¹³C NMR studies of the interaction of gold(I) thiomalate with 6-mercaptopurine and its derivatives

The interaction of gold(I) thiomalate, [Autm]_n with thiolated nucleosides, 6-mercaptopurine (6-MP), 6-mercaptopurine-9-β-D-riboside (6-MPR) and 2-amino-6-mercaptopurine-9-β-D-riboside (2-A-6-MPR) has been studied by ¹H and ¹³C NMR spectroscopy. It has been observed that these thiolated purine bases break the [Autm]_n polymer and form complexes of the type [〉C==S-Au-tm]. The major shift in ¹³C NMR occurs in C-6 resonances of the bases when they react with [Autm]_n. This is indicative of Au(I) binding with these bases through the sulfur atom only. In the case of 6-MP, it was observed that at pD 12, the N-9 proton is deprotonated causing a downfield shift in the C-8 resonance. However, coordination of Au(I) with N-9 or N-7 was not observed. Also, at its higher concentration thiomalate was ejected as a free ligand and due to its oxidation, thiomalic disulfide resonances were observed suggesting that it could behave as a thiolate ligand to gold(I). In cases of 6-MPR and 2-A-6MPR, no free thiomalate resonances in ¹³C NMR were observed even at higher concentrations of these bases indicating that thiomalate was not released from [Autm]_n. At the 2:1 ratio of 6-MP to [Autm]_n, the b₁, b₂ and b₃ resonances of [Autm]_n separated into two peaks indicating the existence of two geometrical isomers for the complex, 6-MP-Au-tm.