Zinc(II), cadmium(II), and mercury(II) thiolate transitions in metallothionein.

The metal-specific absorption envelopes of zinc-, cadmium-, and mercury-metallothioneins and of complexes of these metal ions with 2-mercaptoethanol have been analyzed in terms of Jørgensen's electronegativity theory for charge-transfer excitations by using the spectra of zinc(II), cadmium(II), and mercury(II) tetrahalides as references. By Gaussian analysis the difference absorption spectra of the various forms of metallothionein vs. thionein and of the corresponding 2-mercaptoethanol complexes vs. 2-mercaptoethanol were resolved into three components. For each metal derivative the location of the lowest energy band is in good agreement with the position of the first ligand-metal charge-transfer (LMCT) transition (type t2 leads to a1) predicted from the optical electronegativity difference of the thiolate ligands and of the central metal ion by assuming tetrahedral coordination. There is also a correspondence between the effects of the metal ion on the position of the first LMCT band and the binding energy of the 2p electrons of the sulfur ligands as found by X-ray photoelectron spectroscopic measurements [Sokolowski, G., Pilz, W., & Weser, U. (1974) FEBS Lett. 48, 222]. Due to the lack of exact structural information, the assignment of the two other resolved metal-dependent bands remains conjectural, but it is likely that they include a second LMCT transition (type t2 leads to a1) analogous to that occurring in tetrahalide complexes of group-2B metal ions. The simplicity of the resolved thiolate spectra and their correspondence to those of tetrahedral models support the view that the various metal-binding sites of metallothionein are chemically similar and that the coordination environment of the metal ion has a symmetry related to that of a tetrahedron [Vasák, M. (1980) J. Am. Chem. Soc. 102, 3953].