Biological systems coordination

The high catalytic activity of enzymes has a number of sources. Every enzyme has a particular active site configured so as to secure intimate contact with the substrate molecule (a strictly defined mutual orientation in space, a coordination of the electronic states, etc.). This results in the formation of highly reactive substrate-enzyme complexes. The influence of tfie individual enzymes also rests on the fact that they act as electron shuttles between adjacent redox systems. In biological systems one often sees multienzyme systems for chains of consecutive steps. These systems are usually built into the membranes, which secures geometric proximity of any two neighboring active sites and transfer of the product of one step to the enzyme catalyzing the next step. 

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. 

For the known nickel sites in biological systems, four-coordinate square planar, five-coordinate, and six-coordinate octahedral geometries are found.1840-1846 In general, the flexible coordination geometry of nickel causes its coordination properties in metallo-biomolecules to be critically influenced by the protein structure. 

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. 

The increased knowledge of the role of zinc in biological systems has allowed study of the coordination chemistry of small-molecule mimics and the enzymes themselves.964 A number of texts deal with biological aspects of zinc and zinc enzymes.965- 68... 

Some of the important types of coordination compounds occur in biological systems (for example, heme and chlorophyll). There are also significant applications of coordination compounds that involve their use as catalysts. The formation of coordination compounds provides the basis for several techniques in analytical chemistry. Because of the relevance of this area, an understanding of the basic theories and principles of coordination chemistry is essential for work in many related fields of chemistry. In the next few chapters, an introduction will be given to the basic principles of the chemistry of coordination compounds. 

Following the original paper, reports of the synthesis of new crowns and crown-like molecules proliferated. A typical property of these systems is their ability to form stable complexes with the alkali metal and alkaline earth ions. Prior to the synthesis of the crowns, the coordination chemistry of the above ions with organic ligands had received very little attention. A further impetus to the study of such complexes was the recognition of the important role of Na+, K+, Mg2+ and Ca2+ ions in biological systems. 

Cryptands of the type (217)-(220) tend to form stable complexes with a number of heavy metal ions. Of particular interest is the selectivity of (219) for Cd(n) the complex of this metal is approximately 106-107 times more stable than its complexes with either Zn(n) or Ca(n). This reagent may prove useful for removing toxic Cd(n) from biological systems as well as for other applications involving sequestration of this ion (for example, in antipollution systems). The selectivity observed in the above case appears to arise because (i) the nitrogen sites favour coordination to Zn(n) and Cd(n) relative to Ca(n) and (ii) the cavity size favours coordination of Cd(n) relative to Zn(n). 

CaMo508 provides a link to calcium channels in biological systems. This mixed oxide contains Mo/O columns, of which groups of four define channels in which the Ca2+ cations reside. There is no clearly defined coordination number for these cations Ca-0 distances range from 2.39 A upwards, there being ten oxygens at fairly regularly spaced distances up to 3.04 A from the Ca2+ (117). 

The binding energy per N-site in 44-BP-metal coordination complexes amounts to 60-120 kJ mol i.e., it ranges between strong covalent bonding and weak bonds in biological systems [292],... 

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