Complexation of iron(III) and iron(II) by citrate. Implications for iron speciation in blood plasma

Estimates of the concentrations and identity of the predominant complexes of iron with the low-molecular-mass ligands in vivo are important to improve current understanding of the metabolism of this trace element. These estimates require a knowledge of the stability of the iron–citrate complexes. Previous studies on the equilibrium properties of the Fe(III)–citrate and Fe(II)–citrate are in disagreement. Accordingly, in this work, glass electrode potentiometric titrations have been used to re-determine the formation constants of both the Fe(III)– and Fe(II)–citrate systems at 25 °C in 1.00 M (Na)Cl and the reliability of these constants has been evaluated by comparing the measured and predicted redox potentials of the ternary Fe(III)–Fe(II)–citrate system. The formation constants obtained in this way were used in computer simulation models of the low-molecular-mass iron fraction in blood plasma. Redox equilibria of iron are thus included in large models of blood plasma for the first time. The results of these calculations show the predominance of Fe(II)–carbonate complexes and a significant amount of aquated Fe(II) in human blood plasma.