Glucans, carbon-13 chemical shifts

Our original approach to polysaccharide C-13 n.m.r. spectral analysis consisted of making a minimum number of hypotheses about expected structure-to-spectra relationships (8). By then comparing spectra to known structure for a series of D-glucans, we attempted to establish the validity of these hypotheses and to establish how diverse a structural difference could be accommodated The hypotheses were as follows. Firstly, that each polymer could be considered as an assembly of independent saccharide monomers. Secondly, that these hypothetical saccharide monomers would be 0 alkylated (0 -methylated) in the same positions as the actual saccharide linked residues (it had previously been established that 0-methylation of any a-D-glucopyranosyl carbon atom position resulted in a down-field displacement of vlO p.p.m. for the associated resonance). Thirdly, that each differently substituted residue would have a completely different set of chemical shift values for each carbon atom position (different from the unsubstituted saccharide) but that only the carbon atom positions involved in inter-saccharide linkages would have A6 greater that 1 p.p.m. And, fourthly, that the hypothetical 0-alkylated residues would contribute resonances to the total spectrum proportional to their mole ratio in the polymers. 

The C chemical shifts of C-1 and C-3 carbons of the resilient gel are clearly shifted downfield by 2.8 and 3.2 ppm, respectively, compared with those of a lower molecular weight fraction (DPn=13). Such substantial downfield displacements are limited to the carbons in the glucosidic linkages of (l- 3)-3-D-glucans. 

Figure 6 shows the CPMAS NMR spectrum of curdlan powder, taken by courtesy of Professor Gary Maciel and Dr. Victor Bartuska of Colorado State University. Interestingly, the most mobile C-6 carbon gives rise to the least intense signal in the solid state because of insufficient CP transfer between proton and carbon. It was again found that the C chemical shifts of C-1 and C-3 are displaced downfield appreciably in the solid state compared with the peak-positions of liquid state D-glucan, as shown in Table I. 

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