Multiscale Mechanics of Calcium-Mediated Reinforcement within a Pectin-Cellulose Composite via Integrated MD and Experimental Approach

Cellulose and pectin are the key components of plant primary cell walls (PCWs) responsible for their dynamic growth, as they transition from a flexible structure at early stages to a rigid unit at full growth. However, a fundamental understanding of the pectin-cellulose interface and interactions within PCW and the underlying mechanics of these materials remained a subject of debate, hindering progress in bioinspired and sustainable composite designs to meet the demands of emerging fields, from flexible robotics to regenerative medicine. This study presents a multiscale investigation into the CNC-pectin interface and the influence of calcium ion-mediated cross-links, integrating molecular dynamics (MD) simulations, supported by experimental data of molecular interactions via spectroscopic studies and bulk interactions via viscosity measurements. The MD simulations revealed cross-linking mechanisms of "zipper" and "egg-box", both being present, depending on local composite properties and ionic concentrations, with the zipper model being the dominant mechanism by almost 10 times with relative insensitivity to Ca²⁺, thus providing deeper insights into the long-ongoing discussion on pectin Ca²⁺ interactions. The zipper model is driven by the coordination of Ca²⁺ with deprotonated carboxyl groups (-COO-), while the egg-box model involves both carboxyl and hydroxyl groups (-OH). The confirmation via spectroscopic studies, characterized by consistent shifts in Raman peaks of the carboxylate group, indicating the rearrangement of ester and carboxyl groups of HGA, and concentration-dependent peak enhancement trends of the hydroxyl group in the FTIR study involved in the two models validated the MD outcomes. Furthermore, MD predicted viscosity aligned with bulk properties provides a basis for the future extension of the work on quantifying interface energies. Overall, the study provides fundamental knowledge on Ca²⁺-mediated CNC-pectin interactions, helping to resolve reported experimental discrepancies and offering design guidelines for advanced pectin-based biocomposites.