Synthesis, spectroscopic and X-ray characterization of various pyrazine-bridged platinum(II) complexes: 1H NMR comparative study of their catalytic abilities in the hydrolysis of methionine- and histidine-containing dipeptides

Abstract

Four pyrazine (pz)-bridged Pt(II) complexes, [{Pt(1,3-pd)Cl}2(μ-pz)]Cl2·LiCl (1) (1,3-pd=1,3-propylenediamine), [{Pt(2,2-diMe-1,3-pd)Cl}2(μ-pz)]Cl2·2[Li(H2O)4]Cl·2H2O (2) (2,2-diMe-1,3-pd=2,2-dimethyl-1,3-propylenediamine), [{Pt(1,3-pnd)Cl}2(μ-pz)](ClO4)2·H2O (3) (1,3-pnd=(±)-1,3-pentanediamine) and [{Pt(1,3-pnd)Cl}2(μ-pz)]Cl2·2[Pt(1,3-pnd)Cl2]·2H2O (4) have been synthesized. NMR and UV–Vis spectroscopic characterization has been performed for compounds 1–3, while single-crystal X-ray analysis has been carried out for complexes 2 and 4. Atomic distribution in the crystals of 4 indicated a disorder which could be attributed to the presence at the same crystallographic site of four distinct stereoisomers of the [{Pt(1,3-pnd)Cl}2(μ-pz)]2+ complex cation. The presence of four stereoisomeric products was also observed in complex 3 by 13C NMR spectroscopy. Complexes 1–3 were converted into the corresponding aqua complexes, [{Pt(X)(H2O)}2(μ-pz)]4+ (X is 1,3-pd, 2,2-diMe-1,3-pd and 1,3-pnd, respectively), and 1H NMR spectroscopy was applied for comparison of their catalytic activities with those of the analogous mononuclear [Pt(X)(H2O)2]2+ and pyrazine-bridged [{Pt(en)(H2O)}2(μ-pz)]4+ complexes in the hydrolysis of the N-acetylated l-methionylglycine (Ac-l-Met-Gly) and l-histidylglycine (Ac-l-His-Gly). All reactions were performed in the pH range 2.0–2.5 at 37°C. It was found that all investigated dinuclear Pt(II)-aqua complexes promote selective cleavage of the amide bond involving carboxylic group of the anchoring amino acid methionine in the Ac-l-Met-Gly or histidine in the Ac-l-His-Gly. 1H NMR data indicate that neither the size of the chelated diamine ring (five-membered in ethylenediamine and six-membered in 1,3-propylenediamine) nor the bulky substituents incorporated into the 1,3-propylenediamine ligand have significant influence on the rate of hydrolysis of Ac-l-Met-Gly dipeptide. Meanwhile, the rate of hydrolysis of Ac-l-His-Gly depends on both of these factors and decreases in order en>1,3-pd>1,3-pnd>2,2-diMe-1,3-pd. Moreover, it has been shown that all investigated dinuclear Pt(II)-aqua complexes are better catalytic agents in the hydrolysis of the dipeptides than the analogous mononuclear Pt(II)-aqua complexes. The present findings are expected to play a crucial role in the development of new Pt(II) complexes, which can act as effective catalytic reagents for the selective hydrolysis of peptides containing either methionine or histidine residues.