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(N)2 reaction Gate-Limiting step: addition
of water to the carbonium ion) while the hydrolysis 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-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.
(C) 2001 Elsevier Science Ltd. All rights reserved.
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