Formation constants, hydroxamate siderophore

The overall ferric ion complex formation constants of siderophores cannot be determined directly at neutral pH, because the strong iron binding pulls the equilibrium (Eq. 1) to the right, exhibiting (in the case of hydroxamates) no appreciable dissociation into free ligand and free iron above pH 2. 

In contrast to the hydroxamate siderophores, little or nothing is known about the stability constant for the catechol siderophore, entero-bactin. Prior to determining the formation constant of enterobactin (for which hydrolysis of the ligand presents special problems), the reaction of catechol itself with ferric ion has been investigated (31). 

Hydroxamate- or catecholate-containing siderophores are strongly absorbing species with characteristic spectra (see Table 1) which can be utilized for spectrophotometric determination of the complex formation constant. Iron(III) hydroxamates absorb in the visible region, producing a broad absorption band in the 420-440 nm region. Iron(III) catecholates exhibit pH-dependent absorption maxima. Unfortunately, the overall Fe + ion complex formation constants cannot be determined directly at neutral pH, because the extremely high stability of siderophore complexes precludes direct measurements of the equilibrium of interest, which would yield the desired formation constant for a tris-bidentate siderophore complex, /3no (equation (2)). ... 

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