Characterization of Chitin and Chitosan Molecular Structure in Aqueous Solution.

Molecular dynamics simulations have been used to characterize the structure of single chitin and chitosan chains in aqueous solutions. Chitin chains, whether isolated or in the form of a β-chitin nanoparticle, adopt the 2-fold helix with ϕ and φ values similar to its crystalline state. In solution, the intramolecular hydrogen bond HO3(n)···O5(n+1) responsible for the 2-fold helical motif in these polysaccharides is stabilized by hydrogen bonds with water molecules in a well-defined orientation. On the other hand, chitosan can adopt five distinct helical motifs, and its conformational equilibrium is highly dependent on pH. The hydrogen bond pattern and solvation around the O3 atom of insoluble chitosan (basic pH) are nearly identical to these quantities in chitin. Our findings suggest that the solubility and conformation of these polysaccharides are related to the stability of the intrachain HO3(n)···O5(n+1) hydrogen bond, which is affected by the water exchange around the O3-HO3 hydroxyl group.