Metal complexes of sulphur-containing amino acids

Cadmium(II) ions form complexes with all natural amino acids and peptides. The thermodynamic stabilities of the cadmium(II) complexes of the most common amino acids and peptides are generally lower than those of the corresponding zinc(II) complexes, except the complexes of thiolate ligands. The coordination geometry of the cadmium(II) amino acid complexes is generally octahedral with the involvement of the amino and carboxylate groups in metal binding. In the case of simple peptides, both octahedral and tetrahedral complexes can be formed depending on the steric conditions. The terminal amino group and the subsequent carbonyl-O atom are the primary binding sites and there is no example for cadmium(II)-induced peptide amide deprotonation and coordination. The various hydrophobic and polar side chains do not have a significant impact on the structural and thermodynamic parameters of cadmium(II) complexes of amino acids and peptides. β-carboxylate function of aspartic acid and imidazole-N donors of histidyl residues slightly enhance the thermodynamic stability of cadmium(II)-peptide complexes. The most remarkable effects of side chains are, however, connected to the involvement of thiolate residues in cadmium(II) binding. Stability constants of the cadmium(II) complexes of both L-cysteine and its peptides and related ligands are significantly higher than those of the zinc(II) complexes. Thiolate donor functions can be bridging ligands too, resulting in the formation of polynuclear cadmium(II) complexes.

Cadmium(II) Complexes of Amino Acids and Peptides

The coordination chemistry of mercury has received rather less attention than that of the later transition elements, due in part to the fact that as a d10 metal ion Hg2+ exhibits neither paramagnetism nor ‘d-d’ spectra, the study of which have provided much of the impetus in other areas of coordination chemistry. However, the current interest in the biological properties of mercury clearly points to the need for a more complete understanding of the ability of mercury to bind various donor atoms and of the resulting stereochemistry.

The coordination chemistry of mercury

The local electronic structure at Zn2+ ions during complex formation with histidine and cysteine has been studied by near edge x-ray absorption fine structure spectroscopy at the Zn L-edge. At pH ≅ 5 both histidine and cysteine have carboxylate and amino groups able to form complexes with cations in solution such as Zn2+. Compared to histidine, cysteine has an extra thiolate group which can chelate Zn2+. This investigation shows that histidine is chelating the Zn2+ ion mainly via the amino group, while cysteine chelates primarily via the thiolate group. The nature of the empty molecular orbitals involved in the transitions has been analyzed using density functional theory including the solvent effects. By comparing the calculated results with the experimental observations, we conclude that histidine affects primarily the Zn2+ electronic states of d-symmetry, while cysteine affects both s- and d-states. This mechanism is important for understanding the zinc sulfhydryl bond in zinc finger proteins, where cysteine and histidine in the protein are chelating Zn2+.

Effect of Cysteine vs. Histidine on the Electronic Structure of Zn2+ Upon Complex Formation

CU(II) Catalyzed Hydrolysis of an Unactivated Ester Based on Reversible Conjugate Addition

The proton ionisation constants of cysteine methyl ester have been determined at 25°, 37°, and 50° and I= 0·1M(KCl). Values of ΔH° and ΔS°298 for the various ionisation equilibria have been calculated. Stability constants have been determined potentiometrically at 25° and I= 0·1M for complexes present in aqueous solutions of cysteine methyl ester and the bivalent ions of nickel, zinc, cadmium, mercury, and lead. In the case of nickel(II) the complexes NiL+, NiL2, NiL3–, Ni4L62–, NiHL2+, and NiHL2+ occur. Confirmatory evidence for the existence of polynuclear species in this system has been obtained spectrophotometrically. Polynuclear complexes do not occur with zinc(II) and lead(II), but protonated species (ZnHL2+, ZnHL2+) and hydrolysed species [ZnL(OH)] occur with zinc in addition to simple mononuclear complexes. Protonated complexes also occur with lead(II). A polynuclear complex Cd2L3+ occurs with cadmium, in addition to the simple mono- and bis-species. In the case of mercury(II) it is suggested that the complex HgL+ is completely formed at the commencement of pH-tritrations at a 1 : 2 metal-to-ligand ratio (i.e., log β1 is very large). Using this assumption the titration data can be satisfactorily described by the set of complexes HgL2, Hg2L3+, and Hg3L42+. A number of solid complexes have been isolated and their infrared spectra studied. Of particular interest is the isolation of a number of trinuclear complexes of the type [M{Ni[SCH2CH(NH2)COCH3]2}2]MCl4 where M = Zn(II), Cd(II), and Hg(II).

R. W. Hay

Papers

The interaction of cysteine methyl ester with metal ions. Part I. The nature of the complex species