Dimethyltin(IV) cation induced amide deprotonation of aspartic acid containing dipeptides

Abstract

The co-ordination behaviours of two dipeptides, glycyl-aspartic acid (Gly-Asp) and aspartyl-glycine (Asp-Gly), towards dimethyltin(IV) cations have been investigated by potentiometry and spectroscopic methods (¹H and ¹³C NMR and Mössbauer). The formation of mononuclear complexes has been detected between pH 2 and 10, although the hydrolysed species are also present in the solution. On the basis of pH-metric and NMR data, the carboxylate groups are bound to the metal ion in the acidic pH range and act as anchoring groups for the metal-promoted deprotonation of amide nitrogens between pH 4 and 7. The complexes formed in this way are relatively inert on the NMR timescale in the case of both ligands, allowing their structural characterization by NMR spectroscopy. These species can be described with a trigonal bipyramidal structure having a {CO₂⁻, N⁻_amide, NH₂} co-ordination, where the metal-bound carboxylate is very likely the C-terminal one. In spite of the constitutional differences, Asp-Gly and Gly-Asp form stable complexes with dimethyltin(IV) cation in the neutral pH range with identical structure and geometry.