Chitin - Chemical Modification

Chemical modifications of chitin are generally difficult owing to the lack of solubility because reactions under heterogeneous conditions are accompanied by various problems such as the poor extent of reaction, difficulty in region-selective substitution, structural ambiguity of products and partial degradation due to the severe reaction conditions. Carboxymethyl chitin is one of the most studied chitin derivatives, obtained by adding monochloro-acetic acid to chitin previously treated with sodium hydroxide at different 

0-/N- Carboxyalkylation Acylation Sulfation Schiff s base Enzymatic substitution Metai cheiation Cyanoethyiation Nitration, phosphorylation 

Chitinase/chitosanase Non-specific enzymes (Lipase, protease, lysozyme, carbohydrases) 

The most important chitin derivative is chitosan, obtained by its partial deacetylation under alkaline conditions or by enzymatic hydrolysis in the presence of chitin deacetylase. Because of the semicrystalline morphology of chitin, chitosan obtained by a solid-state reaction has a heterogeneous distribution of acetyl groups along the chain. On the other hand when chitin is treated with concentrated aqueous sodium hydroxide, N-deacetylation proceeds smoothly and homogeneously deacetylated samples are obtained [37]. 

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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. 

Cross-linking agents have been proposed for the improvement of chitin fibres in the wet state. Epichlorohydrin is a convenient base-catalysed crosslinker to be used in 0.067 M NaOH (pH 10) at 40 °C. The wet strength of the fibres was considerably improved, whereas cross-hnking had neghgible effect on the dry fibre properties. Of course, the more extended the chemical modification, the more unpredictable the biochemical characteristics and effects in vivo. Every modified chitin or modified chitosan fibre should be studied in terms of biocompatibiUty, biodegradabiUty and overall effects on the wounded tissues. 

Kurita, K. 1986. Chemical modification of chitin and chitosan. In Chitin in Nature and Technology (R.A.A. Muzzarelli, C. Jeuniaux, and G.W. Gooday, eds), pp. 287-293. Plenum Press, New York. 

The chemical modification of chitin and chitosan is summarized in Table 6.1. 

Gopalan, N.K., Dufresne, A., Gandini, A., Belgacem, M.N. Crab shell chitin whisker reinforced natural rubber nanocomposites. 3. Effect of chemical modification of chitin whiskers. Biomacromolecules 4, 1835-1842 (2003)... 

Nature already produces the desired structures, and isolation of these components mostly requires only physical methods without chemical modification. Examples comprise polysaccharides (cellulose, starch, alginate, pectin, agar, chitin, and inuUn), disaccharides (sucrose and lactose), and triglycerides, lecithin, natural rubber, gelatin, flavors and fragrances, etc. 

Chitin is undoubtedly the most abundant animal polysaccharide on earth. It constitutes the basic element of the exo-skeleton of insects and crustaceans, but it is also found in the outer skin of fungi. Chitin is a regular linear polymer whose structure differs from that of cellulose by the presence of Al-methylamide moieties instead of the hydroxyl groups at C2 (Fig. 1.17). Given the susceptibility of this function to hydrolysis, chitin often bears a small fraction of monomer units in the form of primary amino groups resulting from that chemical modification. 

In order to improve the properties of these unique polysaccharides and to develop new advanced materials, much attention has been paid to their chemical modification. These polymers have two reactive groups suitable for this purpose, namely, primary (C6) and secondary (C3) hydroxyl groups in the case of chitin whereas chitosan has additionally the amino (C2) group on each deacetylated unit. All these functions are susceptible to a variety of classical reactions which can be applied here in a controlled fashion to obtain a vast array of novel materials based on the two polysaccharides which can also be modified by either crosslinking or graft copolymerization. This topic has been extensively studied and thoroughly documented [5-7]. 

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