDW, one for each type of the two complexes that result in an energy gap in the single particle electronic spectrum and coherent phonon s spanning the entire rings. The ground state CDWs seem to have two functions the first is to form a stable optical platform and the second is to suppress radical formation and energy dissipation of the coherent excited state by creating single particle energy gaps. After excitation by a photon, the B850 exciton delocalizes on the ring a second photon can form a two-level exciton polariton that could be an alternative explanation for the splitting of the B850 exciton band. The coherent polariton formed could actively participate in uphill electronic energy transfer EET. Additionally we suggest other possible energy storage mechanisms and entanglement possibilities. We suggest experimental studies to clarify these proposals.