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NMR studies of carbohydrates in the solid state

University of British Columbia

The work described herein entails the application of solid-state n.m.r. methods to both mono- and polysaccharide carbohydrates. Two major n.m.r. methods have been evaluated for studying these substances, namely, the carbon-13 cross-polarization-magic angle spinning method and the deuterium quadrupole echo method. Heptakis-(2,6-di-O-methyl)- β-cyclodextrin and β-cyclodextrin peracetate were prepared and, together with α-and β-cyclodextrins as hosts, were used to encage a variety of organic guest molecules. A number of metal sugar conjugates were prepared according to the procedures documented in this laboratory. Firstly, chelate coordination complexes were synthesized by forming salicylaldimine ligands derived from combinations of amino sugars (methyl 3,4,6-tri-O-acetyl-2-amino-2-deoxy-β-D-glucopyranoside or 1 , 3 , 4 , 6-tetra-O-acetyl-2-amino-2-deoxy-β-D-glucopyranose) and salicylaldehyde, with subsequent complexation of these to copper(II), zinc(ll), and cobalt(Il) ions. Similarly, chitosan Schiff's bases were prepared and complexed to copper(II) ion. Application of the ¹³C-cross-polarization-magic angle spinning method in the spectral assignment of cyclodextrin inclusion complexes and metal sugar conjugates in the solid state has been demonstrated. In all cases, recourse to solution spectra was necessary for correct assignments of individual carbon resonances. The sugar resonances of cyclodextrins are relatively broad and are complicated by the substantial splittings. The rationales for such observations were drawn from comparative studies, based on closely related guest molecules. The smaller guest molecules were least affected by the cavity size of the host molecule heptakis-( 2 , 6-di-O-methyl) -β-cyclodextr in and the isotropic chemical shift values were similar to those measured for solutions. These values suggest that the guest molecules undergo anisotropic motion, which can be detected by the use of the dipolar dephasing technique. Studies of metal sugar conjugates by ¹³C n.m.r. revealed that paramagnetic ions have profound effects on the resolution obtained. The high concentrations of these ions result in broad featureless spectra. On the other hand, the diamagnetic ions proved to have little effect on the isotropic chemical shifts of the sugar chelates. The deuterium quadrupole echo method has been used specifically to study the mobility of the guest molecules encaged in cyclodextrins. The "powder-type" spectra obtained were then compared with results reported in the literature in order to define the types of molecular motion.

Text

2010-08-06

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http://hdl.handle.net/2429/27129

Chemistry

University of British Columbia

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