The depolymerization of chitosan: effects on physicochemical and biological properties.

Chitosan has been extensively used as an absorption enhancer for macromolecules and as gene delivery vehicle. Both properties are molecular weight (MW) dependent. Here, we investigate factors affecting the oxidative depolymerization of chitosan and physicochemical properties of the resulting polymer fractions including their cytotoxicity. The molecular weight of the depolymerized chitosan was influenced by the initial concentration and the source of chitosan. At constant initial concentrations, the molecular weight decreased linearly with the chitosan/NaNO2 ratio and was a function of logarithm of the reaction time. Chitosan with larger molecular weight was more sensitive to depolymerization. No structural change was observed during the depolymerization process by infrared and proton nuclear magnetic resonance spectroscopy. In addition, thermal properties of chitosan fragments were studied by thermal gravimetric analysis and it was found that the decomposition temperature was molecular weight dependent. Furthermore, the solubility of different molecular weight chitosan was assayed as a function of pH and it increased with decreasing molecular weight. The cytotoxicity of chitosan was concentration dependent but almost molecular weight independent according to MTT assay using L929 cell line recommended by USP26. In summary, low molecular weight fractions of chitosan may potentially useful for the design of drug delivery systems due to the improved solubility properties.