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Superoxide dismutase (SOD) enzymes are pivotal in regulating oxidative stress. In order to model Ni containing SOD enzymes, the results of the thermodynamic, spectroscopic and SOD activity studies on the complexes formed between nickel(II) and a NiSOD related peptide, CysCysAspLeuProCysGlyValTyr-NH 2 ( wtCC ), are reported. Cysteine was introduced to replace the first histidine residue in the amino acid sequence of the active site of the NiSOD enzyme. The novel peptide exhibits 3 times higher metal binding affinity compared to the native NiSOD fragment. This is due to the presence of the first cysteine in the coordination sphere of nickel(II). At physiological pH, the (NH 2 ,S - ,S - ,S - ) coordinated complex is the major species. This coordination mode is altered when one thiolate group is replaced by an amide nitrogen of the peptide backbone above pH 7.5. The nickel complexes of wtCC exhibit similar SOD activity to that of the complex formed with the active site fragment of the native NiSOD. The reaction between the complexes and the superoxide anion was studied by the sequential stopped-flow method. These studies revealed that the nickel(II) complex is always in excess over the nickel(III) complex during the dismutation process. However, the nickel(III) species is also involved in a relatively fast degradation process. This unambiguously proves that a protective mechanism must be operative in the NiSOD enzyme which prevents the oxidation of the sulfur atom of cysteine in the presence of O 2 - . The results provide new possibilities for the use of NiSOD mimics in bio- and industrial catalytic processes.

Detailed equilibrium, spectroscopic, and SOD activity studies are reported on nickel(II) complexes formed with the N -terminally free HHDLPCGVY-NH 2 ( NiSODHH ) and HCDLPHGVY-NH 2 ( 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 (NH 2 ,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 (NH 2 ,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.

N-Terminally free but C-terminally amidated peptides Pen-SSACS-NH 2 and CSSA-Pen-S-NH 2 containing l-penicillamine (Pen) in sequence have been synthesized and their nickel(ii), zinc(ii) and cadmium(ii) complexes were studied by potentiometric and spectroscopic measurements. This study is the first example of the synthesis and metal complexes of peptides containing penicillamine in a peptide sequence constructed from natural amino acids. The data were compared to those of the two cysteine counterparts, CSSACS-NH 2 . It was found that the replacement of l-cysteine with l-penicillamine has a significant impact on the complex formation processes with nickel(ii) ions. The major differences include the suppression of polynuclear complex formation, the enhanced metal binding affinity of the amino terminus and the increased tendency for the formation of amide bonded species. The tridentate (NH 2 ,S - ,S - ) coordination was characteristic of the zinc(ii) and cadmium(ii) complexes in the case of all three peptides containing two thiolate functions.