The structure of D-xylo-5-hexulosonic acid and its gadolinium(III) complexes in aqueous medium as studied by nuclear magnetic resonance

Abstract

The solid-state ¹³C n.m.r. spectrum of calcium D-xylo-5-hexulosonate, which exists exclusively in the β-furanose form, resembles that of the minor furanose form in solution. A ¹H two-dimensional nuclear Overhauser enhancement spectrum (NOESY), however, unambiguously shows that the latter component is the α-furanose form. Thus caution is required in comparing liquid- and solid-state n.m.r. spectra. Longitudinal relaxation-rate measurements show that Gd^III binds preferentially to the keto tautomer, in a bidentate fashion via a carboxylate oxygen and O(2). The first co-ordination sphere of Gd^III in the predominant complex is completed by furanose ligands bound in a bidentate manner via the two carboxylate oxygens.