Metal ions complex-forming properties

Chelate Formation. Citric acid complexes with many multivalent metal ions to form chelates (9,10). This important chemical property makes citric acid and citrates useful in controlling metal contamination that can affect the color, stabiUty, or appearance of a product or the efficiency of a process. 

Somewhat better data are available for the enthalpies of hydration of transition metal ions. Although this enthalpy is measured at (or more property, extrapolated to) infinite dilution, only six water molecules enter the coordination sphere of the metal ion lo form an octahedral aqua complex. The enthalpy of hydration is thus closely related to the enthalpy of formation of the hexaaqua complex. If the values of for the +2 and +3 ions of the first transition elements (except Sc2, which is unstable) are plotted as a function of atomic number, curves much like those in Fig. 11.14 are obtained. If one subtracts the predicted CFSE from the experimental enthalpies, the resulting points lie very nearly on a straight line from Ca2 lo Zn2 and from Sc to Fe3 (the +3 oxidation state is unstable in water for Ihe remainder of the first transition series). Many thermodynamic data for coordination compounds follow this pattern of a douUe-hunped curve, which can be accounted for by variations in CFSE with d orbital configuration. 

The metal-ion complexing properties of crown ethers are clearly evident in then-effects on the solubility and reactivity of ionic compounds in nonpolar media. Potassium fluoride (KF) is ionic and practically insoluble in benzene alone, but dissolves in it when 18-crown-6 is present. This happens because of the electron distribution of 18-crown-6 as shown in Figure 16.2a. The electrostatic potential surface consists of essentially two regions an electron-rich interior associated with the oxygens and a hydrocarbon-like exterior associated with the CH2 groups. When KF is added to a solution of 18-crown-6 in benzene, potassium ion (K+) interacts with the oxygens of the crown ether to form a Lewis acid-Lewis base complex. As can be seen in the space-filling model of this... 

Lipids and phospholipids too accumulate at the site of active mineralization. It has been proposed that phosphatidylserine can combine with hydrated metal ions and form a bi-or possibly a tridentate complex417. In forming such a complex, the hydrophilic properties of Ca2+ are decreased and ligands could associate with the... 

Apparently, the most stable complex forms when six chains residing on the same side of the benzene ring surround the metal ion. Three chains on the same side of the aromatic ring can best enclose a cavity favourable to metal ion complexation if the chains are in a 1,3,5 relationship [cf. (2)] as opposed to 1,2,3 [cf. (5)] or 1,2,4. Ligand properties for the 2-chain molecules are found only with the 1,2-isomer (4). 

Very little is known about these dendrimer properties, although it has been noted that ester-terminated PAMAM dendrimers form deep blue complexes with CuS04 solutions, and that NH2-terminated homologues produce deep purple solutions [2, 79]. The well-known coordination properties of the amide bond, which lead to the formation of metal-ion complexes [163], should make this a very rich area for further investigation. 

There are two methods the preformed radiometal-chelate method and the indirect chelator—antibody method. Various antibodies are labelled by the latter, where the bifunctional chelating agent is initially conjugated to a macromolecule, which is then allowed to react with a metal ion, to form a metal-chelate-macromolecule complex. Due to the presence of the chelating agent, the biological properties of the labeled protein may be altered and must be assessed before clinical use. 

The association constant for ion binding of Cyclo-(Pro-Gly)3 b nearly the same as that of antamanide, but the selectivity for Ca of Cydo-(Pro-Gly)3 is inferior to that for Na" " of antamanide (144). Cyclo-(Pro-Gly)3 resembles the K -specific cyclic hexadepdpeptide aiitibiotic enniatin (145), in the aspect that both cycUc compounds form sandwich-type comjdexes with ions. It is very likely that Cyclo-(Pro y)3 transports ions across a membrane vb the formation of a club sandwich-type complex. The metal-ion complex of Cyclo-(Pro<]tly)3 is extractable with water from organic phase. A specific behavior of clo-(Pro-Gly)3 in the ion tran rt throu a membrane b expected from fhb property. 

Properties Stable, orange-red powder. Decomposes at 140C. Insoluble in water soluble in acetone and chloroform. Forms highly colored stable complexes with many metal ions can form a series of NJSf-derivatives. 

Metal cofactors in enzymes may be bound reversibly or firmly. Reversible binding occurs in metal-activated enzymes (e.g., many phosphotransferases) firm (or tight) binding occurs in metalloenzymes (e.g., carboxypeptidase A). Metals participate in enzyme catalysis in a number of different ways. An inherent catalytic property of a metal ion may be augmented by the enzyme protein, or metal ions may form complexes with the substrate and the active center of the enzyme and promote catalysis, or metal ions may function in electron transport reactions between substrates and enzymes. 

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