A p.m.r. study of the composition and sequence of uronate residues in alginates

The sequence and composition of uronate residues in intact alginate samples have been obtained by high-resolution 1H-n.m.r. spectroscopy. The viscosity problem was overcome by a slight, controlled depolymerization of the alginate samples before the 1H-n.m.r. spectra were recorded at 90°. The mannuronate (M)/guluronate (G) molar ratio was obtained from the intensities of the signals for the anomeric protons. A sequence-dependent deshielding of H-5 of the guluronate residues made it possible to determine the fractions of the four possible doublets of nearest neighbours along the chain. The results were wholly consistent with 13C-n.m.r. data. Significant deviation from comparable results obtained by chemical analysis appeared only for samples containing a large fraction of the mixed doublets.