Biological systems ligands

Biological systems Ligand-myoglobin Protein dynamics Bacteriorhodopsin Light harvesting Pigment-protein complexes Photosynthetic reaction centers... 

Fig. 6-4. Chelate compounds in biological systems. Ligand attachment in peroxidase (left) and hemoglobin (right), from G.L. Eich-horn, Federation Proceedings, 20, 48 (1961)...

Vitamin B12 (Fig. 1) is defined as a group of cobalt-containing conoids known as cobalamins. The common features of the vitamers are a corrin ting (four reduced pyrrole rings) with cobalt as the central atom, a nucleotide-like compound and a variable ligand. Vitamin B12 is exceptional in as far as it is the only vitamin containing a metal-ion. The vitamers present in biological systems are hydroxo-, aquo-, methyl-, and 5 -deoxyadenosylcobalamin. 

The ligands were designed to be specific complexing agents for Pu+i from biological systems (25). Another specific complexing... 

The ideas presented here merely scratch the surface of factors that control metal ion selectivity in biological systems. It is hoped that in future the picture will become even clearer, enabling us to learn much more about ligand design and selective metal ion complexation. 

Pyrazolate as a ligand can exhibit various coordination modes, inter alia, monodentate, exo-bidentate, and endo-bidentate. Several review articles have been published in the field.556-559 The use of pyrazole in the modeling of biological systems was also reviewed.560,561 The most common application of pyrazole ligands is to use its exo-bidentate coordination mode linking two metal centers that may be identical or different (compare Section 6.3.4.12.6). Polypyrazolylborates and related ligands are covered in the subsequent section. 

Imidazole is of particular relevance to biological mimic ligands due to the presence of histidine as a coordinating group for zinc in biological systems and has been a particular target for zinc complex formation. 

Fig. 3. Ligand donor atoms. The atoms in open blocks are the most common donors, those that axe singly hatched are also common but are very improbable in a biological system, and the doubly hatched donor, F, is of very rare occurrence...

A major consideration before the ligand exchange equilibria can be considered with reference to biological systems is the stability of a particular oxidation state in the biological medium. Low-spin complexes undergo rapid one-electron oxidation and reduction. As a biological system operates at a low redox potential, say —0.5 to 0.0 volts, reduced, i.e. low valence, states of the metals are to be expected. The metal complexes, Ru, Os, Rh, Ir, Pd, Pt and Au should be reduced to the metallic state in fact but for the slow speed of this reduction. The metals of Fig. 6 will tend to go to the following redox states ... 

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