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[AuCl(4)]). 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(3)N(G1),N(G2),O(G2))Cl] and [Au(Gly-L-Ala-kappa(3)N(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 ((1)H and (13)C) 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 AuCl(4) 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.