Metal ion substitution

Aqvist, J. and Warshel, A. Free energy relationship in metalloenzyme-catalyzed reactions. Calculations of the effects of metal ion substitutions in staphylococcal nuclease, J.Am.Chem.Soc., 112 (1990), 2860-2868... 

Coming to the comparison of the and 0-2 series, the possibilities of isomerization and/or decomposition of the precursor lacunary complexes and, presumably, of their metal ion-substituted derivatives, make it necessary to insure first that no fast conversion occurs, in particular from the a structure to the 2 one. Figure 13(a) compares the cyclic voltammograms of the two lacunary complexes in the pH 3 medium. The main difference appears on the third redox system a single two-electron, reversible wave is obtained for the complex in contrast, the corresponding system for the U2 isomer is clearly constituted by two. 

This observation must be linked with the beneficial accumulation of electrons in the POM framework. It was checked that NO is also electrocatalytically reduced in the same potential domain. Concerning nitrate, it is worth reminding that a comparative study of several metal ion-substituted heteropolyanions including aj- and 2-P2W17M and 2-P2Wi5Mo2M where Mn", Fe", Co", Ni"Cu", Zn" and =y (where =y indicates the absence of M) most heteropolyanions, except those substituted by Cu or Ni do not show any important electrocatalytic ability toward its reduction [160a]. Specifically,... 

Examples of electrochemicaUy initiated metal ion substitution reactions have been described for the case of substitution of high-spin iron by cadmium ions [49, 50], high-spin iron by nickel, and silver by nickel [51] and high-spin iron ions [52]. The reasons for the proceeding of these electrochemicaUy initiated substitution reactions have been ascribed as well to the labilization of the cyanide ions in certain oxidation states of the metal ions of the involved PCMs. 

Substitution of other transition metal ions for Cu, however, was observed (174) to be highly deleterious to superconducting behavior. Table 14 shows the results of 10% metal-ion substitution in the Cu sites. 

Mixed addenda or transition-metal-ion-substituted heteropolyanions containing Co, Mn, and Ru are catalysts for oxidation reactions with tert-butyl hydroperoxide and other oxidants. Typical examples are listed in Table XXX. 

A. Mono-Transition-Metal-Ion-Substituted Heteropolyanions as Inorganic Synzymes... 

The reactivity depends on the lability of the metal ion, indicating that condensation of the amino groups requires their disassociation from the metal ion—substitutionally inert Co(III) salts do not react at all. 

Trivalent transition metal ions substituting for Al3+ occur in several aluminosilicate minerals, including epidotes, yoderite and the Al2Si05 polymorphs andalusite, kyanite and sillimanite, often producing spectacular colours and pleochroism. As a result, numerous optical spectral measurements have been made of several natural and synthetic Al2Si05 minerals (Kerrick, 1990). 

Displacing the Essential Metal Ion in Biomolecules. It is estimated that approximately one third of all enzymes require metal as a cofactor or as a structural component. Those that involve metals as a structural component do so either for catalytic capability, for redox potential, or to confer steric arrangements necessary to protein function. Metals can cause toxicity via substitution reactions in which the native, essential metal is displaced/replaced by another metal. In some cases, the enzyme can still function after such a displacement reaction. More often, however, enzyme function is diminished or completely abolished. For example, Cd can substitute for Zn in the protein famesyl protein transferase, an important enzyme in adding famesyl groups to proteins such as Ras. In this case, Cd diminishes the activity of the protein by 50%. Pb can substitute for Zn in 8-aminolevulinic acid dehydratase (ALAD), and it causes inhibition in vivo and in vitro. ALAD contains eight subunits, each of which requires Zn. Another classic example of metal ions substituting for other metal ions is Pb substitution for Ca in bones. 

NOTES Organic side chains and metal ions can be substituted. The organic side-chain substitution is usually done by gene mutation, but metal ion substitution is done by direct exchange and, in fact, can be done for S and Se. 

The kinetics and mechanism of synthesis and decomposition of macrocyclic compounds are regarded as one of the most important aspects in the chemistry of these compounds. The majority of papers concern metal ions complexing with preliminarily synthesized macrocyclic ligands and metal ion substitutions by other metal ions in the preliminarily prepared complexes. Template synthesis, the most promising approach to the directed preparation of macrocyclic compounds with desired structures [17], plays a still more decisive role in the chemistry of macrobicyclic complexes with encapsulated metal ion. However, the literature contains only scarce data on the kinetics and the mechanism of the template synthesis of macrocyclic compounds because of the difficulties encountered in experimental determinations of kinetic and thermodynamic parameters, such as low product yields, nonaqueous media, high temperatures, and side reactions. 

Aqvist, J., Warshel, A., Free Energy Relationships in Metalloenzyme-Catalyzed Reactions. Calculations of the Effects of Metal Ion Substitutions in Staphylococcal Nuclease, J. Am. Chem. Soc. 1990, 112, 2860-2868. 

In the example described above, studies of a metal-substituted derivative helped in the evaluation of mechanistic possibilities for the enzymatic reaction. In addition, studies of such derivatives have provided useful information about the environment of the metal-ion binding sites. For example, metal-ion-substituted derivatives of CuZnSOD have been prepared with Cu , CuS Zn , Ag, Ni", or Co bound to the native copper site, and with Zn , Cu , Cu Co , Hg Cd, Ni", or Ag bound to the native zinc site. °° ° The SOD activities of these derivatives are interesting only those derivatives with copper in the copper site have a high degree of SOD activity, whereas the nature of the metal ion in the zinc site or even its absence has little or no effect. 

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