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UBC Theses and Dissertations

Selective chemical modification of polysaccharides Yalpani, Mansur


Various synthetic procedures were designed for selectively transforming carbohydrate polymers into versatile products with a wide range of applications using as substrates several abundant and industrially important polysaccharides (alginate, cellulose, chitin, chitosan, guaran, locust bean gum, xanthan gum). Thus, alkyl amide, amine, ester, and hydrazine derivatives of alginic acid as well as similar derivatives of cellulose and xanthan gum were prepared. Selective modification of chitosan afforded N-aryl derivatives which were found to be highly efficient metal-chelating agents. Attachment of carbohydrate moieties to the amino groups of chitosan using reductive alkylation yielded a new class of branched, comb-like, 1-deoxyglucit-1-yl derivatives which exhibited a great diversity of useful properties in terms of solubility, gel-formation, compatibility, and interaction with other polysaccharides. The synthetic principles of this procedure were found to be amenable for adaption to other polysaccharide systems and branching types, as exemplified by a guaran derivative with chain-extended trisaccharide branches. Reductive alkylation of chitosan using ferrocenylaldehyde and sodium cyanoborohydride produced a new type of organometallic polysaccharide derivative. Combined enzymic and chemical modifications were found to be high-yielding for guaran and locust bean gum: specific oxidations using galactose oxidase afforded versatile C6' aldehyde intermediates which were reductively aminated to produce a wide variety of useful derivatives including synthetic glycoproteins and glycopeptide analogues. Various spectroscopic and other instrumental techniques were employed for the characterization of the polysaccharide derivatives and some native polymers in terms of their primary structure, three dimensional shape, and surface structure, as well as the molecular mobility of the hydrated polymer chains. Thus, nitroxide spin-labelling was utilized to monitor the course of many of the modification reactions, and provided evidence for a heterogeneous galactosyl-branch distribution for guaran and locust bean gum which was in agreement with a recently proposed structural model for these polymers. Esr experiments suggested heterogeneities in surface structure for chitosan and cellulose. The structural elucidation of several branched-chain chitosan deriva-tives and of guaran and locust bean gum was accomplished using ¹³C-nmr spectroscopy. Electron microscopy (SEM) of xerogels derived from branched-chain chitosan derivatives revealed a wide variety of ultra-structures ranging from smooth, non-porous to microporous.

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