Chitin, polymorphic forms

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

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 is also found in different crystalline or polymorphic forms. The unit cell dimensions for these different forms are shown in Table 2. They are obtained by different processing conditions. Chitosan is not usually found completely deacety-lated hence, it has some of the characteristics of a random copol5uner. When the deacetylation of the chitin is less than 90%, the crystallization is hindered, and the chitosan has a lower degree of crystallinity than the original chitin. 

Chitin exists in three polymorphic forms, depending on the arrangements of the chains in the microfibrils a-, [)- or y-chltin, in which the molecules have antiparallel, parallel or mixed arrangements, respectively. The most common form is a-chitin [45], being found in shrimp, prawn, crab and lobster shells, while p-chltin is found in mol-lusks such as squids [86] y-chitin is found in the cell walls of fungi [118]. 

On the other hand, chitin obtained from the carapace is a popular adsorbent [11] or a starting material for the production of adsorbing media like the amino-acid substituted glucans [12], and is insoluble in water and does not form a gel in water. Being different from the structure of alginic acid, chitin is poly-y8-(l,4)-A -acetyl-D-glucosamine. Three polymorphic forms have been identified by X-ray methods. Among a-, j8- and y-chitin, or-chitin appears to be the most stable form and consists of a series of sheets of twofold chains [13]. 

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. 

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 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 a cationic amino polysaccharide composed of A-acetyl-o-glucosamine (GlcNAc, 2-acet-amido-2-deoxy-D-glucose 50%-100%) with 3 (1 4) glycosidic bonds between each monomer (Figure 2.1a Beaney et al. 2005). Chitin occurs in three polymorphic solid state forms designated as a, p, and y chitin which differ in their degree of hydration, size of unit cell, and number of chitin... 

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