New Selectivity in Peptide Hydrolysis by Metal Complexes. Platinum(II) Complexes Promote Cleavage of Peptides Next to the Tryptophan Residue

Tryptophan-containing N-acetylated peptides AcTrp-Gly, AcTrp-Ala, AcTrp-Val, and AcTrp-ValOMe bind to platinum(II) and undergo selective hydrolytic cleavage of the C-terminal amide bond; the N-terminal amide bond remains intact. In acetone solution, bidentate coordination of the tryptophanyl residue via the C(3) atom of indole and the amide oxygen atom produces complexes of spiro stereochemistry, which are characterized by ¹H, ¹³C, and ¹⁹⁵Pt NMR spectroscopy, and also by UV−vis, IR, and mass spectroscopy. Upon addition of 1 molar equiv of water, these complexes undergo hydrolytic cleavage. This reaction is as much as 10⁴−10⁵ times faster in the presence of platinum(II) complexes than in their absence. The hydrolysis is conveniently monitored by ¹H NMR spectroscopy. We report the kinetics and mechanism for this reaction between cis-[Pt(en)(sol)₂]²⁺, in which the solvent ligand is water or acetone, and AcTrp-Ala. The platinum(II) ion as a Lewis acid activates the oxygen-bound amide group toward nucleophilic attack of solvent water. The reaction is unimolecular with respect to the metal−peptide complex. Because the tryptophanyl fragment AcTrp remains coordinated to platinum(II) after cleavage of the amide bond, the cleavage is not catalytic. Added ligand, such as DMSO and pyridine, displaces AcTrp from the platinum(II) complex and regenerates the promoter. This is the first report of cleavage of peptide bonds next to tryptophanyl residues by metal complexes and one of the very few reports of organometallic complexes involving metal ions and peptide ligands. Because these complexes form in nonaqueous solvents, a prospect for cleavage of membrane-bound and other hydrophobic proteins with new regioselectivity has emerged.