Metal-ion activation

Page 1170 (Figure 28 5) is adapted from crystallographic coordinates deposited with the Protein Data Bank PDB ID IDDN White A Ding X Vanderspek J C Murphy J R Ringe D Structure of the Metal Ion Activated Diphtheria Toxin Re pressor/Tox Operator Complex Nature 394 p 502 (1998)... 

The increased acidity of the larger polymers most likely leads to this reduction in metal ion activity through easier development of active bonding sites in siUcate polymers. Thus, it could be expected that interaction constants between metal ions and polymer sdanol sites vary as a function of time and the sihcate polymer size. The interaction of cations with a siUcate anion leads to a reduction in pH. This produces larger siUcate anions, which in turn increases the complexation of metal ions. Therefore, the metal ion distribution in an amorphous metal sihcate particle is expected to be nonhomogeneous. It is not known whether this occurs, but it is clear that metal ions and siUcates react in a complex process that is comparable to metal ion hydrolysis. The products of the reactions of soluble siUcates with metal salts in concentrated solutions at ambient temperature are considered to be complex mixtures of metal ions and/or metal hydroxides, coagulated coUoidal size siUca species, and siUca gels. 

T.G. Spiro (ed.). Metal Ion Activation of Dioxygen, Wiley-Interscience, New York, 1980, 247 pp. 

The first mechanism was proposed by Baird et al. [189]. The carbocationic species is schematically shown in Scheme IX. The attack of monomer, well known on the carbocationic center of a metal-ion-activated olefin, proceeded in the normal manner for carbocationic polymerization. This mechanism is based on the following two evidences. Alcoholysis of the polymerization system, TiMe3Cp /B(C6F5)3, resulted in the presence of an alkoxy group at an end group, and vinyl ethers and iV-vinylcarbazole were polymerized by using the same system. 

The metal ion in electroless solutions may be significantly complexed as discussed earlier. Not all of the metal ion species in solution will be active for electroless deposition, possibly only the uncomplexed, or aquo-ions hexaquo in the case of Ni2+, and perhaps the ML or M2L2 type complexes. Hence, the concentration of active metal ions may be much less than the overall concentration of metal ions. This raises the possibility that diffusion of metal ions active for the reduction reaction could be a significant factor in the electroless reaction in cases where the patterned elements undergoing deposition are smaller than the linear, or planar, diffusion layer thickness of these ions. In such instances, due to nonlinear diffusion, there is more efficient mass transport of metal ion to the smaller features than to large area (relative to the diffusion layer thickness) features. Thus, neglecting for the moment the opposite effects of additives and dissolved 02, the deposit thickness will tend to be greater on the smaller features, and deposit composition may be nonuniform in the case of alloy deposition. 

The relationship between free metal ion activity (or concentration, depending on the conventions used for the stability constants) and cjy, t is given by ... 

It follows that metal electrodes of the second kind not only serve as indicator electrodes for their own cations but also react to changes in metal ion activity resulting from precipitation or complexation reactions. For example, a Cl- ion electrode responds to the activity of the... 

Karsten, W.E., Gavva, S.R., Park, S.H. and Cook, P.E. (1995). Metal ion activator effects on intrinsic isotope effects for hydride transfer from decarboxylation in the reaction catalyzed by the NAD-malic enzyme from Ascaris suum. Biochemistry 34, 3253-3260... 

If measurements are to be carried out at low activities (for example in studying complexation equilibria), standard solutions cannot be prepared by simple dilution to the required value because the activities would irreproducibly vary as a result of adsorption effects, hydrolysis and other side reactions. Then it is useful to use well-defined complexation reactions to maintain the required metal activity value [14, 50, 132]. EDTA and related compounds are very well suited for this purpose, because they form stable 1 1 complexes with metal ions, whose dissociation can be controlled by varying the pH of the solution. Such systems are often termed metal-ion buffers [50] (cf. also p. 77) and permit adjustment of metal ion activities down to about 10 ° m. (Strictly speaking, these systems are defined in terms of the concentration, but from the point of view of the experimental precision, the difference between the concentration and activity at this level is unimportant.)... 

Metal Ions in Biology Metal Ion Activation of Dioxygen" Spiro, T. G., Ed. John Wiley and Sons New York, 1980 Vol. 2. 

Coon, M. J. and White, R. J. (1980) Cytochrome P450 a versatile catalyst in mono-oxygenation reactions. In Metal ion activation of dioxygen. Wiley, New York. 

Such a generalization is useful in that it provides clues to the. nature of the participation of metal ions in a reaction, when the order of catalytic effect of the various metals is known. As a first example of such an approach, let us consider the case in which the order of inhibition correlates with the order of complex stability such an order is frequently observed—e.g., urease (39). It may be concluded, in such an instance, that the metals do not activate the reaction, but inhibit it only. As a second example, if metal ions activate a reaction, but they do so in inverse order from that of complex stability, it follows that inhibition competes effectively with activation. Such an order is observed with enolase (Mg+2 >Zn+2>Mn+2>Fe+2>Co+2>Ni+2) (35, 53) presumably the inhibitory effect of the more strongly binding metals is responsible for the selection of the less active Mg+2 in the natural enzyme. Probably such effects are not as important in aconitase, making it possible for a stronger chelating metal to activate that enzyme. 

The potential is determined by the reaction Mn++ne M and the electrode material M is involved in it. The electrodes of this type are called electrodes of the first kind. In principle, these electrodes can indicate the metal ion activities (or concentrations). However, only a few metal ion/metal electrodes work satisfactorily as potentiometric indicator electrodes. An Ag+/Ag electrode is an example of such electrodes. The potential, determined by reaction Ag++e Ag, is... 

Fee, J. A. Superoxide, superoxide dismutases, and oxygen toxicity. In Metal Ion Activation of Dioxygen (Spiro, T. G, ed.). New York-Chichester-Brisbane-Toronto, Wiley-Interscience, 1980, pp. 209-237... 

Coordination of the metal ion activates the carbon-carbon triple bond toward nucleophilic attack to yield a cr-vinyl complex (22), which is a characteristic pathway of metal-catalyzed additions to the acetylenic bond. Protolysis of 22 gives the end product. 

Many important questions can be asked about the binding of metal ions within living cells. For example, What fraction of a given metal ion is free and what fraction is bound to organic molecules To what ligands is a metal bound Since many metal ions are toxic in excess, it is clear that homeostatic mechanisms must exist. How do such mechanisms sense the free metal ion activity within cells How does the body get rid of unwanted metal ions Answers to all these questions depend upon the quantitative differences in the binding of metal ions to the variety of potential binding sites found within a cell. 

If a pathway which involved metal ion activation followed by oxygen transfer in the coordination spere (Reaction 10) were operative, olefin reactivity would be expected to parallel the coordinative ability of the... 

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