Chitins

Chitin is a polysaccharide similar to cellulose except that the OH at C-2 is replaced by an acetamido group (CH3CONH). Chitin is the main component of the hard external covering (exoskeleton) of crustaceans such as lobsters, crabs, and shrimp. Like cellulose, the processing of chitin into polymeric products is limited by its insolubility and decomposition without melting. The availability in huge quantities has encouraged many attempts to find commercial applications of chitin, but very few have been found to be economically feasible. 

Chitin or poly(2-acetamido-2-deoxy-D-glucose) is a substance of considerable biological importance. From the structural point of view, it performs the same function in invertebrates as does cellulose in plants. 

It is found as a component of fungal and bacterial cell-walls, in insect cuticles, and as the shell of crustaceans. Being so similar to cellulose in chemical composition, its structure is important, if for no other reason than that comparison of the two structures might aid in our understanding of each. The similar fibrillar fine-structure (see Fig. 12) of these two polysaccharides is noteworthy, as the lateral forces between molecules are different. Although chitin does not occur in Nature specifically as a fiber, it is frequently found well-oriented in bristles and as tendon material. Samples from invertebrates are usually admixed with protein and carbonate, both of which must be removed before x-ray diagrams of high quality can be obtained. 

Like cellulose, chitin occurs in more than one crystal form. The j3-chitin modification, which contains one firmly bound molecule of water of hydration per 2-acetamido-2-deoxy-D-glucose residue, is usually found in association with animal tissue of the collagen type. a-Chitin, which is more common, usually replaces tissue of the collagen type this form has been examined more thoroughly than the p, and will be discussed in detail. A little studied derivative of chitin, called chitosan, can be obtained in crystalline, oriented form by deacetylating chitin membranes with concentrated sodium hydroxide. Naturally occurring, chitinous membranes, such as insect cuticle, show various degrees of uniplanar orientation. 

The most acceptable structure for a-chitin is the unit cell proposed by Carlstrom, which is shown in Fig. 13. It is orthorhombic, and the cell dimensions are listed in Table I (see p. 422). Some 62 separate reflections are present in the fiber diagram, permitting a relatively high degree of confidence in the final result. The fiber repeat observed is identical with that of cellulose, and, in the early studies, this led Meyer and Mark to postulate that chitobiose is the fiber repeat, before it was isolated as a 

Like cellulose, chitin is a polysaccharide for which the polarized-infrared spectrum has proved a valuable complement to x-ray data. For 

The structure of chitin is identical to that of cellulose, apart from the replacement of the OH group on the C-2 carbon of each of the glucose units with an -NHCOCHj 

Chitin is not only abundant as biomass resources but also as specialty biopolymers for preparing advanced functional materials. Since it is insoluble in common solvents, there are serious difficulties in modification reactions to prepare well-defined derivatives of chitin [168]. 

In 1995 the first in vitro synthesis of chitin was succeeded by a ring-opening polyaddition of a chitobiose oxazoline monomer, c.f.. 

The reaction is done at a pFl of 9-11. There the hydrolysis of the product chitin is much suppressed. The degree of polymerization of synthetic chitin is some 10-20 depending on the reaction conditions (50). Peracetyloxazolines can be obtained from peracetyl saccharides (51). A facile preparation method of chitin cellulose composite films has been described (52). Hereby ionic liquids are used, l-aUyl-3-methylimidazolium bromide and l-butyl-3-methyl-imidazoUum chloride. The former liquid dissolves chitin and the latter Uquid dissolves cellulose. 

solutions of chitin in l-aUyl-3-methylimidazolium bromide and cellulose in l-butyl-3-methylmudazolium chloride are separately prepared by heating each mixture up to 100°C for 24 h. Then, the homogeneous mixture of the two solutions is casted onto a glass plate, followed by standing at room temperature for 2 h. Eventually, the material is subjected to successive Soxhlet extraction first with ethanol for 12 h and then with water for 12 h. Finally, the residue is dried at room temperature to give the desired composite film (50). The thermal stability of the produced films are comparable to their constituents, chitin and cellulose (52). 

Adipic acid-diethylenetriamine copolymer, reaction product with epichlorohydrin. Wet strength resin (30) 

Octylphenolpoly(ethylenglycolether), non-ionic detergent (39) Novamax Georgia-Pacific Chemicals 

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

By treatment of chitin with alkali the acetyl groups can be removed and a polyglucosamine formed. This compound, chitosan, appears to be broken down with nitrous acid even under mild conditions, the sole product being a monosaccharide derivative. 

