The assembly of the cellulose microfibril (CMF) from the cellulose synthase complex (CSC) is not yet understood. Recent progress in imaging the CSC[1] and analyzing the size[2] and structure of cellulose[3,4] make it possible to envision modeling the CMF assembly process because the initial and final states are fairly well constrained. Previous work has tackled this problem using coarse-grained[5] and classical molecular dynamics simulations[6]. We apply dispersion-corrected density functional theory (DFT) calculations that have proven accurate in reproducing cellulose structure, vibrational frequencies and 13C NMR chemical shifts. In this work, methyl-terminated decamers of cellulose (C62O51H106) interactions were modeled edge-on and in layer configuration with and without explicit H2O to calculate the energy of assembly. Results indicate that the layer-layer interaction is more favorable in water, especially considering entropic effects, consistent with predictions of Cousins and Brown[7] based on classical MD simulations. However, the solvent properties of water within the space where CMF assembly occurs are not necessarily equivalent to bulk water due to slower dynamics[8]. Consequently, we investigated the polymer interaction energies as a function of dielectric constant using polarized continuum models. The results do not change significantly between water (ε = 78.4) and methanol (ε = 32.7), but the favorability of side-on interactions become favorable in a chloroform-like solvent (ε = 4.81). Based on this model prediction, it becomes imperative to better analyze the solvent properties of the CMF formation space within the plant cell wall. These results can be used to benchmark force field simulations of the CMF assembly process. Results indicate that the CHARMM force field is reasonably accurate for gas-phase calculations, but improvements in the parameterization are possible.

Density functional theory modeling of cellulose polymer assembly

253rd National Meeting (2017)

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