Solution-state behaviour of algal mono-uronates evaluated by pure shift and compressive sampling NMR techniques

• Published in: Carbohydrate research 2020-09, Vol.495, p.108087-108087, Article 108087
• Main Authors: Rowbotham, Jack S., Aguilar, Juan A., Kenwright, Alan M., Greenwell, H. Christopher, Dyer, Philip W.
• Format: Article
• Language: English
• Subjects: Compressive sampling; Guluronate; Guluronic; Hydrothermal lactonisation; Kelp; Macroalgae; Mannuronate; Mannuronic; Pure shift NMR spectroscopy; Seaweed; Uronic acids
• ISSN: 0008-6215; 1873-426X
• DOI: 10.1016/j.carres.2020.108087

Sodium salts of the algal uronic-acids, d-mannuronic acid (HManA) and l-guluronic acid (HGulA) have been isolated and characterised in solution by nuclear magnetic resonance (NMR) spectroscopy. A suite of recently-described NMR experiments (including pure shift and compressive sampling techniques) were used to provide confident assignments of the pyranose forms of the two uronic acids at various pD values (from 7.5 to 1.4). The resulting high resolution spectra were used to determine several previously unknown parameters for the two acids, including their pKa values, the position of their isomeric equilibria, and their propensity to form furanurono-6,3-lactones. For each of the three parameters, comparisons are drawn with the behaviour of the related D-glucuronic (HGlcA) and D-galacturonic acids (HGalA), which have been previously studied extensively. This paper demonstrates how these new NMR spectroscopic techniques can be applied to better understand the properties of polyuronides and uronide-rich macroalgal biomass. [Display omitted] •1H and 13C NMR of mono-uronic acids HManA and HGulA assigned using 1D pure shift1H and compressive sampling1H-13C HSQC.•The acidity constants of the α- and β-forms of the acids HManA and HGulA were determined.•The 1C4-β-L-gulopyranose and 4C1-α-D-mannopyranose acid forms found to be the most stable at all pD values from 1.4 to 7.5.•Both HManA and HGulA form furanurono-6,3-lactones at pD 1.4; HManA was found to lactonise more readily than HGulA.