108
P, W. KENT
envisaged that in situ the hydrogen bonding takes places between polysaccharide chains and proteins, in the first instance (68).
Comparison of chitins of diverse biological origins by X-ray methods
indicates a wide degree of similarity (98, 99, 103a), e.g., Phycomyces
bhkesleeanus, sinews of Palinurus vulgaris and Periplaneta. In another
study (104a), chitins from crayfish, Aspergillus niger, Psalliota campestris, and ArmiUaria mellea proved to be essentially similar.
B. CHEMICAL STRUCTURE OF CHITIN
Detailed chemical investigation of the structure of chitin has been
beset throughout by the insoluble and inert nature of the substance. It
is an amorphous solid, insoluble in water, dilute acids, or alkalis. In hot
concentrated alkali, chitin undergoes partial deacetylation and degradation, giving a mixture of substances collectively known as chitosan
[recently reviewed by Foster and Webber (11)]. As stated earlier,
sources of chitin frequently include variable amounts of inorganic salts
(CaCO s and calcium phosphate) and proteins. The former have been
most commonly removed by dilute acids or more recently by milder
procedures using ethylenediaminetetraacetic acid (105). In some cases
protein may be largely removable from soft cuticle by aqueous extraction. With hard cuticle, quantitative deproteinization is seldom if ever
achieved even when the chitin sample is taken into solution with lithium
thiocyanate. Chitin can however be dissolved also in concentrated hydrochloric and sulfuric acids, in formic acid, and in phosphoric acid, with
some degree of degradation. The material is reported not to dissolve in
Schweitzer's solution, and only with difficulty in liquid ammonia. It is,
however, known to dissolve in aqueous solutions of some neutral salts in
particular lithium and calcium thiocyanates. It is uncertain whether in
the presence of oxygen and in alkaline conditions the reducing end of a
poly-N-acetylglucosamine chain undergoes oxidative changes similar to
those reported for cellulose, but such a possibility cannot be discounted.
At least two observations (62, 103) indicate that chitin fibers regenerated from solutions have recognizable X-ray diffraction patterns of
«-chitin. Examination of solutions of purifed crab and fungal chitins in
nitric acid have given values (106) of 143 and 141 for the viscosity constant (k χ 10
3 ).
Difficulties in the isolation of pure chitin are reflected also in the
diversity of nitrogen contents reported for different chitin specimens.
The theoretical nitrogen content (6.89%) is seldom obtained, commonly
found values lying in the range 6.45-6.7% for what would be considered
good specimens. The nature of the impurities is not known, but it must
P, W. KENT
envisaged that in situ the hydrogen bonding takes places between polysaccharide chains and proteins, in the first instance (68).
Comparison of chitins of diverse biological origins by X-ray methods
indicates a wide degree of similarity (98, 99, 103a), e.g., Phycomyces
bhkesleeanus, sinews of Palinurus vulgaris and Periplaneta. In another
study (104a), chitins from crayfish, Aspergillus niger, Psalliota campestris, and ArmiUaria mellea proved to be essentially similar.
B. CHEMICAL STRUCTURE OF CHITIN
Detailed chemical investigation of the structure of chitin has been
beset throughout by the insoluble and inert nature of the substance. It
is an amorphous solid, insoluble in water, dilute acids, or alkalis. In hot
concentrated alkali, chitin undergoes partial deacetylation and degradation, giving a mixture of substances collectively known as chitosan
[recently reviewed by Foster and Webber (11)]. As stated earlier,
sources of chitin frequently include variable amounts of inorganic salts
(CaCO s and calcium phosphate) and proteins. The former have been
most commonly removed by dilute acids or more recently by milder
procedures using ethylenediaminetetraacetic acid (105). In some cases
protein may be largely removable from soft cuticle by aqueous extraction. With hard cuticle, quantitative deproteinization is seldom if ever
achieved even when the chitin sample is taken into solution with lithium
thiocyanate. Chitin can however be dissolved also in concentrated hydrochloric and sulfuric acids, in formic acid, and in phosphoric acid, with
some degree of degradation. The material is reported not to dissolve in
Schweitzer's solution, and only with difficulty in liquid ammonia. It is,
however, known to dissolve in aqueous solutions of some neutral salts in
particular lithium and calcium thiocyanates. It is uncertain whether in
the presence of oxygen and in alkaline conditions the reducing end of a
poly-N-acetylglucosamine chain undergoes oxidative changes similar to
those reported for cellulose, but such a possibility cannot be discounted.
At least two observations (62, 103) indicate that chitin fibers regenerated from solutions have recognizable X-ray diffraction patterns of
«-chitin. Examination of solutions of purifed crab and fungal chitins in
nitric acid have given values (106) of 143 and 141 for the viscosity constant (k χ 10
3 ).
Difficulties in the isolation of pure chitin are reflected also in the
diversity of nitrogen contents reported for different chitin specimens.
The theoretical nitrogen content (6.89%) is seldom obtained, commonly
found values lying in the range 6.45-6.7% for what would be considered
good specimens. The nature of the impurities is not known, but it must