Interaction of Dimethyltin(IV)2+ Cation with Gly-Gly, Gly-His, and Some Related Ligands. A New Case of a Metal Ion Able To Promote Peptide Nitrogen Deprotonation in Aqueous Solution

Equilibrium (pH-metric) and spectroscopic ( 1 H, 13 C, and 119 Sn NMR and 119 Sn Mössbauer) studies were performed to characterize the interaction of the dimethyltin(IV) cation with glycine, glycyl-glycine (Gly-Gly), imidazole-4-acetic acid, histamine, histidine, glycyl-histamine, glycyl-histidine (Gly-His), and β-alanyl-histidine (carnosine). For histamine and glycyl-histamine (having only nitrogen donor atoms) no complex formation was detected. The hydrolyzed species of the dimethyltin(IV) cation are always dominant over the complexes formed with the other ligands, except with Gly-Gly and Gly-His. For these two ligands, {COO -,N -,NH 2 } coordinated complexes are dominant in the neutral pH range with a trigonal bipyramidal structure, providing the first example that alkyltin(IV) cations are able to promote the deprotonation of the peptide-nitrogen in aqueous solutions, at unexpectedly low pH. In this process the carboxylate is the anchoring group (assisting by chelate formation), in contrast with any other metal ions which are known to coordinate to amide nitrogen. The metal coordination of the imidazole ring, which is suggested as binding site toward alkyltin(IV) cation in several proteins, was not observed for Gly-His under the conditions used; it is probably the case for the other ligands, too.