Chitosan - Chemical Modification

Among the many chemical derivatives of chitosan mentioned in literature [38], one can differentiate the specific reaction involving the -NH group at the C-2 position, or the non-specific reactions of -OH groups at C-3 and C-6 positions (especially esterification and etherification). The more common and easier reactions involving the amino group at C-2 position are the quaternization and the reductive amination with aldehydes [39]. 

This method results in regioselective carboxymethylation of the amino group, so the product of reaction is a well-defined derivative. Several N-carboxyalkylated chitosans were prepared via Schiff base formation from carboxylic acids having aldehyde or keto groups [47-48]. The resulting 

O-carboxymethy Ichitosan is also used to develop a water-soluble matrix polymer for controlled drug release. OCM-chitosan microspheres containing antibiotic drug pazufloxacin mesilate were prepared by the emulsion method and successively crosslinked with glutaraldehyde [52]. 

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Thus, our results demonstrated that the trends of selectivity for hydrogenation of 1,4-butenediol and cyclopentadiene are in tight connection with preliminary chitosan chemical modification as well as mineral support nature. Pd/chitosan modified with 2-pyridinealdehyde deposited on SiOa demonstrated high selectivity in hydrogenation of cyclopentadiene into cyclopentene. 1,4-butynediol into cis-l,4-butenediol hydrogenation proceeded very selectively over Pd-Pb catalytic systems based on chitosan succinate form. 

Chemical modifications of chitosan assayed to enhance cell specificity and transfection efficiency were reviewed. Also, chemical modifications of chitosan were performed to increase the stabihty of chitosan/DNA complexes [95]. 

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. 

H. Sashiwa, Y. Shigemasa, and R. Roy, Chemical modification of chitosan. 10.1. Synthesis of dendronized chitosan-sialic acid acid hybrid using convergent grafting of preassembled dendrons built on gallic acid and tri(ethylene glycol) backbone, Macromolecules, 34 (2001) 3905-3909. 

Kim T-H, Jiang H-L, Jere D et al (2007) Chemical modification of chitosan as a gene carrier in vitro and in vivo. Prog Polym Sci 32(7) 726-753... 

Novel chitosan based catalytic systems for hydrogenation of unsaturated compounds in the liquid phase were prepared. The catalytic performance of the obtained systems depended significantly on the chitosan forms (as the micro beads or chitosan deposited on the mineral supports), their preparation method and chemical modification of chitosan as well. The obtained chitosan based carriers and catalysts were examined by transmission and diffuse-reflectance FTIR spectroscopy. 

Chemical modification of the obtained chitosan micro beads was carried out by treatment with 2-pyridinealdehyde boiling solution in benzene for 24 h. The resulting chitosan forms were rinsed with benzene, THF and MeOH. 

Deposition of chitosan on SiOz (0.06-0.02 mm) and ZrOz was performed by multiple steeping of a portion of carrier in a solution of chitosan in 1% acetic acid and filtered. Wet or semi-dry chitosan-coated carrier was added directly to methanol or propanol-2 and stirred for 0.5-2h after addition of the calculated amount of glutaraldehyde. Calculated cross-linking extent was 10-15%. Chemical modification of chitosan was performed by carrier treatment with pyridinealdehyde-2 boiling solution in benzene. 

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. 

Chemical modification of the enzyme and its immobilization may cause either increase or decrease of thermostability [12-14] depending on the enz)mie, nature of the carrier, and the method of immobilization. As the data Irom Nagamoto et al. [15] indicates, thermostability of 3-glucosidase immobilized on chitosan increases, and the enzyme works successfully at 70 °C during 12 h. Such considerable increase of thermostability that we observed is due to the covalent binding of the enzyme and the carrier. 

Table 3.4 Chemical modification of chitosan for improved reactivity.

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

However, low solubility, non-specificity, and low transfection efficiency of chitosan limited its clinical trials. Hence, chemical modification of chitosan became necessary to overcome its drawbacks for the clinical trials. 

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