The analysis of aggregation properties and pore structure of hydrated xylans is essential for the tailoring of physical properties for various applications in industries such as packaging, pharmaceuticals, etc., and in order to understand the interaction of hemicelluloses with other plant cell wall constituents. Xylans, typically found in the saturated state with different degrees / types of chemical substitutions, form a major constituent of hemicelluloses in the plant cell wall. Interplay between structure and dynamics was observed through similarities in the local structural and dynamical evolution of hydrated xylans, which converged at timescales close to a microsecond at high hydration levels (at or greater than the moisture content corresponding to saturation). In the highly hydrated systems, phase separation between the aggregated polymer-rich and water-rich regions was observed at steady state starting from an initially homogeneous system. The structural heterogeneity in the hydrated xylans was studied using atom-centred densities corresponding to local structural variations, and the dynamical heterogeneity by parametrically quantifying the decay of the auto-correlation function describing the polymer backbone flexibility. The polymer backbone dynamics of the xylan chains were observed to have fewer dynamical modes, indicating lesser dynamical heterogeneity, at higher levels of hydration compared to the systems at low moisture content. The water dynamics in terms of the self-diffusion coefficients were observed to span several orders of magnitude at all the hydration states, with a significant fraction corresponding to that of the order of bulk water diffusivity at or above moisture saturation. Simulations over a microsecond long were carried out to understand how different substitution patterns involving acetyl and 4-O-methyl glucuronic acid groups affect polymer-water interactions and the pore network within the aggregated xylan structure. The results indicate significant differences in aggregation characteristics for the different xylan chains with accompanying variations in the local hydration environments surrounding xylan carbons.

Structural and dynamical properties of hydrated xylans using molecular dynamics simulations

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