80
1
General Principles
such as fungi, yeasts, green algae, and brown and red seaweeds. It also comprises the major
component of the exoskeleton of insects, where it makes a hard shell-like material that is quite
strong. Chitin is also found in the cuticles of worms and in the shells of crustaceans, such as
mollusks, shrimps, crabs, and lobsters [77].
Chitin, like cellulose, has a highly ordered, crystalline structure in which the chains are intermolecularly hydrogen bonded in an antiparallel arrangement, a parallel arrangement, and
a mixed arrangement of two parallel and one antiparallel repeating arrangement [78]. Also
like cellulose, it is very insoluble in water and most other solvents. In arthropods, the chitinous shell, or exoskeleton, does not grow, and is periodically cast off or molted. After the old
shell is shed, a new, larger shell is produced, providing room for further growth. Chitin is very
rigid, except between some body segments and joints, where it is much thinner and allows
movement of the various parts.
7.2 Properties and Occurrence of Chitosan
Chitosan is a polysaccharide very similar to chitin, except that the N-acetyl-D-glucosamine
is replaced by D-glucosamine in which the N-acetyl group is removed (see > Fig. 6 for the
structure of a segment of chitosan). Chitosan is found occurring naturally mixed with chitin in
the cell walls of some fungi and seaweeds. It is, however, primarily produced chemically by
treating chitin with strong alkali to deacetylate the N-acetyl-amino group [79]. The degree of
deacetylation can range from 60 to 100%, giving a family of chitosans. The free amino group
of chitosan has a pK a value of ∼6.5 and it can be protonated in mildly acidic solutions, giving
a positive charge to the glucosamine residues. The positive charges on chitosan produce very
different physical and chemical properties from chitin. Because of the repulsion of the positive
charges, chitosan chains do not line up and associate to form micelles and fibers, as does
cellulose and chitin. Chitosan, thus, is water-soluble at acidic pH values.
Because of the positive charges on chitosan, it has found a number of applications. It binds
to negatively charged surfaces, such as mucosal membranes, and has been used as a bandage
material for wounds that is biocompatible and biodegradable [80,81]. Positively charged chitosan enhances the transport of polar drugs across epithelial tissues and is used to transport
drugs in humans [82]. It has been used as an enhancer for plant growth, and as an aid in the
defense of plants against fungal infections. Chitosan is used in water purification, as a material in a sand filtration system where it binds fine sediment particles during filtration, greatly
aiding the removal of turbidity; it also removes phosphates by ion exchange, heavy metals by
chelation, and oils by hydrophobic adsorption from water [81]. Chitosan has also been found
useful for the immobilization of enzymes and cells [81,82,83,84].
7.3 Properties and Occurrence of N- Acetyl-D-Glucosamine and
N-Acetyl-D-Muramic Acid in Murein – The Bacterial Cell Wall
The major component of all known bacterial cell walls is a polysaccharide composed of
N-acetyl-D-glucosamine (NAG) linked together by β-(1→4) glycosidic bonds, as in chitin,
but with every other NAG residue substituted at C-3 by an ether linkage to the hydroxyl group
of L-lactic acid to give N-acetyl-D-muramic acid (NAM) [85,86,87,88]. This results in a nine-
1
General Principles
such as fungi, yeasts, green algae, and brown and red seaweeds. It also comprises the major
component of the exoskeleton of insects, where it makes a hard shell-like material that is quite
strong. Chitin is also found in the cuticles of worms and in the shells of crustaceans, such as
mollusks, shrimps, crabs, and lobsters [77].
Chitin, like cellulose, has a highly ordered, crystalline structure in which the chains are intermolecularly hydrogen bonded in an antiparallel arrangement, a parallel arrangement, and
a mixed arrangement of two parallel and one antiparallel repeating arrangement [78]. Also
like cellulose, it is very insoluble in water and most other solvents. In arthropods, the chitinous shell, or exoskeleton, does not grow, and is periodically cast off or molted. After the old
shell is shed, a new, larger shell is produced, providing room for further growth. Chitin is very
rigid, except between some body segments and joints, where it is much thinner and allows
movement of the various parts.
7.2 Properties and Occurrence of Chitosan
Chitosan is a polysaccharide very similar to chitin, except that the N-acetyl-D-glucosamine
is replaced by D-glucosamine in which the N-acetyl group is removed (see > Fig. 6 for the
structure of a segment of chitosan). Chitosan is found occurring naturally mixed with chitin in
the cell walls of some fungi and seaweeds. It is, however, primarily produced chemically by
treating chitin with strong alkali to deacetylate the N-acetyl-amino group [79]. The degree of
deacetylation can range from 60 to 100%, giving a family of chitosans. The free amino group
of chitosan has a pK a value of ∼6.5 and it can be protonated in mildly acidic solutions, giving
a positive charge to the glucosamine residues. The positive charges on chitosan produce very
different physical and chemical properties from chitin. Because of the repulsion of the positive
charges, chitosan chains do not line up and associate to form micelles and fibers, as does
cellulose and chitin. Chitosan, thus, is water-soluble at acidic pH values.
Because of the positive charges on chitosan, it has found a number of applications. It binds
to negatively charged surfaces, such as mucosal membranes, and has been used as a bandage
material for wounds that is biocompatible and biodegradable [80,81]. Positively charged chitosan enhances the transport of polar drugs across epithelial tissues and is used to transport
drugs in humans [82]. It has been used as an enhancer for plant growth, and as an aid in the
defense of plants against fungal infections. Chitosan is used in water purification, as a material in a sand filtration system where it binds fine sediment particles during filtration, greatly
aiding the removal of turbidity; it also removes phosphates by ion exchange, heavy metals by
chelation, and oils by hydrophobic adsorption from water [81]. Chitosan has also been found
useful for the immobilization of enzymes and cells [81,82,83,84].
7.3 Properties and Occurrence of N- Acetyl-D-Glucosamine and
N-Acetyl-D-Muramic Acid in Murein – The Bacterial Cell Wall
The major component of all known bacterial cell walls is a polysaccharide composed of
N-acetyl-D-glucosamine (NAG) linked together by β-(1→4) glycosidic bonds, as in chitin,
but with every other NAG residue substituted at C-3 by an ether linkage to the hydroxyl group
of L-lactic acid to give N-acetyl-D-muramic acid (NAM) [85,86,87,88]. This results in a nine-