■ Complexing properties of the β metallothionein domain with cadmium and/or zinc, studied by differential pulse polarography

An electrochemical study of the complexing properties of the β metallothionein domain with cadmium and/or zinc was performed at pH 8, using differential pulse polarography. The applied technique allows one to obtain well-defined peaks corresponding to the free metal ions as well as providing evidence that different forms of complexation of Cd(II) and Zn(II) with the β domain exist. The influence of sequential additions of Cd and/or Zn to the β domain was investigated. The β domain is capable of complexing cadmium and/or zinc, separately or after simultaneous additions, resulting in complexes with a of M:β ratio of 3:1, but which possess different characteristics. In the case of binary mixtures, Cd + β or Zn + β, pure complexes with a probable stoichiometry of Cd3β and Zn3β are formed, whereas in the case of tertiary mixtures, β + [Cd + Zn] mixed cadmium and zinc complexes Cd2Znβ and CdZn2β prevail. When one of the two metal ions is added to a solution containing the β domain complex with the other cation, it is incorporated into the molecule resulting in a reorganisation, which is illustrated by changes in the polarographic responses. Polarographic results have been compared with those obtained by electrospray ionisation mass spectrometry, providing the molecular mass of compounds under similar experimental conditions. Different polarographic peaks are found for either pure or mixed complexes.