Ergotropy of a Photosynthetic Reaction Center

TL;DR

This paper investigates the maximum work extractable from the Photosystem II reaction center using quantum thermodynamics, revealing how different electron transfer pathways influence energy storage capacity.

Contribution

It introduces a quantum thermodynamic analysis of the reaction center, linking population dynamics to energy extraction potential in biological systems.

Findings

Charge separation pathways affect ergotropy levels.

Certain pathways act as quantum energy capacitors.

Population transitions influence active and passive energy regimes.

Abstract

We theoretically analyze the Photosystem II reaction center using a quantum master equation approach, where excitonic and charge-transfer rates are computed at the Redfield and F\"orster levels with realistic spectral densities. The focus is on ergotropy, the maximum work extractable from a quantum state without energy loss. We compute the ergotropy by constructing passive states in the thermodynamic sense. Among the electron transfer pathways, those involving charge separation between $Chl_{D1}$ and $Phe_{D1}$, as well as a route passing through three sequential charge-separated states, yield higher ergotropy, suggesting greater capacity for work extraction, akin to quantum energy capacitors. A third pathway, bypassing the $Chl_{D1},Phe_{D1}$ pair, shows significantly reduced ergotropy. These differences arise from population-induced transitions between active and passive regimes. Our findings highlight how biological systems may exploit non-equilibrium population structures to optimize energy conversion, connecting quantum thermodynamic principles to biological energy harvesting.