2. CHITIN AND MUCOSUBSTANCES
103
complete within 24 hours. The mineralization may proceed by crystal
growth from generally distributed foci or by growth from some outer
layer or boundary. Kleinholtz (83, 84) points out that molting appears to
be under hormonal control whereas calcification does not. The recycling
of calcium deposits in cuticle during molting and the resorption of Ca
2+
has been intensively studied (85), and it is apparent that at least in part
these cations are conserved for the formation of new cuticle. The means
of calcium transport in the blood of arthropods is not certain, but it
seems likely that some type of protein complexes may be involved.
Using the technique of tilting infra spectra (86-88),
it has been
found that crab chitin crystallites and blow fly cuticle chitin show different uniplanar orientations.
In cell walls, unicellular bristles (Drosophila), and scales (Ephestia
sericarium), there is evidence of parallel alignment of protein and chitin
(poly-N-acetylglucosamine) in the direction of growth. From initial
development both bristles and scales inhibit strong birefringence (89).
This molecular arrangement conforms to the protein and poly-N-acetylglucosamine parallel interactions proposed by Rudall (12a, 90).
2. Macromolecular Structure of Chitin
Investigations of the relative intermolecular arrangements of chitin
chains represents one of the earliest applications of X-ray analysis to a
problem of biological interest. It must be emphasized, however, that the
degree of crystallinity necessary for such work is influenced by the
source and extent of "purification" of the chitin specimen, and that
alkali-treated "pure" chitin may exhibit considerably higher degrees of
hydrogen bonding and hence of intermolecular orientation than is normally present in vivo.
Following early reports by Herzog (91) and Gonell (92), on the
existence of chitin crystallites the general similarity of fibers of this material and cellulose became apparent. Meyer and Mark (93), in 1928,
suggested that the acetylglucosamine residues in chitin were linked and
bonded in the same fashion as the D-glucose residues in cellulose, assuming in each case β-Ό-(1 -> 4) glycosidic bonds. As a result of a more
detailed examination of crustacean chitin, Meyer and Pankow (67)
confirmed earlier work (94) and reported a rhombic unit cell in which
eight 2-acetamido-2-deoxy-D-glucose residues were located, having the
dimensions (51, 62) a = 9.40 Α., Z? = 10.46Ä., and c = 19.25 Ä. Of
these h represents the fiber axis of the «-chitin chain. The acetamido
groups were shown to alternate from one side of the main chain to the
other in adjacent amino sugar residues (due to the ^-linkage); as in
103
complete within 24 hours. The mineralization may proceed by crystal
growth from generally distributed foci or by growth from some outer
layer or boundary. Kleinholtz (83, 84) points out that molting appears to
be under hormonal control whereas calcification does not. The recycling
of calcium deposits in cuticle during molting and the resorption of Ca
2+
has been intensively studied (85), and it is apparent that at least in part
these cations are conserved for the formation of new cuticle. The means
of calcium transport in the blood of arthropods is not certain, but it
seems likely that some type of protein complexes may be involved.
Using the technique of tilting infra spectra (86-88),
it has been
found that crab chitin crystallites and blow fly cuticle chitin show different uniplanar orientations.
In cell walls, unicellular bristles (Drosophila), and scales (Ephestia
sericarium), there is evidence of parallel alignment of protein and chitin
(poly-N-acetylglucosamine) in the direction of growth. From initial
development both bristles and scales inhibit strong birefringence (89).
This molecular arrangement conforms to the protein and poly-N-acetylglucosamine parallel interactions proposed by Rudall (12a, 90).
2. Macromolecular Structure of Chitin
Investigations of the relative intermolecular arrangements of chitin
chains represents one of the earliest applications of X-ray analysis to a
problem of biological interest. It must be emphasized, however, that the
degree of crystallinity necessary for such work is influenced by the
source and extent of "purification" of the chitin specimen, and that
alkali-treated "pure" chitin may exhibit considerably higher degrees of
hydrogen bonding and hence of intermolecular orientation than is normally present in vivo.
Following early reports by Herzog (91) and Gonell (92), on the
existence of chitin crystallites the general similarity of fibers of this material and cellulose became apparent. Meyer and Mark (93), in 1928,
suggested that the acetylglucosamine residues in chitin were linked and
bonded in the same fashion as the D-glucose residues in cellulose, assuming in each case β-Ό-(1 -> 4) glycosidic bonds. As a result of a more
detailed examination of crustacean chitin, Meyer and Pankow (67)
confirmed earlier work (94) and reported a rhombic unit cell in which
eight 2-acetamido-2-deoxy-D-glucose residues were located, having the
dimensions (51, 62) a = 9.40 Α., Z? = 10.46Ä., and c = 19.25 Ä. Of
these h represents the fiber axis of the «-chitin chain. The acetamido
groups were shown to alternate from one side of the main chain to the
other in adjacent amino sugar residues (due to the ^-linkage); as in