Stabilization of the Nickel Binding Loop in NiSOD and Related Model Complexes: Thermodynamic and Structural Features

Detailed equilibrium and spectroscopic characterization of the complex formation processes of the nickel binding loop in NiSOD and its related fragments is reported in the slightly acidic–alkaline pH range. The N-terminally free and protected nonapeptides HCDLPCGVY-NH2 (NiSODM1), HCDLACGVY-NH2 (NiSODM3), and Ac-HCDLPCGVY-NH2 (NiSODM2) and the N-terminally shortened analogues HCDL-NH2 and HCA-NH2 were synthesized, and their nickel(II) complexes were studied by potentiometric and several spectroscopic techniques. EPR spectroscopy was also used to assign the coordinating donor sites after the in situ oxidation of nickel(II) complexes. The terminal amino groups are the primary metal binding sites for nickel(II) ion in NiSODM1 and NiSODM3, resulting in the high nickel(II) binding affinity of this peptide via the formation of a square-planar, (NH2,N–,S–,S–) or (NH2,NImN–,S–) coordinated species in a wide pH range. The latter coordination sphere prevents the formation of the active structure of NiSOD under physi...