Reactions of gold(III) complexes with l-histidine-containing dipeptides: influence of chelated ligand and N-terminal amino acid on the rate of peptide coordination

Abstract

The course of reactions between [Au(en)Cl2]+ (en is ethylenediamine) and [AuCl4]– complexes with an equimolar amount of X-L-His dipeptides (X = Gly, L-Ala and L-Ser) was investigated in aqueous solution at pD range 3.50–4.50 at ambient temperature by 1H NMR spectroscopy. When [Au(en)Cl2]+ was mixed with X-L-His, three 1H NMR detectable Au(III)-dipeptide complexes were observed in solution. In the first complex, the corresponding dipeptide is monodentate coordinated to Au(III) through the N3 imidazole nitrogen, while the second complex was formed by bidentate coordination of the dipeptide through the N3 imidazole and the deprotonated amide nitrogen (NP). The deprotonated amide nitrogen in this complex weakens the Au–N(en) bond in the trans position, leading to the displacement of the coordinated en ligand by the N-terminal amino group of the corresponding dipeptide, yielding the formation of [Au(X-L-His-NA,NP,N3)Cl]. The same coordination mode of these dipeptides was also observed in the reaction with [AuCl4]–. Nevertheless, the tridentate coordination of the dipeptides to [AuCl4]– was 3–6 times faster in comparison to the same process with [Au(en)Cl2]+. For both gold(III) complexes, the following order of the dipeptide reactivity was observed: Gly-L-His > L-Ser-L-His > L-Ala-L-His.