Study of Chemically Modified Non-Starch Polysaccharides

This dissertation, "Study of Chemically Modified Non-starch Polysaccharides" by 袁詩雅, Sze-nga, Yuen, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled STUDY OF CHEMICALLY MODIFIED NON-STARCH POLYSACCHARIDES Submitted by Yuen Sze Nga for the degree of Doctor of Philosophy at The University of Hong Kong in January 2008 Five types of non-starch polysaccharides, namely alginic acid, α-cellulose, guar gum, locust bean gum (LBG) and xanthan gum, were chemically modified by three popular reactions, carboxymethylation, sulfation and acetylation. New analytical methods, based on Raman and Fourier-transform infrared (FTIR) spectroscopy, were developed for determining the degree of substitution (DS) to replace traditional wet chemistry procedures which have many drawbacks. Structural and physical properties of the native and modified polysaccharides were also studied by these spectroscopies, and other techniques including differential scanning calorimetry (DSC), laser light scattering, rheometry, and circular dichroism and nuclear magnetic resonance spectroscopy. Specific marker bands in the Raman and FTIR spectra were selected. A C=O carbonyl -1 -1 -1 stretching vibration at 1607 cm (Raman) and 1315 cm or 1605 cm (IR) were -1 observed after carboxymethylation. Sulfation led to new bands at 980 cm (Raman) and -1 619 cm (IR), attributed to the C-O-S stretching and SO bending vibrations 3 -1 respectively. A C=O carbonyl stretching vibration at 1720 cm (Raman) and a C-O -1 stretching vibration at 1247 cm (IR) were found upon acetylation. Calibration curves were constructed by plotting the intensity ratios of these marker bands relative to an -1 internal standard (vibration of skeletal configuration and linkage at 850-950 cm ) against DS (determined by wet chemistry methods). All calibration curves exhibited a linear fit with significant linear correlation (r>0.96, PThe selected modifications exerted different influences on the structural and physical properties of the polysaccharides. Carboxymethylation decreased the amount of aggregates and changed the polysaccharide conformation, with expanded and stiffer structure in galactomannans, and more flexible coiled conformation in xanthan. Viscosity and viscoelasticity in carboxymethylated derivatives were decreased by extensive breakdown of chemical forces or intermolecular associations. Sulfation also decreased aggregation. After sulfation, molar mass was increased and the aggregates became more extended and stiff. Increases in viscosity and viscoelasticity were observed in sulfated LBG and xanthan. DSC data show that carboxymethylation and sulfation led to increases in water holding capacity (WHC), and the substitute groups were decomposed at temperatures above 300 C. In contrast, acetylation decreased WHC of polysaccharides. Simultaneous thermal decomposition of the backbone chain and acetate groups, and enhanced intermolecular interaction were also observed. The present study illustrates that Raman and FTIR spectroscopy can be used as both quantitative (measuring DS) and qualitative (structural determination) techniques for different polysaccharide derivatives. Together with other complementary techniques, the effects of chemical modifications on the co