A study of the reactions of a methionine- and histidine-containing tetrapeptide with different Pd(II) and Pt(II) complexes: selective cleavage of the amide bond by platination of the peptide and steric modification of the catalyst

¹ H NMR spectroscopy was applied to the study of the reactions of [M(en)(H₂O)₂]²⁺ complexes (M = Pd(II) and Pt(II)) with the N-acetylated methionyl-glycyl-histidyl-glycineamide, MeCOMet-Gly-His-GlyNH₂. All reactions were performed in the pH range 1.5–2.0 with equimolar amounts of the [M(en)(H₂O)₂]²⁺ complex and the tetrapeptide at 60 °C. In all these reactions, a metal(II) complex bound to a methionine residue affects the regioselective cleavage of the amide bond involving the carboxylic group of methionine. The priority in the cleavage of the Met-Gly amide bond in relation to the other amide bonds in this peptide is due to the high affinity of Pt(II) and Pd(II) ions for the sulfur donor atom. The mechanism of these hydrolytic reactions is discussed and, for its clarification, the reaction of the [Pd(en)(H₂O)₂]²⁺ complex with MeCOMet-Gly-His-GlyNH₂ was additionally investigated by potentiometric titration. The steric effects of the various palladium(II) complexes of the type [Pd(L)(H₂O)₂]²⁺, in which L is a chelating diamine (ethylenediamine, en, 2-picolylamine, pic, or 2,2-dipyridylamine, dpa) on the hydrolytic cleavage of the amide bond involving the carboxylic group of histidine in the MeCOMet-Gly-His-GlyNH₂ tetrapeptide were also studied by ¹H NMR spectroscopy. All reactions were performed under the above-mentioned conditions and in the initial stage of these reactions, the MeCOMet-Gly-His-GlyNH₂ was reacted with an equimolar amount of the [Pt(dien)Cl]⁺ complex (dien is diethylenetriamine) and then the monoplatinated [Pt(dien)(MeCOMet-Gly-His-GlyNH₂-S)]²⁺ complex was treated with an equimolar amount of [Pd(L)(H₂O)₂]²⁺. It was found that the rate of hydrolysis of the His-GlyNH₂ amide bond in [Pt(dien)(MeCOMet-Gly-His-GlyNH₂-S)]²⁺ decreased from the en to the pic complex, with finally a total inhibition of this reaction with [Pd(dpa)(H₂O)₂]²⁺. These results are an important step in the study of the regioselective cleavage of peptides and proteins and in the development of new palladium(II) complexes as artificial metallopeptidases.