Secondary plant cell walls are composed from 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. For instance, the effectiveness of both common wood protection treatments and biomass pretreatments for biorefineries typically depend on ionic 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 wall, depending on the hydration index (3 – 30%). From these simulations, we analyze the contacts between different plant cell wall components with each other and their interaction with the ions. The positively charged cations are found to diffuse freely in the secondary cell wall, and preferentially interact with hemicellulose components. Carboxylate groups on our hemicellulose model are negatively charged, forming strong and stable electrostatic interactions with the cation, hindering cation diffusion. Diffusion measurements within our simulations show that diffusivity grows nonlinearly with increasing moisture content within the cell wall. These observations shed light onto the molecular mechanisms for diffusive processes within secondary plant cell walls at atomic resolution. Armed with this mechanism, it is possible to design and engineer pathways to alter the mechano-chemical properties of lignocellulosic biomass, catering to applications in sustainable and biomedical engineering.

Diffusion in plant cell wall models at different relative humidity using molecular dynamics simulations

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