Coordination complexes biological importance

If the plasma membrane is relatively impermeable, how do most molecules enter a cell How is selectivity of this movement established Answers to such questions are important in understanding how cells adjust to a constantly changing extracellular environment. Metazoan organisms also must have means of communicating between adjacent and distant cells, so that complex biologic processes can be coordinated. These signals must arrive at and be transmitted by the membrane, or they must be generated as a consequence of some interaction with the membrane. Some of the major mechanisms used to accomplish these different objectives are listed in Table 41-3. 

The geometry of the coordination compounds can be similarly predicted based on the coordination number of the central atom. Coordination numbers 2 and 3 are both relatively rare and give linear and planar or pyramidal geometries, respectively. The most important coordination numbers are 4, 5 and 6 with the latter being the most important one as nearly all cations form 6-coordinate complexes. Table 2.4 shows the geometries corresponding to the commonest coordination numbers in biological systems. 

A number of reviews are available in this area several complexes with amino adds are covered in recent general reviews on ligands of biological importance.236,237 The coordination chemistry of L-cysteine and D-penicillamine238 and glutathione239 has been reviewed. 

Researchers J, Versieck and L. Vanballenberghe (University Hospital, Ghent. Belgium) have observed, Tin has chemical properties offering potentials for a biological function, The element has a tendency to form truly covalent linkages as well as coordination complexes hence, it was hypothesized that it could well contribute to the tertiary structure of proteins or other biologically important macromolcculcs, such as nucleic acids. 

The kinetic acidity of the methylene protons on coordinated amino acid fragments is enhanced in imine complexes such as those derived from the ligands (116)307 and (117).308 This enhanced acidity is responsible for both racemization309 310 and transamination311-313 processes, which have been studied in detail because of the biologically important pyridoxal-activated enzyme... 

It is necessary to pay attention to the fact that, during recent years, the bioinorganic properties of aluminum coordination compounds have become the objects of detailed study their general aspect [354], metabolism and toxicology [355,356], complex formation with nucleozides of di- and triphosphates and nucleo-zide-bound proteins [357], and x-ray analysis of biologically important complexes... 

Many 119Sn NMR data exist for tin complexes with ligands of biological importance and activity such as in vitro antitumor activity or antimicrobial activity229-232,365 -381. The exact nature of the coordination sphere around the tin is not always given. 

There is a particularly extensive and rich coordination chemistry associated with iron(II) N donor macrocycles. The coordination chemistry of the biologically important iron porphyrin complexes has been of interest since the classic studies of Fischer, and over recent years there has been a resurgence of work on these and also on a wide range of related synthetic macrocyclic complexes. This section concentrates on their coordination chemistry, and where appropriate highlights enhanced ligand reactivity specifically induced by the iron(II) centre. First saturated ligands are discussed and then the unsaturated systems, with the particularly well-studied porphyrins and phthalocyanines being dealt with in separate subsections. 

The iron porphyrins and related compounds constitute an extremely important class of coordination complex due to their chemical behaviour and involvement in a number of vital biological systems. Over recent years a vast amount of work on them has been published. Chapter 21.1 deals with the general coordination chemistry of metal porphyrins, hydroporphyrins, azaporphyrins, phthalocyanines, corroles, and corrins. Low oxidation state iron porphyrin complexes are discussed in Section 44.1.4.5 and those containing nitric oxide in Section 44.1.4.7, while a later section in this chapter (44.2.9.2) is mainly concerned with iron(III) and higher oxidation state porphyrin complexes. Inevitably however, a considerable amount of information on iron(II) complexes is contained in that section as well as in Chapter 21.1. Therefore in order to prevent excessive duplication, the present section is restricted to highlighting some of the more important aspects of the coordination chemistry of the iron(II) porphyrins while the related unusually stable phthalocyanine complexes are discussed in the previous section. 

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