| Résumé | The extracellular fibers attached to the diatom Thalassiosirafluviatilis have been shown to be pure β-(1 → 4)-linked 2-acetamido-2-deoxy-D-glucan. This polysaccharide was given the systematic trivial name chitan to distinguish it from chitin, which is not a chemically distinct species and which has a radically different X-ray diffraction pattern and infrared spectrum. Proton magnetic resonance studies on the acid hydrolysis of chitan have established that the hydrolysis occurred in three distinct steps: (a) the degradation of the polysaccharide to smaller polymeric units, (b) the production of N-acetylglucosamine from the latter, and (c) the conversion of N-acetylglucosamine into glucosamine and acetic acid. X-ray and infrared studies have shown that chitan has a different macrostructure from that of arthropod chitin. Chitan is completely crystalline and can be converted irreversibly into a form indistinguishable from that of chitin by treatment with hot aqueous lithium thiocyanate. Boiling chitan in water converts it into a second crystalline modification, possibly a hydrate, which can be reconverted into the starting material by drying invacuo. The exceptionally sharp infrared spectrum of chitan allows a more unequivocal assignment of a number of bands common to chitan and chitin, and provides information about the nature of the bands at 1 626 and 1 656 cm−1 in the spectrum of chitin. The correlation of the bands in the region of 840 to 890 cm⁻¹ with the configuration at the anomeric center of glycopyranose derivatives is discussed. |
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