Chelates of metals

Many reactions catalyzed by the addition of simple metal ions involve chelation of the metal. The familiar autocatalysis of the oxidation of oxalate by permanganate results from the chelation of the oxalate and Mn (III) from the permanganate. Oxidation of ascorbic acid [50-81-7] C HgO, is catalyzed by copper (12). The stabilization of preparations containing ascorbic acid by the addition of a chelant appears to be negative catalysis of the oxidation but results from the sequestration of the copper. Many such inhibitions are the result of sequestration. Catalysis by chelation of metal ions with a reactant is usually accomphshed by polarization of the molecule, faciUtation of electron transfer by the metal, or orientation of reactants. 

Many studies have been undertaken to establish the structural criteria for the activity of polyhydroxy flavonoids in enhancing the stability of fatty acid dispersions, lipids, oils, and LDL. " As for phenolic acids, the inhibition of oxidation by flavonoids is related to the chelation of metal ions via the... 

In the ligand l,2-bis-(2-pyridylethynyl)benzene the pyridyl N atoms easily attain the appropriate separation for trans-chelation of metal cations. The 1 1 complex of the ligand with palladium(II) chloride has been structurally characterized.171,182... 

Carnosine is an avid chelator of metal ions (Baran, 2000). Complexes with calcium, copper, and zinc ions have been described (Trombley et ah, 2000). It is possible, therefore, that carnosine could exert some sort of control of calcium metabolism in muscle tissue (heart or skeletal). It is also likely that the dipeptide controls the availability of zinc ions in neuronal tissue, especially the olfactory lobe where both carnosine and zinc are enriched (Bakardjiev, 1997 Bonfanti et ah, 1999 Sassoe-Pognetto et ah, 1993). Zinc-camosine complexes, called polaprezinc, are also effective in the repair of ulcers and other lesions in the alimentary tract (Matsukura and Tanaka, 2000). 

Bio)chemical reactions may take place prior to or after the continuous separation module and are intended to enhance or facilitate mass transfer, detection or both. The earliest and simplest approach to integrated analytical steps in continuous-flow systems involves a combination of chemical reactions and continuous separations [4,5]. Such is the case with the formation of soluble organic chelates of metal ions in liquid-liquid extractions with the ligand initially dissolved in the organic stream [6], the formation and dissolution of precipitates [7], the formation of volatile reaction products in gas difiusion [8] and that of volatile hydrides in atomic absorption spectro-... 

Antioxidant/anti-radical activity and chelation of metal ions... 

Antioxidant and anti-radical activity, chelation of metal ion... 

The relationships between heat of sublimation, covalency, and solubility in organic solvents can be extrapolated in other directions. It has been pointed out that there is a correlation of solubility in organic solvents and volatility (29), and that chelates of metals and other organic derivatives are soluble in organic solvents. The heat of sublimation of neodymium-... 

Ethylenediamine tetraacetic acid (EDTA) was introduced originally as a water-softener and as a textile dyeing assistant because of its ability to form very stable, water soluble complexes with many metal ions, including calcium and magnesium. The equilibria involved in chelation of metal ions by EDTA and related ligands have been exhaustively studied, notably by G. Schwarzenbach and his colleagues, and provide the basis for complexometric methods of chemical analysis. EDTA and its metal complexes have also become probably the most familiar examples of agents used in chelation therapy. 

CA s responsive to p02 can be imagined with chelates of metals that can switch between two redox states. The principle has been demonstrated for the system [MnII/m(TPPS)]4/3 [179].The relaxivity of Mnm is dominated by the elec-... 

The general interests in chelation of metals have continued to lead to new forms of polyaminopolycarboxylic acids. Examination of the acid dissociation constant of ethyl-ene-bis-N,N -(2,6-dicarboxy)piperidine (9) showed that they were nearly identical with those of EDTA. However, the formation constants for metal chelates of (9) are lower by 0.5 to 3.7 log 3 units. 

Catsch, A., Harmuth-Hoene, A. E., Mellor, D. P. International Encyclopedia of Pharmacology and therapeutic applications of agents The Chelation of Metal Heavy Metals (ed. Sartorelli, A. C.), Oxford, Pergamon Press 1979... 

For extraction with supercritical carbon dioxide, metals are first chelated with a ligand such as a derivative of dithiocarbamate. It has been found [96] that while the solubility of chelates of metals with sodium diethyl dithio-... 

The metal-free proteins do not exhibit any physical properties which are not common to other globular proteins of medium molecular size. In fact, the first transferrin studied, chicken ovotransferrin, was considered for many years to be a rather nondescript egg-white protein. There are several striking changes in the physical properties of transferrins upon chelation of metal ions, but most of these properties are so similar to those of the metal-free protein that many of the physical studies have not been useful in attempting to understand the mechanism of metal binding. Several important exceptions will be described in a subsequent section. 

Extensive studies have been made of the charge relationships and electrostatic properties of ovotransferrin, and a few studies have been made on other transferrins. Several of these primarily concern the changes seen on chelation of metal ions, which will be discussed below. Most of the transferrins have isoelectric points around pH 6 (Fig. 2), but the isoelectric point may vary considerably with species and may show taxonomic relationships (28). These will be discussed below under genetic relationships. 

Workers in the laboratory of R. C. Warner (133, 139) have examined many different aspects of the charge relationships of chicken ovotrans-ferrins. Fig. 3 is a reproduction of titration curves of ovotransferrin at different temperatures and different ionic strengths. Other data from the exacting experiments of these investigators should be consulted by those interested in the general physical chemistry of proteins. The data particularly pertinent to the chelation of metal ions will be discussed in subsequent sections. 

Azari and Feeney (7) suggested that human serum transferrin and chicken ovotransferrin underwent structural changes on chelation of metal ions which stabilized the molecule to denaturation and proteolysis. This later was extended (8) to include stabilization to chemical cleavage or modification. A similar interpretation also was made by Glazer and McKenzie (55) who further suggested that the iron complex might provide two crosslinks between widely separated sections of the peptide chains. 

From our studies, it can be assumed that Al is likely to play a role in switching off the FMR1 gene by inducing a helical transition from B- to 2-DNA, in addition to the factors involved in the massive expansion of CCG triplet repeats and hypermethylation. This may provide a clue for the discovery of new drugs which convert Z- to B-DNA. It is hypothesized that chelation of metal ions like aluminum, demethylation of the triplet repeats, and acetylation of histones - if achieved - may make it possible to switch on the FMR 1 gene. 

Intracellular distribution of essential transition metals is mediated by specific metallochaperones and transporters localized in endomembranes. In other words, the major processes involved in hyperaccumulation of trace metals from the contaminated medium to the shoots by hyperaccumulators as proposed by Yang et al. (2005) include bioactivation of metals in the rhizosphere through root-microbial interaction enhanced uptake by metal transporters in the plasma membranes detoxification of metals by distributing metals to the apoplasts such as binding to cell walls and chelation of metals in the cytoplasm with various ligands (such as PCs, metallothioneins, metal-binding proteins) and sequestration of metals into the vacuole by tonoplast-located transporters. 

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