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The structure of β-chitin

1961; Elsevier BV; Volume: 51; Issue: 2 Linguagem: Inglês

10.1016/0006-3002(61)90169-x

1878-2434

N. E. Dweltz,

Polysaccharides Composition and Applications

The structure of β-chitin is worked out on the basis that the same sugar occurs in both α-chitin and β-chitin, as is indicated by the conversion of β-chitin to α-chitin by treatment with formic acid or fuming nitric acid. The b-repeat in β-chitin, unlike in α-chitin, is variable. The unit cell for the dry specimen from the "pen" of the cuttlefish "Loligo" is found to be monoclinic (space group P21) with a = 4.7 Å, b = 10.5 Å, c (fibre repeat) = 10.3 Å and the monoclinic angle γ is found to be close to 90°, so that the cell is rectangular. This unit cell contains only one polysaccharide chain with two N-acetylglucosamine residues. The chains are separated by a distance of 4.7 Å perpendicular to the "plane" of the sugar rings and the NH and CO groups of neighbouring amino-acetyl side-chains are hydrogen bonded along the direction of the a-axis, as they are in α-chitin. Along the b-axis direction, however, the short hydroxyl group of one chain is linked by a firmly-bound water molecule to the longer hydroxymethyl group of the adjacent chain. Thus, β-chitin must be considered to be a monohydrate in the dry state with the chemical formula [C8H13O5N·H2O]n. Because the water molecule occurs between two chains, more water can enter there with increasing humidity, separating the chains. In α-chitin, the side chains are directly linked and the difference in the hydrogen bonding along the b-direction explains the different behaviours of the two modifications with water. The calculated density for the hydrate is in fairly good agreement with the observed density, and the calculated intensities of X-ray reflections are also in good agreement with the observations made. The similarity between the infrared absorptions of α- and β-chitin is also explained. The structure is also stereochemically quite satisfactory. After this work was completed, chemical evidence was obtained that the sugar in β-chitin is the same as that in α-chitin, namely glucosamine.