Redox reactions of thionine and leucothionine with iron chelate compounds
Abstract
The kinetics of the reduction of thionine by several iron(II) chelate compounds (FeIIY) 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 FeIIY. A linear relationship has been observed between log kobs and log[KFeIIIY/KFeIIIY], where kobs is an apparent rate constant for the reduction of thionine by FeIIY, and KFeIIIY and KFeIIY are the formation constants of the iron(III) and iron(II) chelates, respectively. Activation parameters for the reduction of thionine by FeIIY 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.