100
P. W. KENT
that the lamination arises from mechanical forces influencing the orientation of chitin crystallites during deposition.
A resume of the chitin-bearing species among the Arthropoda has
been made by Richards (12).
III. Molecular Structure and Function of Chitin
A. PHYSICAL STRUCTURE AND PROPERTIES
Investigations of the precise macromolecular relationships of chitin
are beset by problems due to insolubility of the material and to difficulties of bringing it into solution without degradation. The chemical
evidence favors a linear polysaccharide in which 2-acetamido-2-deoxy-ßD-glucosyl residues are linked through the 4-positions. In general principle, chitobiose comprises the structural repeating unit though there is as
yet no firm evidence about the chain length of chitin nor whether all
chains are identically constituted. The macrostructure thus appears to
have features in common with the fiber structure of cellulose, supplemented with enhanced possibilities of intercalary hydrogen bonding of
adjacent N-acetyl groups. The tensile strength of purified chitin is reported to vary with the source and with moisture content, in the dry
state having values (46, 47) up to 35-38 kg,/mm. and when moist (48),
1.8 kg./mm.; in these respects it is a fibrous material stronger than hair
or silk fibroin. As with cellulose (49) and silk fibroin (50), the density
of chitin (51) (1.41-1.42) also varies with source and moisture content.
Further mechanical properties of chitin, in particular elasticity, hardness,
swelling, and optical properties, have been reviewed by Richards (12,
13).
The optical rotation of chitin has not been extensively studied, although it is known (52) that "solutions" of the material in concentrated
hydrochloric acid exhibit values of [«] D
20 —14°; degradation must be
presumed to have commenced under these conditions, however.
Birefringence [earlier work being reviewed by Frey-Wyssling (53)]
has been much used in examining the lamellar distribution of chitin.
Since the first observation (54) of the birefringent properties of chitinous
tendons, the property has provided extensive data regarding the direction and degree of orientation of anisotropic chitin molecules. It is of
particular importance that observations are concerned with chitin in situ,
that is, not merely with poly- (N-acetylglucosamine), but with the protein-polysaccharide or other complex structures in which it is combined
(see page 119). Whatever may the nature be of such complexes, it is clear
that these form a variety of higher states of aggregation (55-59), ranging
from micelles to microfibers (70-200A.) and in cuticle, to micro-
P. W. KENT
that the lamination arises from mechanical forces influencing the orientation of chitin crystallites during deposition.
A resume of the chitin-bearing species among the Arthropoda has
been made by Richards (12).
III. Molecular Structure and Function of Chitin
A. PHYSICAL STRUCTURE AND PROPERTIES
Investigations of the precise macromolecular relationships of chitin
are beset by problems due to insolubility of the material and to difficulties of bringing it into solution without degradation. The chemical
evidence favors a linear polysaccharide in which 2-acetamido-2-deoxy-ßD-glucosyl residues are linked through the 4-positions. In general principle, chitobiose comprises the structural repeating unit though there is as
yet no firm evidence about the chain length of chitin nor whether all
chains are identically constituted. The macrostructure thus appears to
have features in common with the fiber structure of cellulose, supplemented with enhanced possibilities of intercalary hydrogen bonding of
adjacent N-acetyl groups. The tensile strength of purified chitin is reported to vary with the source and with moisture content, in the dry
state having values (46, 47) up to 35-38 kg,/mm. and when moist (48),
1.8 kg./mm.; in these respects it is a fibrous material stronger than hair
or silk fibroin. As with cellulose (49) and silk fibroin (50), the density
of chitin (51) (1.41-1.42) also varies with source and moisture content.
Further mechanical properties of chitin, in particular elasticity, hardness,
swelling, and optical properties, have been reviewed by Richards (12,
13).
The optical rotation of chitin has not been extensively studied, although it is known (52) that "solutions" of the material in concentrated
hydrochloric acid exhibit values of [«] D
20 —14°; degradation must be
presumed to have commenced under these conditions, however.
Birefringence [earlier work being reviewed by Frey-Wyssling (53)]
has been much used in examining the lamellar distribution of chitin.
Since the first observation (54) of the birefringent properties of chitinous
tendons, the property has provided extensive data regarding the direction and degree of orientation of anisotropic chitin molecules. It is of
particular importance that observations are concerned with chitin in situ,
that is, not merely with poly- (N-acetylglucosamine), but with the protein-polysaccharide or other complex structures in which it is combined
(see page 119). Whatever may the nature be of such complexes, it is clear
that these form a variety of higher states of aggregation (55-59), ranging
from micelles to microfibers (70-200A.) and in cuticle, to micro-