Deacetylation-regulated binding mechanism between chitosan and Pb2+ from nano to macroscale

Abstract

The specific binding capacity between chitosan and heavy metal ions is critical in food safety, which is closely regulated by its degree of deacetylation (DD). However, the underlying mechanism governing the impact of DD remains unclear. Here we employ a combination of experiments and simulations to study the interaction between chitosan and Pb²⁺ across multiscale at different DD. At the molecular level, Pb²⁺ can chelate individual chitosan chains simultaneously via multiple interaction sites, which is enhanced with rising DD and leads to more ordered molecular packing. This molecular arrangement then develops to a nanoscale aggregation process, where the aggregation size, rate, and mechanical stability all show a positive correlation with DD. Such aggregation ultimately leads to enhanced macroscale structures, where the strain and tensile stress are significantly increased by 2 to 3-folds and further rises with increasing DD. These observed effects are governed by a unified mechanism: deacetylation introduces more amino groups that serve as stronger chelation sites than acetyl groups, thereby enhancing Pb²⁺ binding across length scales. These findings provide foundation for the development and application of chitosan in the food industry, such as food preservation, anti-heavy-metal packaging, and food purification.