Ligand in biological systems

Excitation of proteins, nucleic acids, or organic cofactors acting as ligands in biological systems frequently involves individual functional groups or isolated conjugated n-systems. When a metal is coordinated to such moieties, the resulting molecular orbitals are usually classified as predominately located at the... 

The significance of [Fe4S4]n+ (193) and [Mn4X4]"+ (194) (where X = oxo, chloro, or alkoxo ligands) in biological systems, has made the synthesis and properties of iron and manganese alkoxide cubes... 

The ready availability of thiolate ligands in biological systems implies that thiolate complexes dominate Hg(II) coordination in biological systems. Despite some controversy regarding the accuracy of formation constants for Hg-thiolate complexes, extremely large formation constants are consistently reported in the literature. Recent results suggest that Hg(II) ap-... 

Frequently found metal-binding ligands in biological systems are ... 

Table 4.3 Overview of ligands in biological systems yes = proven binding site prob = probable binding site a suspected binding site...

A comparison of values for bi- (n = 3) and hexadentate (n = 1) chelators can be misleading. For example, log P of deferiprone is 35.9 but the log of the third stepwise formation constant given by log(P /p2) is only 9.7 (Motekaitis and Martell 1991). Also, this definition of stability constant does not take into account the different acidities of the ligands and the ability of iron to compete for them with proton. Protonation of the ligand and hydrolysis of the metal, as well as competition with other metals and ligands in biological systems, complicate the interpretation of stability constants. Therefore, in comparing the stability of iron chelates it is useful to introduce the additional terms iTeff and pM. Martell has defined an effective stability constant for Fe complexes based on competition for the... 

A further application of TLRC in the biochemical field is in the assessment of the activity of enzymes, where TLRC can be used to monitor and quantitate the products during the course of an enzyme reaction such as deiodination. Other examples of the use of TLRC in biochemical studies are the qualitative assessment of the complex-forming ability of metals and ligands in biological systems and the complex formation of heavy metal chelates, such as molybdenum species with poly-aminocarboxylic acids. 

This is a consequence of the peculiar properties of Cd " presented above (Sections 1 and 2), and the abundance and variety of potential ligands in biological systems. 

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 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. 

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. 

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