Kavli Affiliate: David T. Limmer
| First 5 Authors: Johanna L. Hall, Shiun-Jr Yang, David T. Limmer, Graham R. Fleming,
| Summary:
Photosystem II (PSII) can achieve near-unity quantum efficiency of light
harvesting in ideal conditions and can dissipate excess light energy as heat to
prevent formation of reactive oxygen species under light stress. Understanding
how this pigment-protein complex accomplishes these opposing goals is a topic
of great interest that has so far been explored primarily through the lens of
the system energetics. Despite PSII’s known flat energy landscape, a thorough
consideration of the entropic effects on energy transfer in PSII is lacking. In
this work, we aim to discern the free energetic design principles underlying
the PSII energy transfer network. To accomplish this goal, we employ a
structure-based rate matrix and compute the free energy terms in time following
a specific initial excitation to discern how entropy and enthalpy drive
ensemble system dynamics. We find that the interplay between the entropy and
enthalpy components differs among each protein subunit, which allows each
subunit to fulfill a unique role in the energy transfer network. This
individuality ensures PSII can accomplish efficient energy trapping in the RC,
effective NPQ in the periphery, and robust energy trapping in the other-monomer
RC if the same-monomer RC is closed. We also show that entropy, in particular,
is a dynamically tunable feature of the PSII free energy landscape accomplished
through regulation of LHCII binding. These findings help rationalize natural
photosynthesis and provide design principles for novel, more efficient solar
energy harvesting technologies.
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