Chemistry Reference
In-Depth Information
Figure 12.1
Chemical structures of chitin and
chitosan. Chitin and chitosan are polymers of
GlcNAc and glucosamine (GlcN) units, respec-
tively. The repetitive units of chitin or chitosan
are disaccharides named chitobiose (indicated
in gray).
disaccharides called cellobiose and chitobiose, respectively (for structural depiction
of cellobiose and monosaccharides, please see Figures 1.4
/
5). However, while cel-
lulose is composed of
-
D
-
glucose [
N
- acetylglucosamine ( GlcNAc )]. The three - dimensional structure of chitin
was investigated extensively in the second half of the last century. It is usually
found in the form of crystalline microfi brils of varying diameters and lengths. On
the basis of X-ray diffraction patterns, chitin is an anisotropic polymer in the
4
C
1
conformation that occurs in three allomorphic crystalline forms named
β
-
D
-glucose, chitin is made of 2- acetamido - 2 - deoxy -
β
α
- ,
β
- and
γ
- chitin [3] . Additionally, non -crystalline, transient chitin states have been reported
in fungi [4]. The crystalline forms mainly differ in the degree of hydration, in the
size of the unit cell and in the number of chitin chains per unit cell.
- Chitin is
the most widely distributed chitin allomorph in Nature, mainly found in the cell
walls of fungi and in the cuticles of arthropods. The sugar chains in the crystal
exhibit an antiparallel orientation with respect to the reducing and nonreducing
ends facilitating tight packaging.
α
α
-Chitin is stabilized by two types of intramo-
lecular hydrogen bonds [C(3)
=
OH · · · O
=
C(5) and C(6)
=
OH · · · O
=
C] and two
types of intermolecular hydrogen bonds [NH· · · O
C(6)]
[5]. Accordingly, two types of hydrogen bonds exist that involve the carbonyl
groups: (i) exclusively intermolecular hydrogen bonding which account for 60%
of all hydrogen bonds, and (ii) a mixture of inter- and intramolecular hydrogen
bonding which account for about 40% of all hydrogen bonds as determined by
13
C
solid-state nuclear magnetic resonance (Figure 12.2) [6]. This rather high number
of intermolecular hydrogen bonds, which resemble those of cellulose II, seems to
explain why
=
C and C(6)
=
OH · · · OH
=
-chitin is thermodynamically very stable and unable to swell after
soaking it in water.
α
-chitin, and is found in squid
pens, in the skin of (pogonophoran and vestimentiferan) tubeworms, and in the
spines of diatoms and some insect cocoons. The sugar chains are arranged in a
parallel orientation reducing the packaging tightness and the number of intermo-
β
-Chitin is less widely distributed in Nature than
α
