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Chitosan modification : toward the rational tailoring of properties

University of British Columbia

The main objective of this work was to develop a method whereby chitosan could be modified to give synthetic analogues of branched polysaccharides. To this end, a variety of allyl glycosides were prepared (99-102, 105, 108,109 and 111) and reductively ozonolyzed, to give the acetaldehydo glycosides 112-119. These aldehydes were then reductively alkylated to chitosan (1), to give branched chitosan derivatives (120-127) of the general structure 157. Pendant residues of α and β-glucopyranose, α and β-D-galactopyranose, 2-acetamido-2-deoxy-D-α and β-glucopyranose, glucopyranuronic acid and β-D-lactose were incorporated by this method, at various levels of substitution. Rheological evaluations of these derivatives by steady-shear viscometry demonstrated a relationship between the degree of substitution and rheological properties, as well as the effect of branch size and functionality on aqueous solution properties. Importantly, many of the trends seen in this study are similar to established explanations for the aqueous solution properties of seed galactomannans. It was also shown that intrinsic viscosities of the derivatives were supportive of observations based on concentrated solution properties. Also, it was demonstrated that these water soluble chitosan derivatives interacted, sometimes in a synergistic manner, with xanthan gum solutions. A similar route, involving the synthesis of 10-undece- nyl-β-D-glycopyranosides (134-136), reductive ozonolysis and reductive amination to chitosan, provided combined hydrophobic/hydrophilic branched chitosan derivatives (140-142) of the general structure 158. This methodology was demonstrated with the 10-undecenyl β-D-glycosides of glucopyranose, gal-actopyranose and lactose. Compounds 140a and 141a. bearing glucose and galactose pendant residues, showed uncommon thermally induced gelation properties in dilute aqueous acid solution. This property was studied by ¹H-nmr relaxation measurements and ¹³C-nmr spectroscopy. It was found that a high degree of substitution was necessary for gel formation, and that the pendant sugar was required, but excess hydrophilicity (such as the disaccharide branch, lactose) precluded gelation. In addition, a derivative (151) was prepared, which contained a metal-chelating moiety and a hydrophilic spacer group. This compound had substantial copper (II) binding capacity, and useful ion-exchange ability. Finally, a chitosan derivative (156) was synthesized, bearing a pendant 1-thio-β-D-glucopyranose moiety, and was shown to be useful for the affinity chromatographic purification of the enzyme β-glucosidase. [Formula Omitted]

Text

2010-08-13

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http://hdl.handle.net/2429/27319

Chemistry

University of British Columbia

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