Effect of pH on Molecular Structures and Network of Glycol Chitosan.

Chitosan is a natural polycationic linear polysaccharide deacetylated from chitin. Glycol chitosan is a derivative of chitosan and has been extensively investigated in the biomaterials and hydrogel field for many bioengineering applications because of their unique material and biological properties. However, the molecular structure and network of glycol chitosan hydrogels remain unclear. Here, we explored the molecular structures and network of glycol chitosan with different protonation percentages by using full atomistic simulations. Hydrogel and xerogel models are constructed to understand the interactions between the water molecules and glycol chitosan chains. We calculated the radius of gyration and radial distribution function of hydrogel and xerogel models to understand the swelling behavior from molecular level. We find that when the pH is close to neutral and becomes basic, greater flexibility of glycol chitosan chains leads to a high swelling ratio. The slight contracting behavior of glycol chitosan chains and the dispersive distribution above 40% protonation can be interpreted to indicate a poor swelling ratio. The protonated amino groups inhibit the hydrogen-bond formation between water molecules and adjacent oxygen-containing groups of glycol chitosan main chains. On the other hand, the glycol groups of glycol chitosan are not affected by the electrostatic interaction, and the number of hydrogen bonds between glycol groups and water molecules does not vary with pH. The van der Waals interaction between glycol chitosan chains is dominant when the protonation percentages are lower than 40%, while the electrostatic interaction of amino groups is dominant when the protonation percentages are higher than 40%. Our results explain the effects of pH on the molecular structures of glycol chitosan and provide useful information regarding the design strategy of novel glycol chitosan and its derivatives for biomedical applications.