Competitive Metal Binding

Continuous affinity spectra are almost exclusively applied to SOM, which does little anion binding thus, we will refer here to cation complexation only. In general, both and H+ bind to the same sites, so that the competitive Langmuir equation can be used as local isotherm for M +. Assuming that each binds to a single site, where H+ also binds, it can be written as 

The general problem of treating continuous affinity spectra for competitive binding is rather complex and has been addressed by several authors (Koopal et al. 1994 Borkovec et al. 1996 Cernik, Borkovec, and Westall 1996 Rusch et al. 1997 Garces, Mas, and Puy 2004 Orsetti, Andrade, and Molina 2009 Rey-Castro et al. 2009 David et al. 2010) we will not discuss all theoretical aspects at this point. 

Koopal et al. (1994) deduced a LF type isotherm for competitive binding, resulting in 

Returning to Equation 11.37, as a way to simplify the problem, it can be assumed that the bidimensional spectrum h can be written as the product of two distributions corresponding to K, g(log Ky), and filog K ). This assumption implies in principle that these distributions are different and independent (uncorrelated). In practice, if g is the distribution deduced for H+ binding in the absence of any interfering ions, the affinity distribution f will carry the H+-M + correlations. Because 

FIGURE 11.5 NICA isotherm plots of the generic affinity spectra for metal binding to humic acid. (Data from Milne, C.J. et al.. Environ. Sci. TechnoL, 37, 958-971, 2003.) 

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The SHM is somewhat simpler than the WHAM and shows good fitting of single-metal binding bnt for competitive metal binding, it is not so satisfactory... 

Two-Metal-lon Catalysis Competitive Metal Binding at the Active Centers... 

The pH of the medium always has a strong effect on metal binding. Competition with protons means that metal complexes tend to be of weak stability at low pH. Anions of carboxylic acids are completely protonated below a pH of 4 and a metal can combine only by displacing a proton. However, at pH 7 or higher, there is no competition from protons. On the other hand, in the case of ethylenediamine, whose pKa values are 10.2 and 7.5 (Table 6-9), protons are very strong competitors at pH 7, even with a strongly com- c... 

Simple comparisons of stability constants in this way have been criticized because no allowance is made for the differences in ligand basicities. Increased basicity as a rule parallels increased metal binding. Also, in the majority of cases competition between hydrogen ion and metal ion occurs (equation 3). The equilibrium constant (K) for reaction (3) is then given by equation (4). 

Inhibition by a variety of metal-binding agents competitive with respect to phosphoryl substrates (118-120) has suggested that an enzyme-bound divalent cation (other than Mg2+) may participate also in the binding of phosphate substrates. Observed inhibition by p-chloro-mercuriphenyl sulfonate and iodoacetate suggests the possibility that sulfhydryl groups may also be involved at, or near, the active enzymic site (119, 120). 

The obvious and fundamental concept that results from such elucubrations, one that is now supported, documented and quantified in several experimental data sets, is that the toxic effect of a trace metal is caused by its interference with the uptake, assimilation or utilization of another, essential trace metal. Trace metals are to various degrees coordinative analogs of each other, toxicity occurs when a metal binds competitively to a ligand site involved in the transport, assimilation or utilization of an essential metal whose function it cannot emulate. (In some cases metals can replace others with reasonably good efficiency). As a result, the study of trace metal toxicity in microalgae must commence with a study of trace metal nutrition, a scientifically sound if paradoxical situation. 

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