The Reversible Reduction of Horse Metmyoglobin by the Iron(II) Complex of trans-1,2-Diaminocyclohexane-N,N,N′,N′-tetraacetate

Abstract

The reduction of metmyoglobin by the iron(II) complex of trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetate (FeCDTA2-) has been investigated. The equilibrium constant, measured spectrophotometrically, is 0.21 with a resulting reduction potential of 0.050 V for Mb0. The rate constant for the reduction is 28 M-1 sec-1 with a ΔH of 13 kcal M-1 and ΔS of -11 eu. Both CN- and OH- inhibit the reduction because of the relatively low reactivity of cyanometmyoglobin (Mb+CN-) and ionized metmyoglobin (Mb+OH-). The rate constant for the reduction of Mb+CN- by FeCDTA2- is 4.0 × 10-2 M-1 sec-1 and that for reduction of Mb+OH- is 4.8 M-1 sec-1. The nitric oxide complex of metmyoglobin is reduced with a rate constant of 10 M-1 sec-1. The kinetics of oxidation of oxymyoglobin by FeCDTA- were studied. The data are consistent with a mechanism where oxidation takes place entirely through the deoxy form. A rate constant of 1.45 × 102 M-1 sec-1 was calculated for the oxidation of deoxymyoglobin by FeCDTA-, in excellent agreement with that calculated from the equilibrium constant and rate constant for reduction. The above data are discussed in terms of a simple outer-sphere reduction reaction. © 1975, American Chemical Society. All rights reserved.