Efficiency of energy funneling in the photosystem II supercomplex of higher plants

TL;DR

This study investigates energy transfer in the photosystem II supercomplex of higher plants, revealing that its structure favors photoprotection over transfer efficiency, with dynamics sensitive to structural changes and modeled using advanced quantum dynamics methods.

Contribution

It provides a detailed analysis of excitonic energy transfer mechanisms in PS-II supercomplexes, highlighting the role of structural features and reorganization processes in transfer dynamics.

Findings

Energy transfer is directed by relaxation to low energy states.

Final transfer step to the reaction center is the slowest, limiting overall efficiency.

Structure favors photoprotection rather than optimized energy transfer.

Abstract

The investigation of energy transfer properties in photosynthetic multi-protein networks gives insight into their underlying design principles.Here, we discuss excitonic energy transfer mechanisms of the photosystem II (PS-II) C$_2$S$_2$M$_2$ supercomplex, which is the largest isolated functional unit of the photosynthetic apparatus of higher plants.Despite the lack of a decisive energy gradient in C$_2$S$_2$M$_2$, we show that the energy transfer is directed by relaxation to low energy states. C$_2$S$_2$M$_2$ is not organized to form pathways with strict energetic downhill transfer, which has direct consequences on the transfer efficiency, transfer pathways and transfer limiting steps. The exciton dynamics is sensitive to small structural changes, which, for instance, are induced by the reorganization of vibrational coordinates. In order to incorporate the reorganization process in our numerical simulations, we go beyond rate equations and use the hierarchically coupled equation of motion approach (HEOM). While transfer from the peripherical antenna to the proteins in proximity to the reaction center occurs on a faster time scale, the final step of the energy transfer to the RC core is rather slow, and thus the limiting step in the transfer chain. Our findings suggest that the structure of the PS-II supercomplex guarantees photoprotection rather than optimized efficiency.