Interaction of N-acetylmethionine with a non-C2-symmetrical platinum diamine complex

The reaction of N-acetylmethionine (N-AcMet) with the complex [Pt(Et2en)(D2O)2]2+ (Et2en = N,N-diethylethylenediamine) was studied by NMR spectroscopy and molecular mechanics calculations. Complexes containing two methionine residues coordinated to the platinum atom were calculated to be relatively high in energy unless the bulk of the methionine residues was directed away from the diethyl group of the Et2en ligand. In contrast, sulfur–oxygen chelates were found to be relatively free of steric clashes. Experimentally, two sets of NMR resonances were observed when [Pt(Et2en)(D2O)2]2+ was reacted with N-AcMet; variable temperature experiments indicated intermediate chemical exchange between the two sets of resonances. NMR studies indicated that the resonances corresponded to [Pt(Et2en)(N-AcMet-S,O)]+ complexes with the sulfur atom trans to the diethyl group of the Et2en ligand. No product with the sulfur atom cis to the diethyl group was observed experimentally even though molecular mechanics calculations suggested that such forms have few steric clashes. The NMR results suggested that the chemical exchange was a result of sulfur chirality inversion. In early stages of the reaction, a [Pt(Et2en)(N-AcMet-S)(D2O)]+ complex was observed, indicating that coordination of the oxygen to form the chelate is relatively slow.