A comparative study of complex formation in the reactions of gold(III) with Gly-Gly, Gly-L-Ala and Gly-L-His dipeptides

Proton NMR spectroscopy was applied to study the reactions of the dipeptides glycyl-glycine (Gly-Gly) and glycyl-L-alanine (Gly-L-Ala) with hydrogen tetrachloridoaurate(III) (H[AuCl4]). All reactions were performed at pH 2.0 and 3.0 and at 40 degrees C. The final products in these reactions were [Au(Gly-Gly-kappa N-3(G1),N-G2,O-G2)Cl] and [Au(Gly-L-Ala-kappa N-3(G),N-A,O-A)Cl] complexes. Tridentate coordination of the corresponding dipeptides and square-planar geometry of these Au(III) complexes was confirmed by NMR (H-1 and C-13) spectroscopy. This study showed that at pH <3.0 the Au(III) ion was able to deprotonate the amide nitrogen atom. However this displacement reaction was very slow and the total concentration of the corresponding Au(III)-peptide complex formed after 5 days was less than 60% for the Gly-L-Ala or 70% for the Gly-Gly dipeptide. The kinetic data of the reactions between the Gly-Gly and Gly-L-Ala dipeptides and [AuCl4](-) were compared with those for the histidine-containing Gly-L-His dipeptide. The differences in the reactivity of these three dipeptides with the Au(III) ion are discussed. (C) 2010 Elsevier Inc. All rights reserved.