An examination of sodium alginate from <i>Sargassum</i> by NMR spectroscopy

Abstract

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 1 H and 13 C NMR spectroscopy, and also in the solid state by MAS 13 C NMR spectroscopy. Its composition is distinctive, relative to that for alginic acid from most other brown algae, in having a lower content of β-D-mannopyranosyluronic acid residues (M) than of α-L-gulopyranosyluronic acid residues (G). That is, M/G is ~0.6, whereas ratios of &gt;1 have been reported for many other species, including Macrocystispyrifera, 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 13 C 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. Keywords: Sargassum alginate, 1 H NMR of fractions, M/G constituent ratios, MAS 13 C NMR, curve-fitting analysis.