Complexation Properties of the Metallothionein Fragment Lys-Cys-Thr-Cys-Cys-Ala [56-61] MT I with Zinc and Cadmium Using Square Wave Voltammetry

Abstract

An electrochemical study using square wave voltammetry of the complexing properties of the peptide Lys-Cys-Thr-Cys-Cys-Ala [56–61] MT 1 (FT), a part of the metallothionein structure, with cadmium and zinc was performed. 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 coordination of Cd(II) and Zn(II) with the peptide exist. FT is capable of complexing Cd2+and Zn2+ separately in a concentration equal to FT, obtaining complexes with a ratio of 1:1. However, the total metal ion concentration bound to the FT is higher when both cations are simultaneously present. The maximum total concentration that can be complexed with FT is a ratio of [Zn]:[Cd]:[FT] like 0.6:1.0:1.0 which leads to the ratio of Zn4Cd7S21, comparable to that of the metallothionein structure. The coordination of Cd(II) with FT not only depends on the Cd:FT ratio, but also on the proportion in which Zn(II) is added. However, the Zn(II) complexes are not affected by the presence of Cd-FT complexes in solution. The complexation equilibrium was investigated going from acid to basic solutions and viceversa. From the slopes of the variation Ep=f (pH), the number of thiol groups at which the cations are bound were evaluated. This study allows us to enable comparisons between the behavior of [Zn]:[Cd]:[FT] with that of Cd,Zn metallothioneins.