Acid hydrolysis of chitosans

The hydrolysis of the O-glycosidic linkages (depolymerization) and the N-acetyl linkage (de-N-acetylation) of partially N-acetylated chitosans were studied in dilute and concentrated HCl. The rate of hydrolysis of the glycosidic linkages was found to be equal to the rate of de-N-acetylation in dilute acid, while the glycosidic linkages was hydrolysed more than 10 times faster than the N-acetyl linkage in concentrated HCl. This can be explained by assuming that the hydrolysis of the N-acetyl linkage is a SN2 reaction (rate-limiting step: addition of water to the carbonium ion) while the hydrolysis of the glycosidic linkages is a SN1 reaction where the rate-limiting step is the formation of the carbonium ion. The specificity of the acid-catalysed cleavage of the different chitosan glycosidic linkages in concentrated HCl was such that the linkages between two acetylated units (A–A) and between an acetylated and a deacetylated unit (A–D) was cleaved with about equal rate and three orders of magnitude faster than the other two linkages (D–A and D–D). The activation energies for acid hydrolysis of two almost fully de-N-acetylated chitosans (FA=0.002 and FA<0.0003) were determined to be 152.2±8.1 and 158.1±9.8 kJ mol−1, respectively, representing the activation energy for hydrolysis of the D–D glycosidic linkage in chitosans. The activation energies for acid hydrolysis of two partially N-acetylated chitosans (FA=0.47 and FA=0.62) were determined to be 130.4±2.5 and 134.3±3.1 kJ mol−1, respectively, representing the activation energy for hydrolysis of the A–A and A–D glycosidic linkage in chitosans.