Rb+ and Na+ spin relaxation in aqueous gellan solutions and implication of selective site binding of alkali metal...

87Rb NMR was applied to investigate the site binding of Rb+ ions in gellan gum gels. The temperature dependence of the transverse and longitudinal relaxation NMR relaxation rates of 87Rb+ ion and 23Na+ ion have been compared in aqueous 5% (w/v) rubidium-type and sodium-type gellan. In each sample, the relaxation rates were sensitive to the conformation (helix or random coil). In rubidium-type gellan, significant line-broadening effects (losses in intensity) were found, which is due to the presence of cation-binding sites in the ordered conformation. In sodium-type gellan, such an enhancement of the relaxation was not observed. These results indicated that gellan gum produced highly selective binding sites for alkali metal ions, in which Rb+ ion bound more strongly than Na+ ion. The 87Rb NMR line shift suggested selective site binding of ions to form the cross-linking domains in gellan gels.