Redox reactions of thionine and leucothionine with iron chelate compounds

Abstract

The kinetics of the reduction of thionine by several iron(II) chelate compounds (Fe^IIY) have been investigated under a nitrogen atmosphere at pH 7.0 and 25 °C by the stopped-flow method. The reactions obey first-order kinetics with respect to the concentration of both thionine and Fe^IIY. A linear relationship has been observed between log k_obs and log[K_Fe^IIIY/K_Fe^IIIY], where k_obs is an apparent rate constant for the reduction of thionine by Fe^IIY, and K_Fe^IIIY and K_Fe^IIY are the formation constants of the iron(III) and iron(II) chelates, respectively. Activation parameters for the reduction of thionine by Fe^IIY have been calculated from the temperature dependence of the rate constants. The oxidation of leucothionine by iron(III) chelates has also been investigated using the same technique. A mechanistic scheme for the thionine–leucothionine redox system is discussed in the light of these results and thermodynamic considerations.