Toward Understanding the Environmental Control of Hydrogel Film Properties: How Salt Modulates the Flexibility of Chitosan Chains

Chitosan is a pH-responsive self-assembling polysaccharide that can be electrodeposited to form a hydrogel film. During electrodeposition a host of dynamic processes occur simultaneously over a hierarchy of length scales. Experiments have shown that the microstructure and properties of the deposited gels are highly dependent on the solution conditions and imposed electrical signals. However, detailed mechanisms are not understood. Here we use molecular dynamics simulations to explore the conformational dynamics of individual chitosan chains composed of 20 glucosamine units in aqueous solution. Simulations yield a total persistence length of about 5 nm, in agreement with the lower range of the experimental estimates and supporting a worm-like-chain model. The surprisingly high flexibility arises from the glycosidic linkage sampling an appreciable population of anti-ψ conformation which is associated with backbone bending and higher flexibility. Significantly, the presence of added salt increases the anti-ψ population and consequently flexibility for both charged and neutral chains. The latter is due to counterion binding and disruption of an intramolecular hydrogen bond that stabilizes the extended conformation. Thus, our data suggest that salt concentration modulates the conformation and dynamics of individual chitosan chains in addition to interchain hydrogen bonding during pH-induced self-assembly. The insight from this work provides a missing piece of the puzzle toward understanding the complex mechanism by which solution conditions control the hydrogel properties of chitosan at the microscopic level which cannot be accessed experimentally.