Copper(ii) and nickel(ii) binding modes in a histidine-containing model dodecapeptide

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Abstract
The formation of complexes of HGGGHGHGGGHG (HG12) with copper(II) and nickel(II) have been studied in aqueous solution under various experimental conditions, including different pH and metal to ligand ratios. The study has been carried out using visible absorption, circular dichroism and electron paramagnetic resonance spectroscopic methods. Moreover, electrospray ionisation mass spectrometry has been used to directly determine the stoichiometry of the copper(II) complexes. The results indicate that HG12 can easily accommodate two metal ions in as many binding sites. The solution structure of the main complex species formed in the reaction of copper(II) with HG12 has been inferred by comparison with the copper(II) complexes formed with the shorter peptide fragments HGGGHG–NH2 (HG6), Ac–HGGGHG–NH2 (AcHG6) and Ac–HGGG–NH2 (AcHG4). With an equimolar metal to ligand ratio, the copper(II) ion binds preferentially in the N-terminal region of HG12. Conversely, Ni(II) ions form identical complexes regardless of whether the metal to ligand ratio is 1∶1 or 2∶1. Finally, the circular dichroism spectra indicate a significant modification of the peptide conformation upon metal binding.
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