Selective monophosphorylation of chitosan via phosphorus oxychloride

Chitosan was selectively monophosphorylated via reaction with phosphorus oxychloride (POCl₃) to enhance water solubility while avoiding polyphosphate formation. The use of POCl₃ resulted in negligible product degradation (i.e., breakdown of O-glycosidic bonds) even after a 3 d reaction period (<5% weight loss). X-ray photoelectron spectroscopy (XPS) characterization of the POCl₃-phosphorylated chitosan (P-chitosan) revealed a phosphorus to nitrogen (P/N) atomic ratio of 0.30. Phosphorus-31 nuclear magnetic resonance (³¹P NMR) spectroscopy verified the monophosphorylation of chitosan's primary and secondary alcohols, and primary amines. The calcium chelation efficiency for the phosphorylated product approached 0.05 mg Ca²⁺ per mg of P-chitosan as measured by inductively coupled plasma-optical emission spectrometry (ICP-OES), indicating improved chelation over native chitosan. This selective monophosphorylation approach proved useful for modifying other biopolymers, including cellulose and alginate.