Complex formation between [(η6-p-cym)Ru(H2O)3]2+ and oligopeptides containing three histidyl moieties

In order to model the metal ion binding capabilities of high molecular mass components of blood the interaction between [(η6-p-cym)Ru(H2O)3]2+ and terminally protected oligopeptides containing three histidyl moieties (Ac-HHH-NH2, Ac-HAHH-NH2, Ac-HAHAH-NH2 and Ac-H*AH*AH*-NH2, where A = L-alanyl, H = L-histidyl, H* = N3-methyl-L-histidyl) were studied by pH-potentiometric, ESI-TOF-MS, circular dichroism and NMR methods at an ionic strength of 0.20 M KCl or KNO3 as well as using density functional theory (DFT) calculations. Protonation constants of the novel peptides are reported. Although for Ac-HHH-NH2 the immediate formation of precipitation with [(η6-p-cym)Ru(H2O)3]2+ hindered any further solution investigations results of the detailed NMR and MS studies revealed that the other three ligands coordinate to the metal ion in rather slow processes via the imidazole moieties forming [(η6-p-cym)RuL]2+ (L = oligopeptide) type species in the slightly acidic, neutral pH-range. At pH ∼7.5 identical binding mode of Ac-HAHH-NH2 and Ac-HAHAH-NH2 in the [(η6-p-cym)RuL]2+ via three imidazole nitrogens was found hindering completely the hydrolysis of the metal ion even at 1:1 metal ion to ligand ratio. At elevated pH MS evidences support the involvement of amide-N donor(s) in metal ion binding too beside partial hydrolysis. 0.20 M KCl medium was found to hinder effectively the hydrolytic processes of the metal ion in the basic pH-range without altering the coordination of the imidazole side chains. Both NMR and DFT results support the imidazole-N1 (“far” or “τ”) over the N3 (“near” or “π”) coordination of the histidyl side chains of all these oligopeptides to the organometallic ruthenium(II) cation.