Assignment of signals of the carbon-13 magnetic resonance spectrum of a selected polysaccharide: Comments on methodology

The mannan from Rhodotorula glutinis contains alternate (1→3)- and (1→4)- linked β-D-mannopyranose residues (1) and its carbon-13 magnetic resonance spectrum displays 12 signals. These were assigned in terms of the positions of their parent nuclei in the sugar rings [but not whether the signals arose from a (1→3)- or (1→4)-linked residue] by preparation of D-mannans from specifically deuterated D-glucoses and observation of α- and β-deuterium isotope-effects. Individual assignments could then be made for carbon atoms of each unit by using the spectra of known oligo- and polysaccharides. The signal displacements of certain ¹³C nuclei observed on O-methylation were compared with those obtained on O-mannosylation in order to determine whether methyl ethers could be used as model compounds for signal assignments in spectra of mannose-containing polysaccharides. The displacements observed were in the same direction and of a similar order of magnitude. An assessment is made of the use of the various techniques in assigning signals of polysaccharides and their possible interpretation in terms of chemical structure.