Binding ability of thiocarbonyl-containing dipeptides. Potentiometric and spectroscopic studies of copper(II) and nickel(II) coordination

Abstract

Complex formation reactions of thioamide-containing dipeptides (Alat-Ala-OMe and Asp-Phet-NH 2) with copper(II) and nickel(II) were studied by the pH-metric and spectroscopic methods. Sulfur donation was established in acidic and neutral media, and it resulted in the enhanced stability of the respective complex species. Already, at pH < 3, the S to metal charge transfer band indicated the {NH 2S} coordination in cupric complexes. The high stability constants of CuA and CuA 2 complexes in comparison to respective complexes with glycine-amide (two and five orders, respectively) also indicate the involvement of sulfur in the metal ion coordination. The thiocarbonyl donor is involved in coordination of nickel(II) ions as well. The stability constants of respective complexes are enhanced, and, according to the spectroscopic data, the NiA 2 species is octahedral. It was also found that the coordination of thiocarbonyl donor although it is very effective, cannot prevent an amide nitrogen deprotonation and its coordination to metal ion in basic solutions. According to absorption and c.d. spectra in basic solutions, metal ions are bound to {NHi 2N −} donor sets, and resulting complexes MA 2H −2 are tetragonal for copper and square-planar for nickel ions.