Quantifying acetylation-induced changes in the plant secondary cell wall structure and dynamics

Lignin and carbohydrate rich secondary plant cell walls are abundantly available in the biosphere, and is a notable renewable feedstock for biofuels and biomaterials. Particularly for construction applications, wood that is resistant to fungal degradation is highly desirable. Chemical modifications, such as acetylation, have been successfully demonstrated to inhibit wood decay by microorganisms. It is well known that acetylation reduces wood moisture content, which slows down motions within the cell wall when measured by X-ray fluorescence microscopy experiments and molecular simulations. The open question is whether acetylation inhibits decay strictly by reducing moisture content, or if specific interactions with the acetyl group hinder motion within the cell wall and further inhibit decay. We investigate these hypotheses directly through molecular simulation, acetylating exposed hemicellulose and lignin hydroxyl groups in existing models for secondary plant cell wall structure to 5-18% weight-percent gain. By comparing diffusive behavior for cell wall polymers, water, and select ions (Na+ and Fe3+ ), we can track the dynamics within the cell wall and identify the causal mechanisms for reduced transport and uptake of these metal ions by acetylated cell walls. We find that the change from hydroxyl to acetyl group alone does not account for reduced transport, with only modest changes in diffusion when acetylated cell walls are expanded to provide constant moisture level. The most substantial changes in diffusion occur where the additional acetylation displaces water, reducing the moisture content for the cell wall. Utilizing these simulations, we further analyze the interactions between ions and cell wall polymers and the evolution of dynamic water pockets within the structure. Ions interact more frequently with the acetyl group than the hydroxyl groups they replace, yielding to increased ion interactions on aggregate upon acetylation. Collectively, these findings elucidate the molecular mechanism through which acetylation affects secondary plant cell walls at atomic resolution.