An examination of sodium alginate from Sargassum by NMR spectroscopy

Alginic acid, obtained from Sargassum brown algae that drift from the Sargasso Sea to beaches in Cuba, has been examined as an aqueous solution by H-1 and C-13 NMR spectroscopy, and also in the solid state by MAS C-13 NMR spectroscopy. Its composition is distinctive, relative to that for alginic acid from most other brown algae, in having a lower content of beta-D-mannopyranosyluronic acid residues (M) than of alpha-L-gulopyranosyiuronic acid residues (G). That is, M/G is similar to 0.6, whereas ratios of >1 have been reported for many other species, including Macrocystis pyrifera, which was examined here as well. In acquiring the high-resolution spectra, partial depolymerization was used to give solutions of reduced viscosity; as is well known, this procedure may cause separation into fractions and the formation of sediments, examples of which are cited. Proton resonances of the reducing end units of chain-shortened segments of the polymers exhibited unusual, marked, line broadening at elevated temperature, which may reflect thermal dehydration. MAS C-13 experiments offered the advantage of information about the intact alginate samples. Although the spectra consisted of broad, heavily overlapping peaks, curve-fitting analysis afforded qualitative and quantitative data that supplement those obtained with solutions of the polymers.