Sequence dependence of the reactivity of histidyl containing peptides with palladium(II) and platinum(II) complex ions. An NMR study

Abstract

The reactions of the pentapeptide His-Pro-Gly-Ala-His with the complex salts [Pd(dien)(D₂O)][NO₃]₂, [Pt(dien)(D₂O)][NO₃]₂, [Pd(en)(D₂O)₂][NO₃]₂, cis-[Pt(NH₃)₂(D₂O)₂][NO₃]₂, K₂PdCl₄ and K₂PtCl₄ were studied in aqueous solutions, as a function of pD, by means of ¹H, ¹³C and ¹⁹⁵Pt NMR spectroscopies in one and two dimensions. In order better to understand the behavior of the ternary systems of the pentapeptide with the complex ion [Pd(dien)(D₂O)][NO₃]₂ or [Pt(dien)(D₂O)][NO₃]₂, the reactions of the tetrapeptide Pro-Gly-Ala-His with these complexes were also studied as a function of pD. The ¹H and ¹³C NMR assignments were made by two dimensional homo- and hetero-nuclear experiments at various pD’s for both peptides. The complexes [Pd(dien)(D₂O)][NO₃]₂ or [Pt(dien)(D₂O)][NO₃]₂, react with Pro-Gly-Ala-His in acidic or alkaline media respectively to form mixtures of two isomers in which histidyl imidazole co-ordinates the metal through either N1 or N3. In neutral solution, the imidazole ring bridges two Pd(dien) moieties, bound to N1 and N3. In the presence of [Pd(dien)(D₂O)][NO₃]₂ the histidyl-5 residue of the pentapeptide behaves in the same way as the one in the tetrapeptide, whereas the histidyl-1 residue reacts in acidic media through both imidazole N1 and N3 bridging two Pd(dien) moieties. The complex [Pd(en)(D₂O)][NO₃]₂ or K₂PdCl₄ reacts with His-Pro-Gly-Ala-His in strongly acidic media (pD < 1.5) to form 1∶1 adducts with the histidyl-1 residue co-ordinated selectively to the metal ion forming NH₂, N3 chelates. Similar products are formed by the reaction of the pentapeptide with cis-[Pt(NH₃)₂(D₂O)₂][NO₃]₂ or K₂PtCl₄ in slightly alkaline or acidic media respectively.