Work

Deciphering the phosphorylation of chitosan through complementary 1H and 31P{1H} DOSY NMR

Year: 2025

Type: article

Abstract: Chemical modification of chitosan through phosphorylation has gained significant attention for expanding its applications. However, confirming whether phosphorylating agents form covalent bonds with the chitosan backbone or remain as non-covalently associated species has remained challenging using conventional analytical techniques. The diffusion-ordered spectroscopy (DOSY) NMR of the complementary probes 1 H and 31 P nuclei was used for distinguishing between the results of three phosphorylating agents: phosphoric acid, phosphorous acid, and dimethyl phosphite. While conventional FTIR and common 1D/2D NMR spectroscopy experiments confirmed the presence of phosphorus-containing groups in all samples, DOSY NMR analysis revealed critical differences in molecular behavior. Chitosan backbone protons exhibited consistently low self-diffusion coefficients (4-9 × 10 -12 m 2 /s) across all samples. Phosphorus species in samples treated with phosphoric acid and phosphorous acid displayed significantly higher diffusion coefficients (394-548 × 10 -12 m 2 /s), indicating non-covalent association and freely diffusing in solution. In contrast, dimethyl phosphite treatment produced a 31 P resonance at 30.3 ppm with a diffusion coefficient of 11 × 10 -12 m 2 /s, closely matching the chitosan backbone protons values and providing strong evidence for covalent phosphorylation. This work establishes DOSY NMR of complementary probes as a reliable, quick, and simple method for distinguishing between covalent or non-covalent modification of biopolymers.

Source: Carbohydrate Polymer Technologies and Applications

DOI: 10.1016/j.carpta.2025.101021