Study of complexing properties of the α-metallothionein domain with cadmium and/or zinc, using differential pulse polarography

A study of the complexing properties of the alpha -metallothionein domain with cadmium and/or zinc was performed at pH 8, using differential pulse polarography. The applied method allows one to distinguish between the chemical form of the compound, free metal ions and complexes with the alpha -domain as well as providing evidence that different forms of complexation exist. This technique was used to monitor changes of different species as a function of different parameters. Polarographic results have been compared with those obtained by electrospray ionisation mass spectrometry providing the molecular mass of compounds at different proportions of metal ions to the alpha -domain, under similar experimental conditions. The alpha -domain is capable of complexing cadmium and/or zinc, separately or after simultaneous additions of metal ions. In the case of binary mixtures, Cd + alpha or Zn + alpha pure complexes are formed, whereas in the case of tertiary mixtures alpha + [Cd + Zn] mixed cadmium and zinc complexes prevail. When one of the two ions is added to a solution containing the alpha -domain complexed with the other cation, it is incorporated into the molecule resulting in a reorganisation, which is illustrated by changes in the polarographic responses. Two different conditions can be clearly distinguished: an excess of ligand and when the metal ions are in excess. For these two conditions two different and characteristic features of polarograms were obtained. So, several stable forms of cadmium and zinc complexation with the alpha -domain exist, depending on the composition of the solution. The combination of results obtained using two different techniques was very useful for the assignment of different cadmium and zinc electrochemical responses. (C) 2000 Elsevier Science B.V. All rights reserved.