Beyond polymer-polymer interactions, the simulations can directly quantify cation interactions with the cell wall microenvironment, tracking both Na + and Fe 3+ interactions at molecular detail (Fig. ). The cations largely make interactions with polar moieties such as hydroxyls, carboxyls, and the acetyl groups within the biopolymers, and examples of these interactions are readily findable within our trajectories (Fig ). Panels C and D in Fig. quantify the interaction shift that takes places in the constant size systems with increasing acetylation for Na + (Fig. ) or Fe 3+ (Fig. ), with the equivalent measures for the "expanded" simulation presented in Figs. and. Without acetylation, ions can only strongly interact with hydroxyl or carboxyl groups, either in approximately equal proportion (Na + ), or heavily weighted towards the carboxyls (Fe 3+ ). As the hydroxyl groups are replaced by acetyl groups in the acetylation process, the total interaction count increases, sometimes substantially. For instance, at 18% WPG Na + ions have nearly twice the contacts with acetyl groups in lignin or hemicellose than compared to hydroxyl groups at 0% WPG. Now some of this increase is clearly due to the increased size for the acetyl group compared to the hydroxyl group. However there are clearly many ions near the acetyl groups. There are only 150 carboxyl groups in our cell wall model (Fig. ), which is comparable to the number of cations present in the system to neutralize it. Thus, carboxyl groups are rare enough that their ion coordination is supported by other supporting interactions such as water or moieties from surrounding functional groups.