A proton nuclear magnetic resonance relaxation study of the glycine, alanine, and lactate complexes of gadolinium(III) in aqueous solution

The 1H nuclear magnetic resonance spin-lattice and spin–spin relaxation rate enhancements induced by the gadolinium(III) ion were measured in solutions of glycine, alanine, and sodium lactate containing different amounts of Gd(III). The proton relaxation rates in the Gd(III) complexes were calculated from these data, and were used to calculate metal–hydrogen atom distances. Comparison of these data with corresponding distances calculated from literature X-ray crystallographic data for model compounds shows that in the two amino acid complexes the Gd(III) ion is coordinated in a four-membered ring through the two oxygen atoms of the carboxylate group. By contrast, in the lactate complex coordination is via a five-membered ring involving one oxygen atom of the carboxylate group and the α-hydroxyl oxygen.