Detailed equilibrium, spectroscopic, and SOD activity studies are reported on nickel(II) complexes formed with the N-terminally free HHDLPCGVY-NH₂ (NiSODHH) and HCDLPHGVY-NH₂ (NiSODHC) peptides mimicking the nickel binding loop in NiSOD. In these model peptides, cysteine was incorporated in different positions in order to gain better insight into the role of the cysteine residues in NiSOD. The results are compared with those obtained with the wild-type fragment of NiSOD. The complex formation equilibria of nickel(II) with the two peptides exhibit different features. In the case of NiSODHH, the ligand field of the (NH₂,N_Im,N_Im,S^-) donor set is not strong enough to cause spin pairing and an octahedral paramagnetic complex is formed under physiological conditions. In contrast, NiSODHC forms a square-planar diamagnetic complex with (NH₂,N^-,S^-,N_Im) donors which exhibits remarkable SOD activity. Our results unambiguously prove that the presence of cysteine in the secondary position of the peptide chain is crucial to establish the square-planar geometry in the reduced form of NiSOD, while the distant cysteine affects the redox properties of the Ni(II)/Ni(III) couple. Compared to the model systems, the Ni(II) complex with the wild-type fragment of NiSOD exhibits superior SOD activity. This confirms that both cysteinyl residues are essential in the efficient degradation of superoxide ion. The enzyme mimetic complexes are also capable of assisting the decomposition of superoxide ion; however, they show considerably smaller catalytic activity due to the absence of one of the cysteine residues.

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