Influence of chloride ligands on the structure of Zn- and Cd-metallothionein species

It is now commonly accepted that non-proteic ligands contribute to the structure and stability of metal-metallothionein (M-MT) species, although this contribution may differ substantially depending on the MT and the metal ions involved. Conversely, literature data are unconnected, lacking correlation studies between the contribution of inorganic ligands to the M-MT complexes and the corresponding CD and UV-vis fingerprints. To contribute towards filling this gap, we have analyzed the influence of chloride anions in the Zn- and Cd-MT complexes of mammalian MT1 and MT4 isoforms. Starting from the initial hypothesis that the shoulders appearing at 240 nm in the UV-vis difference spectra during the Cd(II) titrations of Zn-MTs would be indicative of chloride participation in these metal-MT complexes, we can now propose that, while their absence definitely rules out these ligands being involved in metal coordination, their presence should not necessarily be attributed to the formation of metal-Cl bonds. Instead, we identified a global blue shift for the UV-vis difference spectral envelope as the most liable indication of chloride participation in the binding sites of the M-MT species. Following this criterion, we determined that chloride anions are bound to the Cd7-MT1 and Cd4-αMT1 complexes but not in the isostoichiometric Zn complexes, nor in the Zn- or Cd-complexes of the homologous MT4 peptides.