Three crystalline allomorphs have been described, a, p and y, with structures available only for the first two. These are based on X-ray and electron diffraction, so that hydrogen atoms are not located. 

The dihedral angles about the glycosidic bonds in a-chitin were the same as those in p-chitin, but in a-chitin there are two chitin chains in the unit cell. To account for an X-ray reflection which should not have been diffracted by crystals of the derived space group, it was proposed that the hydroxymethyl group was disordered. Subsequently, however, it was shown that this reflection was a double diffraction artefact. The detailed structure of a-chitin is therefore uncertain. 

The formation of D-glucose phenylosazone from chitosamine showed its relationship to D-glucose and D-mannose and Irvine and Hynd7 succeeded in preparing both D-mannose and D-glucose from chitosamine. 

Attempts to decide whether D-glucosamine is 2-amino-D-glucose or 2-amino-D-mannose have provided a series of fascinating investigations8-13 the balance of evidence therefrom favoring the D-glucose configuration. 

Final proof of the structure of D-glucosamine as 2-amino -D-gluco-pyranose (I) was furnished as indicated below independently by both chemical14, 16 and x-ray methods.1 By the action of ammonia on 2,3- 

Based on the fact that methyl 2,3-anhydro-a-D-mannopyranoside (II) is hydrolyzed17 by sodium methoxide to give, on subsequent methyla-tion, a mixture of two methyl trimethyl hexosides (methyl 3,4,6-trimethyl-a-D-altropyranoside and methyl 2,4,6-trimethyl-a-D-glucopyranoside) it was anticipated that the action of ammonia on II would follow essentially the same course. This was found to be the case, for with ammonia II yielded compounds which were converted to methyl 4,6-dimethyI-3-aeetamido-a-altropyranoside (III) and methyl 4,6-dimethyl-2-acetamido-a-D-glucopyranoside (IV) (10% yield). 

Thus it is proved that D-glucosamine is related configurationally to D-glucose. This same conclusion was reached independently by Cox and Jeffrey1 from x-ray analyses of a-n-glueosamine hydrochloride and hydrobromide. The isomorphism of these substances enabled direct synthetic methods17 to be employed without any previous stereochemical assumptions and the atomic positions were determined with high precision. 

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For a review of the isolation of chitin from natural sources and some of its uses see the November 1990 issue of the Journal of Chemical Education (pp 938-942)... 

The same questions about the safety of organic flocculants have been raised ia other countries. The most drastic response has occurred ia Japan (7,77) and Swit2edand (77) where the use of any synthetic polymers for drinking water treatment is not permitted. Alum and PAC are the principal chemicals used ia Japan (7). Chitin, a biopolymer derived from marine animals, has been used ia Japan (80,81). Maximum allowed polymer doses have been set ia Prance and Germany (77). 

Insect Growth Regulators. These compounds (40—45), unlike most conventional insecticides, interfere with biochemical processes that are unique to arthropods eg, molting, ecdysis, and formation of the chitinous exoskeleton. Therefore, they are selective insecticides with very low mammalian toxicity. 

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. 

Nikkomycins. The nikkomycins (141—159), isolated from S. tendae are nucleoside-peptide antibiotics (1,4,244,245) as shown in Table 8. Nikkomycins X and Z are stmcturaHy identical to neopolyoxins A and C, respectively. Compound (141) is a competitive inhibitor of chitin synthetase. Two new nikkomycins, nikkomycin pseudo-Z and pseudo-J (158, 159), contain a C-glycosidic bond between C-5 of uracil and C-1 of... 

It iaterferes with the synthesis of the hyphal walls, the biosynthesis of nucleic acids, and the synthesis of chitin. The iateraction with microtubules has also been described. The sensitivity of a cell seems to depend particularly on the abiUty to form griseofulvin—nucleic acid complexes. Further information concerning griseofulvin is available (21). 

Plant stmctural material is the polysaccharide cellulose, which is a linear P (1 — 4) linked polymer. Some stmctural polysaccharides iacorporate nitrogen iato thek molecular stmcture an example is chitin, the material which comprises the hard exoskeletons of kisects and cmstaceans. Chitki is a cellulose derivative whereki the OH at C-2 is replaced by an acetylated amino group (—NHCOCH ). Microbial polysaccharides, of which the capsular or extracellular (exopolysaccharides) are probably the most important class, show more diversity both ki monomer units and the nature of thek linkages. 

