Ergotropy of a Photosynthetic Reaction Center

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.