INTERACTIONS OF PARAMAGNETIC METAL-IONS WITH GELLAN GUM STUDIED BY ESR AND NMR METHODS

The characterization of the gellan-paramagnetic metal ion complex has been investigated by ESR and NMR methods in aqueous solution of sodium -type gellan. The resonance of the carboxyl carbon in the D-glucuronat e unit in C-13-NMR spectra of gellan is selectively broadened with the addition of the Mn(II) ion in sol state. This is due to the paramagne tic interaction between the carbon and the Mn(II) ion. The resonances of other units of gellan are not affected, showing that Mn(II) ions in teract with the carboxyl groups in the D-glucuronate unit. Water proto n relaxation measurements indicate that water molecules are well coord inated to the gellan-Mn(II) ion complex in gel state. ESR measurements of the Mn(II) ion give the dissociation constant of 3.2 x 10(-5) and 6.5 x 10(-4) M, and the number of the binding site of 0.12 and 0.33 pe r repeated unit in sol and gel states, respectively. The latter sugges ts that in sol state a Mn(II) ion is surrounded by several residues in a randomly coiled gellan polymer and in gel state a Mn(II) ion binds two or three D-glucuronate units on neighboring gellan molecules. Copy right (C) 1996 Elsevier Science Ltd