Todd A. Eberspacher

A Cytochrome c Oxidase Model Catalyzes Oxygen to Water Reduction Under Rate-Limiting Electron Flux

We studied the selectivity of a functional model of cytochrome c oxidases active site that mimics the coordination environment and relative locations of Fea3, CuB, and Tyr244. To control electron flux, we covalently attached this model and analogs lacking copper and phenol onto self-assembled monolayer–coated gold electrodes. When the electron transfer rate was made rate limiting, both copper and phenol were required to enhance selective reduction of oxygen to water. This finding supports the hypothesis that, during steady-state turnover, the primary role of these redox centers is to rapidly provide all the electrons needed to reduce oxygen by four electrons, thus preventing the release of toxic partially reduced oxygen species.

Role of a distal pocket in the catalytic O2 reduction by cytochrome c oxidase models immobilized on interdigitated array electrodes

Five iron porphyrins with different superstructures were immobilized on self-assembled-monolayer (SAM)-coated interdigitated-array (IDAs) gold–platinum electrodes. The selectivity of the catalysts i.e., limited formation of partially reduced oxygen species (PROS) in the electrocatalytic reduction of dioxygen, is a function of 2 rates: (i) the rate of electron transfer from the electrode to the catalyst, which is controlled by the length, and conjugation of the linker from the catalyst to the electrode and (ii) the rate of bound oxygen (superoxide) hydrolysis, which correlates with the presence of a water cluster in the gas-binding pocket influencing the rate of oxygen binding; these factors are controlled by the nature of the porphyrin superstructure. The structurally biomimetic Tris-imidazole model is the most selective.

Interaction of nitric oxide with a functional model of cytochrome c oxidase.

Cytochrome c oxidase (CcO) is a multimetallic enzyme that carries out the reduction of O2 to H2O and is essential to respiration, providing the energy that powers all aerobic organisms by generating heat and forming ATP. The oxygen-binding heme a3 should be subject to fatal inhibition by chemicals that could compete with O2 binding. Near the CcO active site is another enzyme, NO synthase, which produces the gaseous hormone NO. NO can strongly bind to heme a3, thus inhibiting respiration. However, this disaster does not occur. Using functional models for the CcO active site, we show how NO inhibition is avoided; in fact, it is found that NO can protect the respiratory enzyme from other inhibitors such as cyanide, a classic poison.

Hybrid Bilayer Membrane: A Platform To Study the Role of Proton Flux on the Efficiency of Oxygen Reduction by a Molecular Electrocatalyst

In this report, we present a novel platform to study proton-coupled electron transfer (PCET) by controlling the proton flux using an electrode-supported hybrid bilayer membrane (HBM). Oxygen reduction by an iron porphyrin was used as a model PCET reaction. The proton flux was controlled by incorporating an aliphatic proton carrier, decanoic acid, into the lipid layer of the HBM. Using this system, we observed a different catalytic behavior than obtained by simply changing the pH of the solution in the absence of an HBM.

Ferrocene embedded in an electrode-supported hybrid lipid bilayer membrane: a model system for electrocatalysis in a biomimetic environment.

An electrode-supported system in which ferrocene molecules are embedded in a hybrid bilayer membrane (HBM) has been prepared and characterized. The redox properties of the ferrocene molecules were studied by varying the lipid and alkanethiol building blocks of the HBM. The midpoint potential and electron transfer rate of the embedded ferrocene were found to be dependent on the hydrophobic nature of the electrolyte and the distance at which the ferrocene was positioned in the HBM relative to the electrode and the solution. Additionally, the ability of the lipid-embedded ferrocenium ions to oxidize solution phase ascorbic acid was evaluated and found to be dependent on the nature of the counterion.

Functional models for the oxygen binding/activating hemeproteins, myoglobin and cytochrome c oxidase

Abstract Hemoproteins (hemoglobin and myoglobin) reversibly bind O 2 for transport and storage; other heme proteins (cytochrome c oxidase) reduce O 2 to water and couple the energy from that reaction to create a proton gradient for the eventual synthesis of ATP. This paper will describe the design and synthesis of iron porphyrins which are functional models for these oxygen binding and activating heme proteins.