Metal-chelating substances

Inhibitors of COMT are frequently competitive substrates, such as pyrogallol. No potent irreversible inhibitors of the enzyme are available. Figure 8 shows some of the substances which are effective inhibitors in vitro. 8-hydroxyquinoline is slightly more potent than pyrogallol as an inhibitor, but other metal chelating substances, such as EDTA, are inactive. The tropolone compounds, 4-methyl-tropolone and 4-isopropyltropolone ( -thujaplicin) are isosteric with catechols and can probably form a complex with magnesium ions and the active centre of the enzyme. 3,4 Dihydroxy-a-methylpropiophenone (U-0521) has recently been found to be a powerful competitive inhibitor both in vitro and in vivo. 

Pentanedione is widely used in extraction processes for the separation and purification of metals because of its abiUty to form covalent metal chelates. It is also used as an intermediate in the production of heterocycHc substances and dyes, as a fuel additive (324), and in metal plating and resin modification. 

In metal chelate adsorption chromatography a metal is immobilised by partial chelation on a column which contains bi- or tri- dentate ligands. Its application is in the separation of substances which can complex with the bound metals and depends on the stability constants of the various ligands (Porath, Carlsson, Olsson and Belfrage Nature 258 598 I975 Loennerdal, Carlsson and Porath FEES Lett 75 89 1977). 

In the case of inorganic solutes we are concerned largely with samples in aqueous solution so that it is necessary to produce substances, such as neutral metal chelates and ion-association complexes, which are capable of extraction into organic solvents. For organic solutes, however, the extraction system may sometimes involve two immiscible organic solvents rather than the aqueous-organic type of extraction. 

Metal-chelate affinity chromatography is a powerful purification technique whereby proteins or other molecules can be separated based upon their ability to form coordination complexes with immobilized metal ions (Porath et al., 1975 Lonnerdal and Keen, 1982 Porath and Belew, 1983 Porath and Olin, 1983 Sulkowski, 1985 Kagedal, 1989). The metal ions are stabilized on a matrix through the use of chelating compounds which usually have multivalent points of interaction with the metal atoms. To form useful affinity supports, these metal ion complexes must have some free or weakly associated and exchangeable coordination sites. These exchangeable sites then can form complexes with coordination sites on proteins or other molecules. Substances that are able to interact with the immobilized metals will bind and be retained on... 

In 1976, Marion Bradford introduced the first Coomassie dye-based reagent for the rapid colorimetric detection and quantitation of total protein. The Coomassie dye (Bradford) protein assay reagents have the advantage of being compatible with most salts, solvents, buffers, thiols, reducing substances, and metal chelating agents encountered in protein samples. 

We will not go in depth into the subject of antioxidants (12), which is more a part of preformulation than a stress test, but the autoxidation mechanism does suggest that oxidation can be inhibited by peroxy radical scavengers (chain-breaking antioxidants) like phenol antioxidants, by heavy metal chelating agents, and by peroxide inactivating substances (preventive antioxidants). 

Affinity Chromatography was initially defined as a method based on specific and reversible molecular interactions between biologically active substances. However, the method has extended to non-biological stationary phases, such as metal-chelate complexes. This technique is used for separation and purification of proteins and other biologic materials, such as viruses and cells.47,48 A survey of various stationary phases and affinity interactions is given in figure 8.2. 

Based upon the use of nonionic surfactant systems and their cloud point phase separation behavior, several simple, practical, and efficient extraction methods have been proposed for the separation, concentration, and/or purification of a variety of substances including metal ions, proteins, and organic substances (429-441. 443.444). The use of nonionic micelles in this regard was first described and pioneered by Watanabe and co-workers who applied the approach to the separation and enrichment of metal ions (as metal chelates) (429-435). That is, metal ions in solution were converted to sparingly water soluble metal chelates which were then solubilized by addition of nonionic surfactant micelles subsequent to separation by the cloud point technique. Table XVII summarizes data available in the literature demonstrating the potential of the method for the separation of metal ions. As can be seen, factors of up to forty have been reported for the concentration effect of the separated metals. 

Rocks colonized by lichens weather faster than uncolonized rock surfaces, as a direct result of metal chelation by lichen substances and organic acids (Fig. 15.3). Fomina et al. and Smits (Chapters 10 and 13, this volume) provide comprehensive discussions of the effects of fungi on mineral weathering processes, and therefore only a brief summary of the effects of lichens on this phenomenon will be presented here. Essentially, lichen colonization can influence the rate of silicate weathering through... 

The theory of the H bond remains one of the frontiers of our knowledge of chemical bonding. H bonding compounds are one of the classes of chemical substances which involve weak bonds and display unusual valence properties (e.g., the polyhalogens, metal chelates, boranes, AbCle). The extent to which these situations are similar cannot be stated until theoretical developments advance considerably. 

If the differences in the internal and solvation entropies of these substances are neglected, the only remaining entropy would be the translational entropy, and becomes the entropy of formation of one mole of solute at unit molality, corresponding to the formation of one metal chelate ring. For the formation of x chelate rings the translational entropy increase is given by ... 

The tendency of substances with the same ligand or organic group such as metal chelates or organometallic compounds to separate offers possibilities for multielement analysis. Gas-chromatographic techniques can be utilized as reference methods, especially for the ultra-trace analysis of ppb- and lower levels. 

The group of metal chelates with /3-diketones and similar ligands can be extracted from weakly acid aqueous solutions under set pH conditions with an organic solvent, in which the chelate-forming substance is soluted. Excess reagent can be re-extracted in the aqueous phase with diluted sodium hydroxide the metal chelate remains in the organic solvent. Polar metal chelates require packed columns with deactivated supporting materials because of their temperature sensitivity. 

For non-volatile metal chelates the practibility of indirect determination overcomes if the substances are to be decomposed (pyrrolized) in defmited products. Instances for this approach are given, using Ru and Ni. The metal chelates are first separated by thin-layer chromatography (Table 1.1/IV). 

Numerous papers deal with the separation of ions, inorganic substances and metal chelates an layers using paper and thin-layer chromatography However, only... 

---