Ligands in biology

In this section, it has been stressed that strain and conflict of interest between the central atom and ligands is often a feature of catalysts. There is reason to believe that this applies in biological systems as well. The ligands in biological coordination compounds are usually very complex, and the subtleties of their conformational requirements can impose strain on the central atom and its immediate environment. Equally, coordination to a central atom can affect the reactivity of the ligand. Some examples will illustrate these points. 

Evans, G., 3ohnson, P.E., Brushmiiler and Ames, R.W., 1979. Detection of labile zincbinding ligands in biological fluids by modified gel filtration chromatography. Anal. Chem., 51 839-843. 

As shown in Figure 10.12, four of the six chelation sites of the cobalt atom of cobalamin are occupied by the nitrogens of the corrin ring and one by the nitrogen of the dimethylbenzimidazole side chain. The sixth site may be occupied by the following ligands in biologically active vitamers ... 

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...

Zinc associated with purified enolase molecules appears to involve complex formations with or without chelation. Most of the zinc associated with the enolase molecules can be removed by electrodialysis and restored to the molecule, even after denaturation. In contrast, one of the zinc atoms is tightly bound and cannot be removed by electrodialysis, nor can it be replaced in the enolase molecule after its denaturation with 4 M urea. The last of these observations illustrates one important property of the protein ligand in biological chelation. 

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. 

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