We observe that at reduced moisture levels present in the constant system, the diffusion coefficients for Na + and water reduce by almost an order of magnitude compared with the unacetylated control (Fig. ). By contrast, the diffusion change upon acetylation for the expanded system is much smaller, decreasing by only 50 % with the same change in acetylaton (Fig. ). The pattern is consistent for both Na + and water, with Na + trailing diffusion of water by a consistent factor of 8-10x. Naturally, a Na + ion is heavier than water, and so some slower diffusion is expected. However, the molecular weight difference is too small to account for the difference between water and Na + diffusion. Instead, interactions between the cell wall polymers and cations are likely slowing ion diffusion, as has been noted previously for unacetylated systems. Figure : Specific interactions between ions and moieties on plant cell wall biopolymers. (A) Within our simulation, we can identify poses where Na + ions (yellow spheres) interact directly with acetyl groups, hydroxyl group, and carboxyl groups in hemicellulose (green) and lignin (orange). (B) The interaction between the three secondary cell wall polymers (lignin, hemicellulose and cellulose) for each degree of acetylation are quantified using Eq. 6, and graphed in Fig. . (C-D) The interactions for specific lignin and hemicellulose moieties with Na + (C) and Fe 3+ (D) ions for each of the different acetylation degrees are quantified using the same approach as (B). In panels C and D, we are measuring the contacts for the "constant" simulation set, with the equivalent measures for the "expanded" simulation presented in Figs. and. Standard error estimates for each condition are a result of subdividing the 1 µs trajectory, where the first 50 ns is considered for system equilibration and not included in the analysis.