Biomedical Engineering Reference
In-Depth Information
Scheme 7.6
Chemical structures of chitin and chitosan.
Chitin is the second most abundant biopolymer after cellulose and is found in
ordered fi brils in cell walls of fungi and yeast and in the exoskeleton of crustaceans
and insects. The main commercial sources of chitin are shrimp and crab shells,
a waste product of the seafood production. For the production of pure chitin, the
shells are deproteinized under basic conditions and subsequently demineralized
under acidic conditions to remove CaCO
3
. It is important to note that chitin shows
three different crystalline structures depending on its function in nature. The
outer skeletal chitin in crustaceans consists of
α
- chitin, squid pen consists of
β
chitin, and fungi contain
-chitin. Chitin shows a high biocompatibility, an excel-
lent biodegradability, and a low immunogenicity. A major problem is the low
solubility of chitin in water and almost all common organic solvents due to its
high crystallinity [76] .
N
- deacetylation in concentrated alkali solution at high tem-
peratures or using the enzyme chitin deacetylase (EC 3.5.1.41) leads to chitosan.
The chemical
N
- deacetylation can be performed in two different ways. In a hetero-
geneous process, chitin is treated with 10-60% sodium hydroxide solution at
70-150 °C for up to 6 h. Chitosan prepared by this method is approximately 90%
deacetylated [77]. A milder homogenous process leads to water-soluble chitosan
which is 50% deacetylated by storing an alkaline solution of chitin for 77 h at room
temperature [78]. The predominant thermochemical chitosan production is envi-
ronmentally unsafe and hard to control, leading to broad range of products with
a lower molecular mass due to partial hydrolysis of the polymeric chain. The use
of chitin deacetylase (ED 3.5.1.41), which could be isolated and cloned from
various fungi and insects, can circumvent some of these problems. It was shown
that a 97% deacetylation of chitosan is possible using partially deacetylated chi-
tosan as substrate. The enzymatic deacetylation of crystalline or amorphous chitin
is still less effective yielding a 0.5-9.5% deacetylated product [79]. Naturally occur-
ring chitosan is very rare and can be found together with chitin in several fungi.
Since chitosan is rare, chitosan-degrading enzymes are less abundant. Lysozyme
does also, in addition to its natural substrate (the glycosidic linkage of certain
bacterial cell walls peptidoglycans), hydrolyze chitin and chitosans. Lysozyme is
present in many tissues and secretions such as tears, saliva, and blood [80, 81]. In
a detailed study, the enzymatic (lysozyme, chitinase, etc.) digestibility of various
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