Acid hydrolysis of chitosans

The hydrolysis of the O-glycosidic linkages (depolymerization) and the N-ac etyl linkage (de-N-acetylation) of partially N-acetylated chitosans were st udied in dilute and concentrated HCl. The rate of hydrolysis of the glycosi dic linkages was found to be equal to the rate of de-N-acetylation in dilut e acid, while the glycosidic linkages was hydrolysed more than 10 times fas ter 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(N)2 reaction G ate-Limiting step: addition of water to the carbonium ion) while the hydrol ysis of the glycosidic linkages is a S(N)1 reaction where the rate-limiting step is the formation of the carbonium ion. The specificity of the acid-ca talysed cleavage of the different chitosan glycosidic linkages in concentra ted HCl was such that the linkages between two acetylated units (A-A) and b etween an acetylated and a deacetylated unit (A-D) was cleaved with about e qual 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 fu lly de-N-acetylated chitosans (F-A = 0.002 and F-A < 0.0003) were determine d to be 152.2 +/- 8.1 and 158.1 +/- 9.8 kJ mol(-1), respectively, represent ing the activation energy for hydrolysis of the D-D glycosidic linkage in c hitosans. The activation energies for acid hydrolysis of two partially N-ac etylated 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 activa tion energy for hydrolysis of the A-A and A-D glycosidic linkage in chitosa ns. (C) 2001 Elsevier Science Ltd. All rights reserved.