Chemical shifts of polysaccharides

Carbon-13 Chemical-Shifts of Polysaccharides, Disaccharides, and Relevant Monomers"... 

The literature indicates that the proton NMR spectra of polysaccharides display severe overlap of signals in the chemical shift region of 3.5-5.5 ppm and it is difficult to assign them using ID H NMR spectrum alone [90,127]. This difficulty can be overcome by the fact that polysaccharides display a wide range of chemical shifts when compared to their H shifts. Carbon chemical shifts of polysaccharides appear in the range of 60-110 ppm. 

Experience has indicated that the chemical shifts of the monosaccharides are similar to those of the monosaccharide residues within the polysaccharide, except for substituent effects.37,38 These effects, caused by the attachment of any substituent to a sugar moiety, cause an increase in the chemical shift of the carbon atom directly involved in the linkage this increase is usually accompanied by a decrease of smaller magnitude (or, sometimes, an increase) in the chemical shifts... 

An extensive NMR study of various 1 4-linked disaccharides with D- and L- monosaccharides mixed in disaccharides has been reported [66]. Each group of disaccharides exhibits specific changes in chemical shifts due to glycosidations of the C-4 hydroxy group, which are different between the groups. The differences in chemical shifts were correlated to the preferred conformations of the disaccharides and used to predict the and NMR chemical shifts of 1 4-linked oligo- and polysaccharides. 

Table 9.3 NMR chemical shifts of the polysaccharide from S. gordonii 38 in D2O at 25°C.

Chemical Shifts of C-1 Signals of Polysaccharides containing 3-D-Galactofuranosyl units. 

Positions of substituted groups (e.g., methyl, acetyl, sulfate substitutions on hydroxyl groups of polysaccharides) can be distinguished by chemical shifts of corresponding protons and carbons. For example, according to the study by Duus et al. [129], the chemical shifts of substituted saccharides generally downfield shift up to 0.5 ppm for H and 6-7 ppm for when compared to the unsubstituted saccharides. This observation is supported by other literatures [90,133]. 

A useful review of the C n.m.r chemical shifts of 60 naturally occurring steroidal saponins includes a tabulated compilation of data for methyl glyco-pyranoside and -furanoside pairs and a discussion of n.m.r. methodology. A review of the n.m.r. spectroscopy of natural macromolecules including nucleic acids and polysaccharides has appeared. 

The results of the high-temperature enhancement of C n.m.r. chemical shifts of dextrans have been correlated with those of methylation analysis. The diagnostic nature of the 70—75 p.p.m. spectral region with regard to the type of dextran branching, and an increase in resolution of the polysaccharide spectra at higher temperatures, are reported. 

To resolve this limitation of the current protocol, enzymatic digestion (79) or an acid methanolysis procedure could be employed to more completely liberate GIuA (20) from polysaccharides. Since this NMR method has been developed to rapidly characterize biomass samples with minimal sample treatment, the added complexity of additional treatment or derivatization is not consistent with the objectives of the present work. It should also be noted that we have not resolved the effect of 4-0-methylation on the chemical shift of the GluA anomeric protons. Typically wood xylans contain 4-O-methyl-GluA in their structure and not unsubstituted GluA which generally occurs in microbial polysaccharides. It is anticipated that a substituent on 04 in the GlucA structure will have only a minor effect on the chemical shift of the Cl protons. 

Widmalm and coworkers have developed a software called CASPER for H and C NMR chemical shift assignments polysaccharides. Polysaccharides have a high intrinsic flexibility and in particular the glycosidic linkage is difficult to describe in different models. It has been devised several times in the force models and MD simulations have been important in refining the distribution of the conformational populations. 

Yeasts may be identified, and classified, according to the H-l portion of the p.m.r. spectrum of the cell-wall, D-mannose-containing polysaccharide. This portion has served most frequently as a fingerprint for identification of unknown yeasts, but, sometimes, the presence of certain signals of a particular chemical-shift could indicate the presence of certain structures (for example, /3-D-linkages).121 For some... 

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