Structural Considerations for Iron-Specific Chelators

The stability constant of a complex M L i, where M is metal, L ligand, and the subscripts are the corresponding stoichiometric coefficients is defined as  

A comparison of values for bi- (n = 3) and hexadentate (n = 1) chelators can be misleading. For example, log P of deferiprone is 35.9 but the log of the third stepwise formation constant given by log(P /p2) is only 9.7 (Motekaitis and Martell 1991). Also, this definition of stability constant does not take into account the different acidities of the ligands and the ability of iron to compete for them with proton. Protonation of the ligand and hydrolysis of the metal, as well as competition with other metals and ligands in biological systems, complicate the interpretation of stability constants. Therefore, in comparing the stability of iron chelates it is useful to introduce the additional terms iTeff and pM. Martell has defined an effective stability constant for Fe complexes based on competition for the 

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