Profiling sulfate content of polysaccharides in seaweed species using a ligand-assisted ¹H-NMR assay

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This study aimed to use an existing ligand (imidazole)-assisted ¹H-nuclear magnetic resonance (¹H-NMR) spectroscopy protocol to determine sulfate content of seaweed sulfated saccharides. Twenty seaweed samples – spanning nine species and multiple geographic locations – were analyzed. Extracts were obtained by immersing freeze-dried seaweeds in boiling water, followed by alginate precipitation. Remaining supernatants contained natural crude polysaccharide (CP) fraction, including sulfated polysaccharides. To eliminate free CP carboxy groups that can interfere with the complex formation of imidazole with CP sulfate groups, the carboxy groups were chemically reduced to hydroxymethyl groups prior to ¹H-NMR spectroscopy. Resultant sulfate content estimates were comparable to those obtained via a conventional barium-rhodizonate assay. Findings indicate that the imidazole-assisted ¹H-NMR spectroscopy is an appropriate method for estimation of seaweed CP sulfate content.