Mass spectrometry study of tetraglycine associated with selected metal (II) ions: manganese, iron, cobalt, nickel, copper and zinc

The research presented here is focused on the interactions between transition metal cations and peptides. The binding affinity of transition metal cations Mn2+, Fe2+, Co2+, Ni2+ , Cu2+ and Zn2+ with tetraglycine (GGGG) peptide have been determined by experimental and computational approaches. Molden and Gaussian were the two computational software programs used to develop peptide-metal ion complex, optimize and perform frequency calculations on the tetraglycine peptide associated with different metal ions such as Mn 2+, Fe2+, Co2+, Ni2+, Cu2+ or Zn2+. Molden has a powerful Z-matrix editor which gives full control over the geometry and allows building molecules from scratch, including polypeptides. The geometry optimized molecular structure of tetraglycine peptide associated with different metal ions was obtained by using the MPWIPW91 level of theory and the 6-311+G (d,p) basis set. The stock solutions of metal ions Mn2+, Fe2+, Co2+, Ni2+, Cu2+ and Zn 2+and the tetraglycine peptide of 1x10-2 M were prepared and the stock solution of peptide and different metal ions were mixed in the ratio of 5:1 (metal: peptide) and were set aside for few minutes in order to allow the metal ions to bind to the peptide. Then the solutions were further diluted to 1x10-5 M and the samples were analyzed using mass spectrometry in positive ion mode and negative ion mode. The binding sites and pseudo apparent association constants KA were calculated and their binding affinities were compared by using the following equations.;Positive ion mode: 1. [GGGG +H] + + M 2+ ↔ [(GGGG-H) +M2+] + K A = [(GGGG-H) +M2+] + / ([GGGG +H] + x [M2+]).;Negative ion mode: 2. [GGGG -H] - + M2+ ↔ [(GGGG-3H) +M2+] - K A = [(GGGG-3H) +M2+] - / ([GGGG -H] + x [M2+]).;Where, KA = Pseudo apparent association constant for the formation of the metal ion - tetraglycine complex from the protonated peptide [GGGG +H] + or deprotonated peptide [GGGG-H] - and metal ion M2+. M2+ = is either Mn2+, Fe2+, Co2+, Ni2+, Cu2+ or Zn2+..