Chitin crystallinity

Figure 14.1 Chemical structure of chitin and schematic images of chitin-protein matrix and chitin crystalline structures.

Liquid crystalline behavior occurs in the exocuticle of certain classes of beetles. The bright iridescent colors that are reflected from the surface of Scarabaeid beetles originates from a petrified chiral nematic stmctural arrangement of chitin crystaUites in the exocuticle (38). It is suggested that this chiral nematic texture forms spontaneously in a mobile, Hquid crystal phase that is present during the initial stages of the exocuticle growth cycle. 

The solubility of chitin is remarkably poorer than that of cellulose, because of the high crystallinity of chitin, supported by hydrogen bonds mainly through the acetamido group. Dimethylacetamide containing 5-9% liCl (DMAc/IiCl), and N-methyl-2-pyrrohdinone/LiCl are systems where chitin can be dissolved up to 5%. The main chain of chitin is rigid at room temperature, so that mesomorphic properties may be expected at a sufficiently high concentration [67,68]. 

Poly(HASCL) depolymerases are able to bind to poly(3HB)-granules. This ability is specific because poly(3HB) depolymerases do not bind to chitin or to (crystalline) cellulose [56,57]. The poly(3HB)-binding ability is lost in truncated proteins which lack the C-terminal domain of about 60 amino acids, and these modified enzymes do not hydrolyze poly(3HB). However, the catalytic domain is unaffected since the activity with water-soluble oligomers of 3-hy-droxybutyrate or with artificial water-soluble substrates such as p-nitrophenyl-esters is unaffected [55, 56, 58, 59]. Obviously, the C-terminal domain of poly(3HB) depolymerases is responsible and sufficient for poly(3HB)-binding [poly(3HB)-binding domain]. These results are in agreement ... 

This conclusion10 has been reached mainly from crystallographic studies on chitin and on acetylchitobiose obtained from chitin by acetoly-sis.107 The chitobiose was isolated in the form of its crystalline octa-acetate which could also be obtained108 by acetylation of the hydrolyzate produced by the action of fuming hydrochloric acid on chitin. A hen-decaaeetylchitotriose also was obtained by this procedure. 

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. 

Highly crystalline 0-chitin is obtained from pogonophore tubes and the spines of certain diatoms, and is analogous in both crystallinity and morphology to the cellulose obtained from Valonia cell walls. Intensity data (10) were obtained for 61 observed non-meridional reflections for a specimen of dispersed (sonicated) crystallites of pogonophore tube (Oligobrachia... 

Models for the crystal structures of the many crystalline forms of amylose [516], mannan [517], chitin [515] and many of the bacterial polysaccharides [518] have been proposed. All included the inter-residue 0(3 )H 0(5) intrachain hydrogen bond when stereochemically feasible. For mannan, a three-center bond has been proposed on the basis of infrared measurements [519]. Intrachain interresidue hydrogen bonds also feature in the structures proposed for the bacterial polysaccharides whenever stereochemically possible. 

Tablets are still considered as the dosage forms of choice for reasons such as low manufacturing cost and good stability. Many direct compression dilutents have been reported,but every dilutent has some disadvantages [295]. Crystalline cellulose (MCC) has been widely used as a tablet dilutent in Japan. Chitin and chitosan, on account of their versatility, were reported to be useful dilutents for pharmaceutical preparations [296-298].

From their investigations, it appears that the fluidity of combined powders with chitin and chitosan was greater that that of the powder with crystalline cellulose. The reported hardness of the tablets follows the order chitosan tablets >MCC> chitin. In the disintegration studies, tablets containing less than 70 % chitin or chitosan have passed the test. Moreover, the ejection force of the tablets of lactose/chitin and lactose/chitosan was significantly smaller than that of lactose/MCC tablets [301]. However, no reports are available on CDR formulations using these studies. 

Crystalline, flake chitosan from King crab exhibited patterns similar to those of chitin, but the 002 peak (b is the fiber axis) shifted from 0.962 nm for chitin to 0.857 nm for flake chitosan. The position of this peak depends on the content of water and is lowered to 0.745 nm on drying at 134°. It was postulated that water molecules that enter the lattice are loosely bound between the chains along the 001 direction. 

The solubility of magnesium silicate in different media is shown in Table 2.4. The dissolution mechanism of ot-chitin in N,N-dimefhylacetamide (DMAc)/ 5% LiCl can be aftribufed to the formation of a weak complex between Li ions and the carbonyl oxygens of the DMAc, which solvates the polyelectrolyte formed between the CD ions and labile proton groups (OH and NHCOCH3) of the chitin chain, disrupting the extensive intra- and intermolecular hydrogen bonds of the crystalline sheet structure of a-chitin. 

Comparing the solubility behavior of a- and p-chitins (although the later exists in a crystalline-hydrated structure, which is much looser than that of the ot-chitin), p-chitin shows lower solubility due to the penetration of wafer between the chains of the lattice. Based upon chitin molecule-solvent conformation and solubility mechanisms, p-chitin starts gelling at a lower concentration than a-chitin. Table 2.6 illustrates the solubility of chifin and structurally related compounds in a saturated CaCl2 2H20-methanol solvent system. 

---