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 Zn-3(cyt), has often been used as a reductant, in oxidative-quenching reactions. This article seems to be the first report of the use of Zn-3(cyt) as an oxidant, in two reductive-quenching reactions. Conjugate bases (anions) of ethylenediaminetetraacetic acid (EDTA) quench Zn-3(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 Zn-3(cyt) at pH 7.0 with the rate constant of (1.5 +/- 0.3) x 10(8) M(-1) s(-1). This fast quenching is not caused by electrostatic association of Zn-3(cyt) and [Fe(CN)(6)](4-) nor by energy transfer from the former to the latter. The complex [Fe(CN)(6)](4-) is not detectably contaminated by the similar complex [Fe(CN)(6)](3-), which might act as an oxidative quencher. The evidence supports reductive quenching of Zn-3(cyt) by the [Fe(CN)(6)](4-) ion. The anion radical Zn(cyt)(-) is not detected, probably because it is rapidly consumed in the back-reaction with the [Fe(CN)(6)](3-) ion. To act as reductive quenchers for Zn-3(cyt), chemicals must have favorable electrostatic properties, redox potential, and reactivity. These requirements are discussed so that further studies of this new reaction may be possible.