Metal complexes ionic binding

Molecular structure/biospecific adsorption Surface charge/ionic binding Metals complex formation/coordination complex Molecular size and shape/size exclusion Hydrophobicity/hydrophobic complex formation... 

The luminescence decays are somewhat nonexponential for the ionically bound metal complexes nonexponentiality is exacerbated by the presence of 02.We suggest that nonexponential decays reflect a persistent microheterogeneity around the complex. This was our first clear evidence of spectroscopically different binding sites. In this case, oxygen enhances heterogeneity detection by differentially quenching different sites. 

A. (The gas phase estimate is about 100 picoseconds for A at 1 atm pressure.) This suggests tliat tire great majority of fast bimolecular processes, e.g., ionic associations, acid-base reactions, metal complexations and ligand-enzyme binding reactions, as well as many slower reactions that are rate limited by a transition state barrier can be conveniently studied with fast transient metliods. 

So far, as in Equation (3.33), the hydrolyses of ATP and other high-energy phosphates have been portrayed as simple processes. The situation in a real biological system is far more complex, owing to the operation of several ionic equilibria. First, ATP, ADP, and the other species in Table 3.3 can exist in several different ionization states that must be accounted for in any quantitative analysis. Second, phosphate compounds bind a variety of divalent and monovalent cations with substantial affinity, and the various metal complexes must also be considered in such analyses. Consideration of these special cases makes the quantitative analysis far more realistic. The importance of these multiple equilibria in group transfer reactions is illustrated for the hydrolysis of ATP, but the principles and methods presented are general and can be applied to any similar hydrolysis reaction. 

The theory and application of this fluorescence method have been discussed in detail by LePecq and others (3,8). The assay requires that there is sufficient ionic strength to minimize ionic binding (e.g., O.IM sodium chloride), that the pH is 4-10, that no heavy metals are present, that the fluorescence is not enhanced on binding to other excipients (e.g., proteins) and that at least portions of the nucleic acids are not complexed. These requirements can usually he met when dealing with recombinant products in some cases the samples must he manipulated to create the appropriate conditions. In the intercalative method of dye binding, proteins rarely interfere with the assay, and procedures have been developed to remove the few interferences they may cause (e.g., the use of heparin or enzymatic digestion of the protein 9). 

Free, ionic species of metals are at their highest concentrations at lower pH, so metals tend to be more bioavailable under these conditions.121128 At acidic pH, more protons are available to saturate metal-binding sites.99 For example, metals are less likely to form insoluble precipitates with phosphates when the pH of the system is lowered because much of the phosphate has been protonated. Under basic conditions, metal ions can replace protons to form other species, such as hydroxo-metal complexes. Some of the hydroxo-metal complexes are soluble, such as those formed with cadmium, nickel, and zinc, whereas those formed with chromium and iron are insoluble. 

It occurred to us that ionic interactions might be a highly suitable binding motif to enforce the formation of heterobidentate ligand combinations [48[. The assembly ligand 14 /IS has been formed from the well-known TPPMS (14, monosulfonated triphenylphosphine sodium salt) and 3-(diphcnylphosphinyl)aniline hydrochloride (IS) by a simple ion-exchange reaction (Scheme 10.6). The coordination behavior ofthe ion-pair 14 /I S has been tested with various transition metal complexes. Other... 

In zinc metalloenzymes. zinc is a selective stoichiometric constituent and is essential for catalytic activity. It is frequently present in numerical correspondence with the number of active enzymatic sites, coenzyme binding sites, or enzyme subunits Removal of zinc results in loss of activity. Inhibition by metal complexing agents is a characteristic feature of zinc metalloenzymes. However, no direct relationship holds between the inhibitory effectiveness of these agents and their affinity for ionic zinc. Although zinc is the only constituent of zinc metalloenzymes in vivo, it can be replaced by other metals m vitro, such as cobalt, nickel, iron, manganese, cadmium, mercury, and lead, as m the case of carboxy-peprida.ses. 

Past studies have shown a decrease in the toxicity of metals in the presence of organic matter, due to the binding of natural ligands that are believed to bind metals and reduce the concentration of free ionic species. The free ion activity model proposes that the free metal ion (as opposed to total metal ion concentration) is the dominant species available for organisms, and assumes that the colloidally complexed species should be less available. 

Generic authenticity and transferability are not virtues of the ionic models. Virtually all inorganic chemists would chafe at the notion that metal-ligand interactions can be attributed to electrostatic interactions alone. For example, ionic interactions cannot describe the binding of N2 or O2 to metal complexes ionic models cannot lead to the square planar structure observed for PtCl4 a purely ionic binding model is not in concert with the observation... 

Table 6-9 Important Systems Logarithms of Binding Constants for Some 1 1 Metal Complexes at 25°C Ionic Radii in Nanometers for Some... 

Figure 6.9 Possible mechanism for the field-driven ionic binding to thiobis(ethyl aceto-acetate) moieties arranged in a compact monolayer on gold. It includes a field assisted enolization in acid solution followed immediately by a binding of a metal ion. At negative E the first step may include penetration of the ion into the monolayer to form a weak diketone complex. ...

In a study utilizing SEC and fluorescence, retention time increased with increased concentration of Cu " which in this mode of HPLC would indicate that there had been a decrease in molecular size (3). It was also hypothesized that ionic interactions between the humic-metal complex and charged surfaces such as free silanol groups in the column packing may have also contributed to the increased retention time. In addition to changes in retention time, there was a decrease in peak area with increased copper concentration for both the UV and fluorescence chromatograms. This may have been due to an irreversible binding (in the time-scale of the separation) of copper-humic complexes to the stationary phase in the presence of increased Cu. ... 

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