Heavy Metal Ion-Specific Effect of Chitosan Across Length Scales

Heavy metal ions have a strong effect on macromolecules from their molecular structures to macroscopic behaviors. However, the underlying mechanism that connects different length scales remains poorly understood. In this study, we combine single-molecule experiments, theoretical calculations, simulations, and macroscopic analyses to investigate the interaction between a typical heavy metal ion, Pb²⁺, and a natural polysaccharide chitosan. We find that Pb²⁺ not only chelates with single chitosan chains and restricts their conformational flexibility, but also chelates with and bridges multiple chitosan chains simultaneously. This molecular process induces the aggregation of chitosan into large-scale structures through a kinetic pathway, where Pb²⁺ enhances its mechanics by facilitating cross-linking and crystallization. Our findings provide a bottom-up framework for the regulation of Pb²⁺ on chitosan, which fills the gap in understanding the multiscale mechanism of the ion-specific effect of macromolecules.