Acid hydrolysis of chitosans

Abstract

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 S N2 reaction (rate-limiting step: addition of water to the carbonium ion) while the hydrolysis of the glycosidic linkages is a S N1 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 ( F A=0.002 and F A<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 ( F A=0.47 and F A=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.