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) C2S2M2 supercomplex, which is the largest
isolated functional unit of the photosynthetic apparatus of higher
plants.Despite the lack of a decisive energy gradient in C2S2M2, we
show that the energy transfer is directed by relaxation to low energy states.
C2S2M2 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.Comment: 23 pages, 6 figure