Thioether Complexes of Palladium(II) and Platinum(II) as Artificial Peptidases. Residue-Selective Peptide Cleavage by a Palladium(II) Complex

We report the synthesis and characterization of perchlorate salts containing the following three novel complex cations each with a bidentate thioether ligand:  binuclear cis-[Pt(CH₃SCH₂CH₂CH₂SCH₃)(μ-OH)]₂²⁺, mononuclear cis-[Pt(CH₃SCH₂CH₂CH₂SCH₃)(H₂O)₂]²⁺, and mononuclear cis-[Pd(CH₃SCH₂CH₂CH₂SCH₃)(H₂O)₂]²⁺. Despite their analogous compositions, the mononuclear Pt(II) and Pd(II) complexes differ in the selectivity with which they promote the hydrolysis of polypeptides. The complex cis-[Pt(CH₃SCH₂CH₂CH₂SCH₃)(H₂O)₂]²⁺ promotes slow but selective cleavage of Met−Pro peptide bonds at pH 2.0. The selectivity of the complex cis-[Pd(CH₃SCH₂CH₂CH₂SCH₃)(H₂O)₂]²⁺ is pH-dependent. At pH 2.0, this Pd(II) complex promotes residue-selective hydrolysis of the X−Y bond in X-Y-Met and X-Y-His sequences; the rate is enhanced when residue Y is proline. At pH 7.0, this kinetic preference becomes sequence-selective in that the Pd(II) complex exclusively cleaves the X−Pro bond in X-Pro-Met and X-Pro-His sequences. The enhanced reactivity of the X−Pro amide group is attributed to the high basicity of its carbonyl oxygen atom. Binding of the metal(II) atom enhances the electrophilicity of the carbonyl carbon atom and promotes nucleophilic attack by a solvent water molecule. The bidentate thioether ligand disfavors the formation of hydrolytically unreactive complexes, allowing the Pd(II) complex to promote the cleavage reaction.