Reductive Quenching of the Triplet State of Zinc Cytochrome c by the Hexacyanoferrate(II) Anion and by Conjugate Bases of Ethylenediaminetetraacetic Acid

The long-lived triplet state of zinc cytochrome c, designated 3Zn(cyt), has often been used as a reductant, in oxidative-quenching reactions. This article seems to be the first report of the use of 3Zn(cyt) as an oxidant, in two reductive-quenching reactions. Conjugate bases (anions) of ethylenediaminetetraacetic acid (EDTA) quench 3Zn(cyt) at pH 6.5 with the observed rate constant that is 2 times greater than the rate constant for natural decay of this excited state. Electrostatic attraction between these quenchers and Zn(cyt) is a necessary but not sufficient condition for quenching. A transient species observed at 690 nm has the absorbance and the time profile expected of the anion radical Zn(cyt)-. Detection of this species is possible because of the rapid decomposition of EDTA upon oxidation. The complex [Fe(CN)6]4- quenches 3Zn(cyt) at pH 7.0 with the rate constant of (1.5 ± 0.3) × 108 M-1 s-1. This fast quenching is not caused by electrostatic association of 3Zn(cyt) and [Fe(CN)6]4- nor by energy t...