Study of complexing properties of the α and β metallothionein domains with cadmium and/or zinc using electrospray ionisation mass spectrometry Original Research Article

A study of the complexing properties of the α and β metallothionein domains with cadmium and/or zinc was performed, at pH 8, using electrospray ionisation mass spectrometry (ESI-MS). The applied method allows one to determine the molecular mass, and hence, the stoichiometry of compounds at different proportions of metal ions to the α and β domains. The α and β domains are capable of complexing cadmium and/or zinc, separately or after simultaneous or sequential additions of metal ions in the ratios α:M=1:4 and β:M=1:3. In the case of binary mixtures, α or β+Cd or Zn pure complexes are formed: Cd2α, Cd4α and Cd5α, Zn3α and Zn4α for the α domain and Cd3β and Cd4β, Zn3β for β domain, whereas in the case of tertiary mixtures α or β+[Cd+Zn], mixed cadmium and zinc complexes prevail: Cd3Zn1α, Cd2Zn2α, Cd1Zn3α and Cd2Zn1β and Cd1Zn2β. These mixed complexes co-exist with pure cadmium complexes Cd4α and Cd4β and Cd3β in an excess of metal ions, particularly cadmium. So several stable forms of complexation of cadmium and zinc with the α and β domains exist, depending on the composition of the solution. When one of the cations is added to a solution containing the α or β domain complexed with the other cation, it is incorporated into the molecule resulting in its reorganisation, which is illustrated by changes in the abundance of pure and mixed complexes.