Transition metal ions in biological systems

Only transition-metal ions are redox-active and hence are possible sources of radicals. Ions such as Mg2+, Ca2+, Zn2+ etc. are not important in this context. By far the most important is iron, with copper, molybdenum, cobalt and nickel also participating on a more minor scale. Of course, any transition metal ion may be ingested, and hence may be a fortuitous source of radical formation and damage. We focus attention on iron for illustrative purposes. 

The concentrations of iron as simple solvated ions, Fe2+aq or Fe3+aq, are maintained at extremely low levels because of their damaging ability in the presence of oxygen and hydrogen peroxide. In transferrin, an important iron scavenger, the iron is well-protected and is not involved in redox chemistry. Also, in the major storage proteins, ferritin and haemosiderin, the iron is present in crystalline material inside the protein shell, and is well protected from reaction. 

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A number of texts are now available on bioinorganic chemistry but ref. 26 provides a useful emphasis and account of the role of transition metal ions in biological systems. A number of texts - and reviews " on copper proteins are now available and individual chapters on the different functions of the copper proteins are contained in the various volumes of Metal Ions in Biological Systems edit by H. Sigel in particular, volumes 12 and 13 are almost exclusively devoted to c( per. In biological systems copper is the third most abundant transition metal element, with an occurrence of 80-120 mg in a normal human body (70 kg), compared to values of 4-5 g for iron and 1.4-2.3 g for zinc. It generally occurs in the copper(II) oxidation state, but is believed to involve a copper(I) oxidation state in deoxyhemocyanin (Section 53.3.5) and the copper(IIl) oxidation state has been invoked in mechanistic studies (Section 53.5). ... 

Tables 1.2-1.6 list some of the important geometries assumed by metal ions in biological systems. Common geometries adopted by transition metal ions that will...

Tables 1.2-1.6 list some of the important geometries assumed by metal ions in biological systems. Common geometries adopted by transition metal ions that will be of most concern to readers of this text are illustrated in Figure 1.3. It is important to remember that in biological systems these geometries are usually distorted in both bond length and bond angle.

M. T. Beck, Prebiotic Coordination Chemistry The Possible Role of Transition Metal Complexes in the Chemical Evolution , in Metal Ions in Biological Systems , ed. H. Sigel, Dekker, New York, 1978, vol. 7. 

Group 11 (or IB) contains copper, which is the third most common transition metal found in biological systems. Copper in solution has two stable oxidation states, cuprous (Cu1+) and cupric (Cu2+) ion. The ability of copper to easily accept and donate electrons explains its important role in oxidation-reduction reactions and... 

In this chapter, the unique features of transition metals in biological systems are discussed from the point of view of structural roles, spectroscopic properties, electron transfer, hydrolytic and redox catalysis, and metal-responsive gene expression. The following chapters provide more detail on these subjects. Several important examples not discussed elsewhere in this volume will be presented. The goal of this chapter (and this volume) is to acquaint the reader with the wide range of roles played by metal ions in biological systems and thereby to demonstrate why metals are such useful cofactors and why scientists from such broad disciplines are drawn to study their properties. 

Kozelka, J. "Molecular Modeling of Transition Metal Complexes with Nucleic Acids and Their Constituents." In Metal Ions in Biological Systems Sigel A., Sigel. H., (eds.) Marcel Dekker, Inc. New York, Basel, Hong Kong, 1996 Vol. 33 pp 2. 

Interactions of histidine and other imidazole derivatives with transition metal ions in chemical and biological systems. R. J. Sundberg and R. B. Martin, Chem. Rev., 1974, 74, 471-517 (517). 

R.J. Sundherg u. R.B. Martin, Interaction of Histidine and Other Imidazole Dervatives with Transition Metal Ions in Chemical and Biological Systems, Chem. Rev. 74, 471 -517 (1974). 

Not all complexes are purely electrostatic. In fact, many metal complexes in biological systems have covalent interactions as well. In these cases, the ligand donates a pair of electrons (acting as a Lewis base) to the metal, which functions as a Lewis acid. Therefore, metals can be evaluated based on their abilities to accept electron pairs. Alkali metal ions (Na+, K+) and alkaline earth metals (Mg2+, Ca2+) tend to not form stable complexes with Lewis base ligands. Transition metal ions, particularly those with vacant ri-orbitals, will form more stable complexes with Lewis base-acting ligands. 

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