5 Structure of Chitin and Chitosan
Chitin is the second-most abundant natural biopolymer (just after cellulose) on
earth and it is found in the structure of a wide number of invertebrates (crustacean
exoskeletons, insect cuticles) and the cell walls of fungi, among others [93–95].
Depending on source, chitin is available in three different polymeric configurations,
namely α-chitin, β-chitin, and γ-chitin. Usually, α-chitin is extracted from the
exoskeletons of crustaceans, particularly shrimps and crabs [96, 97]. β-Chitin can
be extracted from squid pens, and γ-chitin from fungi and yeast [98]. α-Chitin is the
most common form, whereas β-chitin is more reactive [99] and shows a higher
affinity for solvents [100]. β-Chitin is easily converted to α-chitin by alkaline
treatment followed by flushing in water [101]. Basically, chitin and chitosan are
described as a family of linear polysaccharides consisting of varying amounts of
β-(1 ! 4)-linked residues of N-acetyl-2-amino-2-deoxy-D-glucose (A residues)
and 2-amino-2-deoxy-D-glucose residues (D residues). The structure of chitin and
chitosan is shown in Fig. 6. Chitosan samples have a low amount of D units and
hence the polymer is insoluble in acidic aqueous media. On the other hand, the
amount of D units in chitosan samples is high enough to allow the polymer to
dissolve in acidic aqueous media.
6 Properties of Chitin and Chitosan
Due to the presence of variable compounds in the natural sources from which chitin
and chitosan are extracted, the physical and chemical properties of both chitin and
chitosan vary remarkably. Chitosan has been described as a nontoxic, biodegradable,
and biocompatible polymer with very interesting biological properties, such as
permeation-enhancing and mucoadhesive properties, anticoagulant and antimicrobial activity and so on. The properties of chitosan are greatly affected by the
conditions under which it is processed, because it is the process conditions that
Table 8 Physicochemical characteristics of chitin and chitosan and the determination methods
Physicochemical
characteristic
Determination methods
Degree of deacetylation Infrared spectroscopy [76–78], first derivative UV-spectrophotometry
[79, 80], nuclear magnetic resonance spectroscopy (
1
HNMR) and
(
13
CNMR) [81–83], conductometric titration [83], potentiometric
titration [84], differential scanning calorimetry [85]
Average M w and/or M w
distribution
Viscometry [86], gel permeation chromatography [87], light
scattering [88]
Crystallinity
X-ray diffraction [89]
Moisture content
Gravimetric analysis [90]
Ash content
Gravimetric analysis [90]
Protein
Bradford method [91]
100
J. Dutta
Chitin is the second-most abundant natural biopolymer (just after cellulose) on
earth and it is found in the structure of a wide number of invertebrates (crustacean
exoskeletons, insect cuticles) and the cell walls of fungi, among others [93–95].
Depending on source, chitin is available in three different polymeric configurations,
namely α-chitin, β-chitin, and γ-chitin. Usually, α-chitin is extracted from the
exoskeletons of crustaceans, particularly shrimps and crabs [96, 97]. β-Chitin can
be extracted from squid pens, and γ-chitin from fungi and yeast [98]. α-Chitin is the
most common form, whereas β-chitin is more reactive [99] and shows a higher
affinity for solvents [100]. β-Chitin is easily converted to α-chitin by alkaline
treatment followed by flushing in water [101]. Basically, chitin and chitosan are
described as a family of linear polysaccharides consisting of varying amounts of
β-(1 ! 4)-linked residues of N-acetyl-2-amino-2-deoxy-D-glucose (A residues)
and 2-amino-2-deoxy-D-glucose residues (D residues). The structure of chitin and
chitosan is shown in Fig. 6. Chitosan samples have a low amount of D units and
hence the polymer is insoluble in acidic aqueous media. On the other hand, the
amount of D units in chitosan samples is high enough to allow the polymer to
dissolve in acidic aqueous media.
6 Properties of Chitin and Chitosan
Due to the presence of variable compounds in the natural sources from which chitin
and chitosan are extracted, the physical and chemical properties of both chitin and
chitosan vary remarkably. Chitosan has been described as a nontoxic, biodegradable,
and biocompatible polymer with very interesting biological properties, such as
permeation-enhancing and mucoadhesive properties, anticoagulant and antimicrobial activity and so on. The properties of chitosan are greatly affected by the
conditions under which it is processed, because it is the process conditions that
Table 8 Physicochemical characteristics of chitin and chitosan and the determination methods
Physicochemical
characteristic
Determination methods
Degree of deacetylation Infrared spectroscopy [76–78], first derivative UV-spectrophotometry
[79, 80], nuclear magnetic resonance spectroscopy (
1
HNMR) and
(
13
CNMR) [81–83], conductometric titration [83], potentiometric
titration [84], differential scanning calorimetry [85]
Average M w and/or M w
distribution
Viscometry [86], gel permeation chromatography [87], light
scattering [88]
Crystallinity
X-ray diffraction [89]
Moisture content
Gravimetric analysis [90]
Ash content
Gravimetric analysis [90]
Protein
Bradford method [91]
100
J. Dutta