Diffusion in Intact Secondary Cell WallModels of Plants at Different Equilibrium Moisture Content

02 February 2023, Version 1
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Secondary plant cell walls are composed of carbohydrate and lignin polymers, and collectively represent a significant renewable resource. Leveraging these resources depends in part on a mechanistic understanding for diffusive processes within plant cell walls. Common wood protection treatments and biomass conversion processes to create biorefinery feedstocks feature ion or solvent diffusion within the cell wall. X-ray fluorescence microscopy experiments have determined that ionic diffusion rates are dependent on cell wall hydration as well as the ionic species through non-linear relationships. In this work, we use classical molecular dynamics simulations to map the diffusion behavior of different plant cell wall components (cellulose, hemicellulose, lignin), ions (Na+ , K+ , Cu2+ , Cl- ) and water within a model for an intact plant cell wallat various hydration states (3 - 30 wt.% water). From these simulations, we analyze the contacts between different plant cell wall components with each other and their interaction with the ions. Generally, diffusion increases with increasing hydration, with lignin and hemicellulose components increasing diffusion by an order of magnitude over the tested hydration range. Ion diffusion depends on charge. Positively charged cations preferentially interact with hemicellulose components, which include negatively charged carboxylates. As a result, positive ions diffuse more slowly than negatively charged ions. Measured diffusion coefficients are largely observed to best fit piecewise linear trends, with an inflection point between 10-15% hydration. These observations shed light onto the molecular mechanisms for diffusive processes within secondary plant cell walls at atomic resolution.

Keywords

molecular diffusion
plant cell wall
ion diffusion
molecular dynamics
equilibrium moisture content

Supplementary materials

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Supplementary animations
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The supporting information includes supplementary animations. In Animation S1, we show a periodic visualization for our cell wall model, highlighting the density of the cellulose (blue), lignin (orange), and hemicellulose (green) components. In Animation S2, we visualize a single simulation unit cell at 5% hydration using the same coloration scale, whereas Animation S3 visualizes the 30% hydration simulation. Animation S4, S5, and S6 show sodium, potassium and copper ions diffusing around the central cellulose fibril (blue), along with nearby carboxylated hemicellulose units.
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