Spectroelectrochemistry of ferrioxamine B,ferrichrome, and ferrichrome A

Thin-layer spectroelectrochemical techniques were used to determine the entropy change for the reduction of the three siderophores ferrioxamine B, ferrichrome, and ferrichrome A. The entropy changes were found to be large and negative. The _ΔS° values obtained are: ferrioxamine B. pH 10.2, _ΔS° = ?33.3 ± 0.4 eu; pH 9.0, _ΔS° = ?26.9 ± 0.9 eu; pH 8.0, _ΔS° = ?23.3 ± 1.2 eu; ferrichrome, pH 10.0, ΔS° = ?42.6 ± 0.5 eu; pH 9.1, ΔS° = ?35.8 ± 0.4 eu; pH 7.3, ΔS° = ?74.5 ± 3.4 eu; ferrichrome A, pH 10.1, _ΔS° = ?35.6 ± 0.9 eu; pH 9.1, _ΔS° = ?34.3 ± 0.9 eu; pH 7.9, _ΔS° = ?31.7 ± 0.9 eu. These values are adjusted to the scale on which S°_H + = 0. The large decreases in entropy upon reduction are attributed to an increase in the solvent ordering around the ferrous complex. Upon reduction, the rigid structure of the ferric chelate is loosened and previously sequestered amide groups are made available for solvent interactions. This increased interaction with solvent causes an increase in the order of the water around the molecule and this is responsible for the observed entropy changes. Variations in ΔS° values and the pH dependencies of these values are attributed to structural peculiarities of the individual siderophores.