Protonation constants ligand

Although the conventional form of tabulation, formation constants are not meaningful alone in judging the relative abihty of ligands to compete with one another for Fe at a given pH. This is due to differences in ligand protonation constants (their number and absolute value), which define the amount of free, uncomplexed, and unprotonated hgand... 

Table I. Ligand protonation constants of hydroxamic acids...

Tabelle ID. Ligand protonation constants and complex protonation constants of tricatecholate ligands... 

In order to convert the resultant proton dependent equilibrium constant (Eq. 3) into the conventional formation constant (Eq. 4) it is necessary to know the ligand protonation constants. 

In order to convert Kg into the conventional, i.e. proton Independent, formation constant, it is necessary to know the six ligand protonation constants of enterobactin. Unfortunately, the ester groups of enterobactin are extremely susceptible to base-catalyzed hydrolysis, which precludes Independent measurement of ligand pKa s. However, we have measured the protonation constants of the bidentate ligand 2,3-dihydroxy-N,N-dlmethylbenzamide as log Ki = 12.1 and log K2 + 8.4. By using these values as estimates for the enterobactin protonation constants, we have estimated the... 

The formation constants of the ferric complexes of these synthetic catecholate ligands have been determined spectrophotometri-cally by competition with EDTA, as described above for entero-bactln. The first three (most acidic) ligand protonation constants have been determined by potentlometrlc titration of the free ligand. The second, more basic, set of protonation constants are too large to be determined readily potentlometrically. Thus the proton-dependent stability constant is expressed as... 

Such constants are valid over the pH range in which the final three phenolic protons are essentially undlssoclated, l.e. up to j 10.5. As with enterobactln, one can estimate the higher ligand protonation constants based on values reported for simple catechols and then convert K3 into K l (Kml [FeL]/([Fe][L])), which Is written In terms of the fully deprotonated form of the ligand. Values of log K3 and log are listed In Table II. 

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