Metal ions as activators

It is also known from experiments with [y- OJATP and 0-labeled transition-state analogs of metaphosphate monoanion that P is coordinated to Mn(II) in the active complex and that the remaining three ligands to Mn(II) are water molecules (2i). The stereochemical results by this technique are in agreement with those obtained using the epimers of ADPaS and ADP/3S as substrates and various metal ions as activators (27). 

The mechanism and rate of hydrogen peroxide decomposition depend on many factors, including temperature, pH, presence or absence of a catalyst (7—10), such as metal ions, oxides, and hydroxides etc. Some common metal ions that actively support homogeneous catalysis of the decomposition include ferrous, ferric, cuprous, cupric, chromate, dichromate, molybdate, tungstate, and vanadate. For combinations, such as iron and... 

Cleavage of a peptide bond is an example of a nucleophilic attack. The nucleophile in the reaction is either an activated water molecule or part of the side-chain of an amino acid, and peptidases are described as having either a water nucleophile or a protein nucleophile. Peptidases with a water nucleophile either utilize one or two metal ions as ligands for the water molecule, in which case the peptidase generally acts... 

Under aqueous conditions, flavonoids and their glycosides will also reduce oxidants other than peroxyl radicals and may have a role in protecting membranal systems against pro-oxidants such as metal ions and activated oxygen species in the aqueous phase. Rate constants for reduction of superoxide anion show flavonoids to be more efficient than the water-soluble vitamin E analogue trolox (Jovanovic et al, 1994), see Table 16.1. 

Nucleophilic substitution reactions, to which the aromatic rings are activated by the presence of the carbonyl groups, are commonly used in the elaboration of the anthraquinone nucleus, particularly for the introduction of hydroxy and amino groups. Commonly these substitution reactions are catalysed by either boric acid or by transition metal ions. As an example, amino and hydroxy groups may be introduced into the anthraquinone system by nucleophilic displacement of sulfonic acid groups. Another example of an industrially useful nucleophilic substitution is the reaction of l-amino-4-bromoanthraquinone-2-sulfonic acid (bromamine acid) (76) with aromatic amines, as shown in Scheme 4.5, to give a series of useful water-soluble blue dyes. The displacement of bromine in these reactions is catalysed markedly by the presence of copper(n) ions. 

Metal alkoxides undergo alkoxide exchange with alcoholic compounds such as alcohols, hydro-xamic acids, and alkyl hydroperoxides. Alkyl hydroperoxides themselves do not epoxidize olefins. However, hydroperoxides coordinated to a metal ion are activated by coordination of the distal oxygen (O2) and undergo epoxidation (Scheme 1). When the olefin is an allylic alcohol, both hydroperoxide and olefin are coordinated to the metal ion and the epoxidation occurs swiftly in an intramolecular manner.22 Thus, the epoxidation of an allylic alcohol proceeds selectively in the presence of an isolated olefin.23,24 In this metal-mediated epoxidation of allylic alcohols, some alkoxide(s) (—OR) do not participate in the epoxidation. Therefore, if such bystander alkoxide(s) are replaced with optically active ones, the epoxidation is expected to be enantioselective. Indeed, Yamada et al.25 and Sharp less et al.26 independently reported the epoxidation of allylic alcohols using Mo02(acac)2 modified with V-methyl-ephedrine and VO (acac)2 modified with an optically active hydroxamic acid as the catalyst, respectively, albeit with modest enantioselectivity. 

The major uses of non-ionizing solvents in chemical analysis are twofold. They may be used simply to provide media for the dissolution and reaction of covalent materials, or they may play a more active part in a chemical process. For example, oxygen-containing organic solvents can be used to effect the solvent extraction of metal ions from acid aqueous solutions the lone pair of electrons possessed by the oxygen atom forming a dative bond with the proton followed by the extraction of the metal ion as an association complex. 

Figure 2.18 shows the range covered by different tunable solid state laser systems based on transition metal ions. As observed, a good variety of matrices have shown tunable laser action on the basis of Cr + as an active ion. The fundamental aspects determining the tunability of those Cr + based systems will be the subject of Section 6.4 in Chapter 6. 

The presence of transition metal ions as cations for the Keggin polyanion is necessary in order to develop an active and selective catalyst. 

Using Eqn. 9-4 for the activation energy, we obtain the anodic transfer current, r, of metallic ions as given by Eqn. 9-6 ... 

Some of these factors contain metal ions as redox-active components. In these cases, it is usually single electrons that are transferred, with the metal ion changing its valency. Unpaired electrons often occur in this process, but these are located in d orbitals (see p.2) and are therefore less dangerous than single electrons in non-metal atoms ( free radicals see below). 

Still with the prebiotic scenario, the conditions developed by Limtrakul et al. (1985) are interesting as a consequence of evaporation, high local concentrations may have arisen, and under such conditions the incompletely hydrated metal ions may activate a dehydration leading to peptide condensation. From this, the technique of the salt induced peptide condensation (SIPC) has been developed (Oie et al, 1983 Suwannachst and Rode, 1999 Rode et al, 1999). 

The catalytic effect is achieved through the weak Lewis acid properties of the metal ion as the active site in the metal chelate compound. The residual Lewis acid activity of aquo metal ions and incompletely coordinated metal ions in complexes and chelates in aqueous solution is actually very weak compared to that of the hydrogen ion on the other hand, metal ions and complexes are available in solution at high pH values, where the concentration of hydrogen ions is so low that their catalytic effect cannot be significant. 

The three in vitro activities of integrase require divalent metal ions as cofactors. The only two metals that support these activities are Mn2+ and Mg2+. Since quite high metal concentrations must be added to assays (1-10 m Mfor optimal activity), it has been presumed that Mg2+ is the ion used in vivo. 

Organelles within cells have their own ion-concentrating mechanisms. Thus, mitochondria can concentrate K+, Ca2+, Mg2+, and other divalent metal ions as well as dicarboxylic acids (Chapter 18). The entrance and exit of many substances from mitochondria appear to occur by exchange diffusion, i.e., by secondary active transport. Such ion exchange processes may also occur in other membranes. 

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