Transition Metal Complexes in Biological Systems

Hemoglobin consists of four protein chains, each with a bound heme complex. (Illustration by Irving Geis. Rights owned by Howard Hughes Medical Institute. Not to be used without permission.) B, In the 

Carbon monoxide is toxic because it binds to Fe ion in heme about 200 times more strongly than O2, which prevents the heme group from functioning  

Like O2, CO is a strong-field ligand, which results in a bright red color of the blood. Because the binding is an equilibrium process, breathing extremely high concentrations of O2 displaces CO from the heme and reverses CO poisoning. 

Valence bond theory pictures bonding in complex ions as arising from coordinate covalent bonding between Lewis bases (ligands) and Lewis acids (metal ions). Ligand lone pairs occupy hybridized metal-ion orbitals to form complex ions with characteristic shapes. 

CHAPTER 22 The Transition Elements and Their Coordination Compounds 

For Review and Reference (Numbers in parentheses refer to pages, unless noted otherwise.) 

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Crystal Field Theory 752 Transition Metal Complexes in Biological Systems 757... 

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. 

The inertness snggests the reason that Mo is the only essential transition metal fonnd in biological systems that is not a first-row transition metal. Most second- and third-row transition metals appear to be too strongly bound and inert to engage in metabolic processes (Hoeschele et al. 1991). However, the inertness of some second- and third-row elements, such as Pt(II), can be useful for the chemotherapeutic activity of these complexes. The inertness probably plays a role also in the mutagenicity and mild carcinogenicity observed for cisplatin and the mutagenicity of other inert complexes, especially Cr(III) complexes. 

The electron transfer rates in biological systems differ from those between small transition metal complexes in solution because the electron transfer is generally long-range, often greater than 10 A [1]. For long-range transfer (the nonadiabatic limit), the rate constant is... 

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. 

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. 

EPR spectroscopy is widely used to study the structure and geometry of d-transition metal ions (TMI) in inorganic complexes, in biological systems and in catalysts. Information obtained through analysis of the spectrum varies from simple identification of the metal centre to a thorough description on the electronic structure of the complex. At a simple level, the main interactions experienced by the TMI that consequently influence the EPR spectrum are (i) the electronic Zeeman effect and (ii) interaction between the electron and the nuclear spin. In the first case, the interaction is expressed via the g tensor which carries information on electronic structure in the second case, the metal hyperfine interaction is expressed via the A tensor. 

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