PAS-POSITIVE REACTION OF CHITIN

The PAS-positive reaction of chitin requires periodic oxidation of hydroxyacetyl-amino groups of N-acetyl-glucosamine units. Periodic oxidation of N-acyl-hydroxyamino acids and of different sugars shows that hydroxy-acetyl-amino groups are not clived under PAS conditions. Thus, chitin, considered as poly-N-acetylglucosamine, is PAS-negative. The PAS reaction of different chitinous structures (especially insect cuticles) is studied. When it is not linked to protein or only weakly, chitin is PAS-positive. Acetylation and deacetylation tests and oxidation with sodium bismuthate suggest that the positive reaction is due to hydroxy-amino groups of glucosamine residues included in the poly-N-acetylglucosamine chain. In stabilized insect cuticles, chitin is linked to form a glycoprotein and reacting groups are masked: PAS reaction is negative. Generally, hot dilute alkali is used to remove the protein fraction. But the Tables 1 and 2 show that chitin undergoes deacetylation and becomes PAS-positive if purification by alcali is too long or concentration too high. It appears that KOH N 24 h at 100° C does not cause deacetylation, and removes enough protein to unmask reacting groups. After such treatment, hard sclerotized cuticles remain PAS-negative, and endocuticles become PAS-positive. The groups responsible for PAS reaction are identified as hydroxy-amino groups by complementary tests. These results confirm that chitin may contain glucosamine, and suggest that during the stabilization process the hydroxy-amino groups are blocked by strong protein or water linkage, or simply acetylated. These two hypothesis are discussed. © 1969 Springer-Verlag.