TEMPO-mediated oxidation of chitin, regenerated chitin and N-acetylated chitosan

A commercial chitin, regenerated chitin prepared from chitin solutions in 6.8% NaOH and N-acetylated chitosans with degrees of N-acetylation (DNAc) of 77–93% were subjected to oxidization in water with NaClO and catalytic amounts of 2,2,6,6-tetramethylpiperidinyloxy radical (TEMPO) and NaBr. When regenerated chitin with DNAc of 87% and N-acetylated chitosan with DNAc of 93% were used as starting materials, water-soluble β-1,4-linked poly-N-acetylglucosaminuronic acid (chitouronic acid) Na salts with degrees of polymerization of ca. 300 were obtained quantitatively within 70 min. On the other hand, the original chitin and N-acetylated chitosan with DNAc of 77% did not give water-soluble products, owing to incomplete oxidation. The high crystallinity of the original chitin brought about low reactivity, and the high C2-amino group content of the N-acetylated chitosan with DNAc of 77% led to degradations rather than the selective oxidation at the C6 hydroxyls. The obtained chitouronic acid had low viscosities in water, and clear biodegradability by soil microorganisms.