Copper(II) and zinc(II) ion binding properties of a MAP type branched ligand with histidines as surface functionalities.

A novel branched peptide type ligand consisting of three lysines as branching units and four histidines as functional groups has been designed and prepared by solid phase peptide synthesis. The N-terminal histidines offer eight primary protonation/metal ion binding sites: four amino and four imidazole groups, capable of binding up to two metal ions. We examined the protonation equilibria, and the complex formation processes with copper(II) and zinc(II) ions in c(M):c(L) = 1:1 and 2:1 initial concentration ratio in aqueous solutions. The composition, speciation and the solution structure of the complexes have been determined by combined pH-potentiometric titrations, visible absorption, circular dichroism and NMR spectroscopy. In the case of both metal ions, mono- and dimetallic complexes formed with bis-histamine type coordination as the main species. While above pH 8 the precipitation of a neutral complex was observed for both metal ions, in the copper(II) containing systems it dissolved in alkaline solutions (pH > 11.0). The resulting complex in an equimolar system displays deprotonated amide-nitrogen coordination, with fused five-membered chelate rings around the metal ion in CuLH(-3). At the same time, only one copper(II) is able to coordinate in the same manner in the Cu(2)LH(-5) species. The second metal ion is probably surrounded by two amide nitrogens and two others either from amino or imidazole donor groups.