Homogeneous catalytic O2 reduction to water by a cytochrome c oxidase model with trapping of intermediates and mechanistic insights

Abstract

An efficient and selective four-electron plus four-proton (4e(-)/4H(+)) reduction of O(2) to water by decamethylferrocene and trifluoroacetic acid can be catalyzed by a synthetic analog of the heme a(3)/Cu(B) site in cytochrome c oxidase ((6)LFeCu) or its Cu-free version ((6)LFe) in acetone. A detailed mechanistic-kinetic study on the homogeneous catalytic system reveals spectroscopically detectable intermediates and that the rate-determining step changes from the O(2)-binding process at 25 °C room temperature (RT) to the O-O bond cleavage of a newly observed Fe(III)-OOH species at lower temperature (-60 °C). At RT, the rate of O(2)-binding to (6)LFeCu is significantly faster than that for (6)LFe, whereas the rates of the O-O bond cleavage of the Fe(III)-OOH species observed (-60 °C) with either the (6)LFeCu or (6)LFe catalyst are nearly the same. Thus, the role of the Cu ion is to assist the heme and lead to faster O(2)-binding at RT. However, the proximate Cu ion has no effect on the O-O bond cleavage of the Fe(III)-OOH species at low temperature.

Fig. 1.

(A) X-ray structures of the fully reduced bimetallic heme a₃/Cu_B center in CcO from bovine heart (Fe^II⋯Cu^I = 5.19 Å) (3) [figure adapted from (8)]. (B) heme/Cu synthetic model for CcO (⁶LFeCu). (C) Cu-free version of synthetic model for CcO (⁶LFe).

Fig. 2.

(A) UV-vis spectral changes during the four-electron reduction of O₂ by Fc* (1.0 mM) catalyzed by ⁶LFeCu (1.0 μM) in the presence of TFA (1.0 mM) in air-saturated acetone ([O₂] = 2.2 mM) at -60 °C. The insert shows the time profile of the absorbance at 780 nm relative to Fc^∗+ formation. Note: a tiny amount of Fc^∗+ is formed (see the nonzero absorbance) during the mixing time following combining catalyst and ferrocene solutions (in order to obtain a homogeneous solution) and before the first spectrum is recorded. Thus, for practical reasons, this first spectrum corresponds to time = 0. (B) Plots of k_obs vs. [cat] for ⁶LFeCu and at -60 °C. (C) Plots of k_obs vs. [Fc*] for ⁶LFeCu and ⁶LFe at -60 °C. (D) Plots of k_obs vs. [TFA] for ⁶LFeCu and at -60 °C. (E) Plots of k_obs vs. [O₂] for ⁶LFeCu and ⁶LFe at -60 °C.

Fig. 3.

(A) Stopped-flow measurement of the absorbance changes during catalytic O₂-reduction by Fc* (200 μM) with ⁶LFeCu (2.0 μM), TFA (4 mM) in O₂-saturated acetone at 25 °C. The insert shows the time profile of the absorbance at 780 nm relative to Fc^∗+ formation. See the Fig. 2A legend for the explanation of the nonzero ferrocenium concentration initially observed. (B) Plots of k_obs vs. [cat] for ⁶LFeCu and ⁶LFe at 25 °C. (C) Plots of k_obs vs. [Fc*] for ⁶LFeCu and ⁶LFe at 25 °C. (D) Plots of k_obs vs. [TFA] for ⁶LFeCu and ⁶LFe at 25 °C. (E) Plots of k_obs vs. [O₂] for ⁶LFeCu and ⁶LFe at 25 °C.

Fig. 4.

Plots of k_obs vs. temperature for the four-electron reduction of O₂ by Fc* (1.0 mM) catalyzed by ⁶LFeCu or ⁶LFe (1.0 μM) in the presence of TFA (1.0 mM) in an air-saturated acetone. (B) Plots of the absorbance maximum vs. temperature corresponding to the Soret band of steady-state in the case of ⁶LFeCu and ⁶LFe. (C) Arrhenius plots obtained by conversion of plots (A).

Fig. 5.

The proposed mechanism of the four-electron reduction of O₂ to water by Fc* in the presence of TFA in acetone catalyzed by: (A) the heme/Cu bimetallic center model of CcO (⁶LFeCu), (B) the Cu-free version (⁶LFe). In fact, it has not yet been established as to which side of the heme the O₂-binding occurs.