Photosystem II (PSII) and light-harvesting complex II (LHCII) are essential proteins involved in light harvesting, and PSII-LHCII super-complexes have been observed to form disordered packings, ordered arrays, and mixtures thereof. Since this organization impacts excitation energy transfer to reaction centers, understanding structural correlations across layers could elucidate possible energetic consequences of these hierarchical arrangements. Moreover, the extent to which their order persists throughout thylakoid stacks is difficult to measure experimentally. Here we propose a simple, Ising-like model of supramolecular organization in thylakoid stacks. The LHCII-LHCII attraction across stromal spacing is modeled as the nearest-neighbor coupling, and the steric constraint on PSIIs across the luminal spacing under low light conditions is implemented as a constraint preventing adjacent vertical occupied states within a thylakoid pair. The mean field results resemble a cross-section of a three-state Potts model and reveal the presence of a disordered phase and a horizontal stripe phase. We intend to find a mapping between the model parameters and the physiochemical properties of photosynthetic membranes and their associated proteins. Furthermore, we aspire to model ways to account for differences between low and high light conditions, as well as to couple a lattice model to mechanical membrane fluctuations.

Modeling protein organization in thylakoid membrane stacks via a lattice gas model

253rd National Meeting (2017)

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