Structure and iron-binding properties of ELT. Data on the iron-binding properties of ELT were obtained specifically for this article and are not previously published elsewhere. The iron-binding properties of other chelators are based on previously published data.^11,19  (A) Structure of ELT and its iron complexes are shown. The free ligand possesses 2 tautomers (i and ii). Three major iron(III) complexes have been identified: iii (FeELT), iv (FeELT₂H), and v (FeELT₂). (B) The speciation of iron(III) in the presence of ELT as a function of pH. [Fe]_total=1 μM; [ELT]_total=10 μΜ is shown at steady state (ie, when sufficient time has elapsed for the reactions to go to completion). These proportions are calculated from the iron-binding constants for iron-chelate complexes of the respective chelators shown in Table 1 and determined as described in “Materials and methods.” Titration with iron(III) yielded 3 equilibrium constants: K_FeL=25.6, K_FeL2H=43.4, and K_FeL2= 34.9. ELT has 3 pK_a values (the pH at which half the molecules are ionized) of 2.6, 8.7, and 11.1. Using these data, a pFe value of 22.0 (the strength of iron(III) binding, being the negative log of the unbound iron(III) concentration under defined conditions (1 µM iron[III] and 10 µM chelator¹⁰ ) was determined, which is greater than that of DFP (20.4) and very similar to that of DFX (23.1). Competition between ELT and other chelators for iron(III) are shown for (C) 1 µM DFO, (D) 3 µM DFP, and (E) 2 µM DFX. These show the predicted proportions of each ELT iron complex when mixed with a second chelator, after reactions have gone to completion (in steady state). Thus, for example, at 1 µM ELT, more than 99% of iron(III) will be bound to DFO (C), whereas under the same conditions, about half the iron will be bound to DFP (D), and about 70% to DFX (E).