Last years the concern of the scientists and contributors to chitin, chitosan and chitincontaining connections has increased. It is connected to their widespread occurrence in the nature, paiticulai properties, and also feasibility in many areas of a national economy. The raw sources for obtaining chitincontaining of products are the testas of crabs, lobsters, shrimps, and also cabbage-weeds, funguses. 

Element stmctures of chitincontaining sorbents are determined using standard methods. Behind the data of an element stmcture the contents of chitin in ChCS was calculated. The analysis of morphological frame of ChCS was conducted by a electron-microscopic method on a raster supermicroscope at increase from 500 up to 1000 times. For matching is samples ChCS were conducted IR reseai ch in the field of 400 - 4000 cm f... 

Two hundred grams of eleaned and dried crab shells (Note 1) ground to a fine powder is placed in a 2-1. beaker, and an excess of dilute (approximately 6 N) commercial hydrochloric acid is added slowly to the powdered material until no further action is evident. Much frothing occurs during the addition of the acid, and care must be exercised to avoid loss of material due to foaming over the sides of the beaker. After the reaction has subsided, the reaction mixture is allowed to stand from 4 to 6 hours to ensure complete removal of calcium carbonate. The residue is then filtered, washed with water until neutral to litmus, and dried in an oven at 50-60°. The weight of dried chitin is usually about 70 g., but with some lots of crab shells it may be as low as 40 g. 

To 40 g. of dry chitin in a 500-ml. beaker is added 200 ml. of concentrated hydrochloric acid (c.p., sp. gr. 1.18), and the mixture is heated on a boiling water bath for 2.5 hours with continuous mechanical agitation. At the end of this time solution is complete, and 200 ml. of water and 4 g. of Norite are added. The beaker is transferred to a hot plate, and the solution is maintained at a temperature of about 60° and is stirred continuously during the process of decolorization. After an hour the solution is filtered through a layer of a filter aid such as Filter-Cel. The filtrate is usually a pale straw color however, if an excessive color persists, the decolorization may be repeated until the solution becomes almost colorless. The filtrate is concentrated under diminished pressure at 50° until the volume of the solution is 10-15 ml. The white crystals of glucosamine hydrochloride are... 

FIGURE 7.29 Like cellulose, chitin, man-nan, and poly(D-mannuronate) form extended ribbons and pack together efficiently, taking advantage of multiple hydrogen bonds. 

Thus, based on material applications, the following polymers are important natural rubber, coal, asphaltenes (bitumens), cellulose, chitin, starch, lignin, humus, shellac, amber, and certain proteins. Figure 4 shows the primary structures of some of the above polymers. For detailed information on their occurrence, conventional utilization, etc., refer to the references cited previously. 

A number of other polysaccharides, such as glycogen, dextran, chitin, etc., possess interesting structures for chemical modification [103,104]. Dextran has been used as a blood plasma substitute. Although it can be converted to films and fibers, chitin s relatively small resource restricts its commercialization. 

M. Poulicek, M. S. Voss-Foucart, and Ch. Jeuniaux, Chitin in Nature and Technology (M. Muzzarelli, Ch. Jeuniaux, and G. W. Gooday, eds.) Plenum Press, New York (1986). 

Chitosan, having a similar chemical backbone as cellulose, is a linear polymer composed of a partially deacety-lated material of chitin [(l-4)-2-acetamide-2-deoxy-/3-D-glucan]. Grafting copolymer chains onto chitosan can improve some properties of the resulting copolymers [48-50]. Yang et al. [16] reported the grafting reaction of chitosan using the Ce(IV) ion as an initiator, but no detailed mechanism of this initiation has been published so far. 

Chirality center, 292 detection of, 292-293 Eischer projections and, 975-978 R,S configuration of, 297-300 Chitin, structure of, 1002 Chloral hydrate, structure of, 707 Chloramphenicol, structure of, 304 Chlorine, reaction with alkanes, 91-92,335-338 reaction with alkenes, 215-218 reaction with alkynes, 262-263 reaction with aromatic compounds, 550 Chloro group, directing effect of, 567-568... 

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