Opening the : NMR spectroscopic analysis of the interactions between s-block cations and kelp monosaccharides

• Published in: Dalton transactions : an international journal of inorganic chemistry 2021-10, Vol.5 (38), p.13246-13255
• Main Authors: Rowbotham, Jack S, Christopher Greenwell, H, Dyer, Philip W
• Format: Article
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d0dt04375c

The best-known theory accounting for metal-alginate complexation is the so-called " Egg Box " model. In order to gain greater insight into the metal-saccharide interactions that underpin this model, the coordination chemistry of the corresponding monomeric units of alginate, l -guluronate ( GulA ) and d -mannuronate ( ManA ) have been studied herein. GulA and ManA were exposed to solutions of different s-block cations and then analysed by 1 H and 13 C NMR spectroscopy. It was found that the α / β ratio of the pyranose anomeric equilibria of GulA showed large pertubations from the starting value ( α / β = 0.21 ± 0.01) upon contact with 1.0 M Ca 2+ , Sr 2+ , and Ba 2+ ( α / β = 1.50 ± 0.03, 1.20 ± 0.02, and 0.58 ± 0.02, respectively) at pD 7.9, but remained almost constant in the presence of Na + , K + , and Mg 2+ ( α / β = 0.24 ± 0.01, 0.19 ± 0.01, and 0.26 ± 0.01, respectively). By comparison, no significant changes were observed in the α / β ratios of ManA and related mono-uronates d -glucuronate ( GlcA ) and d -galacturonate ( GalA ) in the presence of all of the metal ions surveyed. Analysis of the 1 H and 13 C coordination chemical shift patterns indicate that the affinity of α -GulA for larger divalent cations is a consequence of the unique ax-eq-ax arrangement of hydroxyl groups found for this uronate anomer. The α-form of kelp-derived monosaccharide l -guluronate (α- GulA ) has a unique axial-equatorial-axial (ax-eq-ax) arrangement of hydroxyl groups that can coordinate to divalent cations ( e.g. Ca 2+ ), stabilising the structure relative to its β-anomer.