Preparation and characterisation of oligosaccharides produced by nitrous acid depolymerisation of chitosans.

Two chitosans with widely different chemical composition (fraction of N-acetylated units (F(A))<0.001 and F(A)=0.59), were degraded by nitrous acid, to obtain the reactive 2,5-anhydro-D-mannose- (M-) unit at the new reducing end. The fully N-acetylated and fully N-deacetylated oligomers were separated by size-exclusion chromatography. Both the chemical structure and purity were studied by one- and two-dimensional 1H and 13C NMR methods. The fully N-acetylated oligomers were found to be stable, whereas the N-deacetylated oligomers reacted intermolecularly by a Schiff base reaction between the 2-amino group on the N-deacetylated units and the M-units, facilitating the cleavage of the glycosidic bond next to the M-unit and the formation of 5-hydroxymethylfurfural (HMF).