Chitin polymorphism

The Fourier-transform infrared (FT-IR) spectra of a-and p-chitm are shown in Fig. 2.15. For a-chitin, the amide I band is split at about 1650 and 1620 cm (Fig. 2.15A), whereas it is a single sharp band at about 1657 cm for 3-chitin (Fig. 2.15B). The amide II band appears at about 1555 and 1559 cm for a- and (3-chitin, respectively. Both polymorphs show strong absorption bands in the 3100-3285 cm region which corresponds to the N-H group. The bands in the 2840-2960 cm region are due to CH, CH2, and CH3 in both chitin polymorphs. The FT-IR vibrational modes of a- and (3-chitins are summarized in Table 2.8. 

Focher B., Naggi A., Torri G., Cosani A., Terbojevich M., Stmctural dififeiences between chitin polymorphs and their precipitates from solutions - Evidence from CP-MAS 13C-NMR, FT-IR and FT-Raman spectroscopy, Carbohyd. PolyrtL, 17,1992,97-102. 

Most spectroscopic and titration methods have been employed to determine these values and were listed by Rathke (36). The structural differences between chitin polymorphs have been studied by nmr and ir (37,38). Conductometric titrations have been confirmed by solid state nmr to be an accurate but easy method to determine the degree of deacetylation of chitosan (39). 

Chitin, polymorphic forms of, 631-633 Chloramphenicol, 473-474 Chlorophenoxyalkylcarboxylic acid herbicides, 781-783... 

In the present work, we extend the method to compensate for the hydrogen bonds present in carbohydrates. The hydroxylated character of carbohydrate polymers influences between-chain interactions through networks of hydrogen bonds that occur during crystallization. Frequently, several possible attractive interactions exist that lead to different packing arrangements, and several allomorphic crystalline forms have been observed for polysaccharides such as cellulose, chitin, mannan and amylose. The situation is even more complex when water or other guest molecules are present in the crystalline domains. Another complication is that polysaccharide polymorphism includes different helix shapes as well. 

Chitin is the 2-acetamido derivative of cellulose and serves as the fibrous component of skeletal tissues in many lower animals. At least two polymorphic forms of chitin have been recognized, (26) of which the a- and 0-forms are the best characterized. The unit cells and space groups of a- and 0-chitins are given in Table 1. Both have approximately the same fiber repeat as cellulose, and apparently have the same 2j helical conformation. 

Chitosan, a polymer of j8-(I 4)-linked 2-amino-2-deoxy-D-glucose residues, is formed on deacetylation of chitin. As pointed out already, this polysaccharide takes an extended conformation similar to that of cellulose. Deacetylation of chitin is very easily evaluated in view of the NMR spectra, as illustrated in Fig. 24.5. The three polymorphs of chitosan, ten-don-chitosan (from crab shell), L-2 (from shrimp shell), An-nealed (from crab shell chitosan annealed at 22°C in the presence of water) are easily distinguished, consistent with the data for the polymorphs as obtained by a powder X-ray diffraction data [38, 39]. The observed non-equivalence of two chitosan chains, as viewed from the splittings of the C-1 and C-... 

In this Section will be considered a proposal that polymorphism of chitins and cellulose results from differences in chain folding. To illustrate this hypothesis, RudalF used the two polymorphs of chitin, which he referred... 

Fig. 3.3 Three polymorphic configuration of chitin a a-Chitin, b j3-Chitin, c y-Chitin...

In the solid state, chitosan is a semicrystalline polymer. Its morphology has been investigated and many polymorphs are mentioned in the literature. Single crystals of chitosan were obtained using fully deacetylated chitin of low molecular weight. The dimensions the orthorhombic unit cell of the most common form were determined as a = 0,807 nm, h = 0,844 nm, c = 1,034 nm the unit cell contains two antiparallel chitosan chains, but no water molecules (Dash et al., 2011). 

Chitin is found in nature as highly crystalline ordered networks. Two major polymorphs have been firmly established for chitin in nature, namely a and (3-chitin. Each can readily be distinguished on the basis of its X-ray diffraction patterns [55], solid state C CP-MAS NMR [56], infrared [57] and Raman spectroscopy [58]. 

---