Deployment of solar fuels derived from water requires robust oxygen-evolving catalysts made from earth abundant materials. Copper has recently received much attention in this regard. Mechanistic parallels between Cu and single-site Ru/Ir/Mn water oxidation catalysts, including intermediacy of terminal Cu oxo/oxyl species, are prevalent in the literature; however, intermediacy of late transition metal oxo species would be remarkable given the high d-electron count would fill antibonding orbitals, making these species high in energy. This may suggest alternate pathways are at work in copper-based water oxidation. This report characterizes a dinuclear copper water oxidation catalyst, {[(L)Cu(II)]-(μ-OH)}(OTf) (L = MeTMPA = bis((6-methyl-2-pyridyl)methyl)(2-pyridylmethyl)amine) in which water oxidation proceeds with high Faradaic efficiency (>90%) and moderate rates (33 s at ∼1 V overpotential, pH 12.5). A large kinetic isotope effect (k/k = 20) suggests proton coupled electron transfer in the initial oxidation as the rate-determining step. This species partially dissociates in aqueous solution at pH 12.5 to generate a mononuclear {[(L)Cu(II)(OH)]} adduct (K = 0.0041). Calculations that reproduce the experimental findings reveal that oxidation of either the mononuclear or dinuclear species results in a common dinuclear intermediate, {[LCu(III)]-(μ-O)}, which avoids formation of terminal Cu(IV)═O/Cu(III)-O intermediates. Calculations further reveal that both intermolecular water nucleophilic attack and redox isomerization of {[LCu(III)]-(μ-O)} are energetically accessible pathways for O-O bond formation. The consequences of these findings are discussed in relation to differences in water oxidation pathways between Cu catalysts and catalysts based on Ru, Ir, and Mn.

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Mitochondria are an alternative biocatalyst to microbes and enzymes in bioelectrochemical applications, including biosensors and biofuel cells. However, the bioelectrocatalytic mechanism is not well understood and there are contradictory reports in the literature. The electrochemical communicating species for mitochondria has previously been reported to be ubiquinone, cytochrome c, or cytochrome c oxidase. In this paper, it is determined that the primary electrochemical communicating species in mitochondria is ubiquinone by comparing yeast wild-type and mutants without cytochrome c and cytochrome c oxidase. The electrochemical results compared healthy and unhealthy yeast mitochondria, as established by a Seahorse oxygen consumption assay. Healthy mitochondrial electrochemical response increased by 20 ± 7% for reduction and 51 ± 3% for oxidation while unhealthy mitochondrial response increased 3 ± 50% for reduction and 39 ± 3% for oxidation when uncoupled with DNP. These responses provide a better understanding of mitochondrial systems in vitro and their relationship to in vivo systems as well as insights into the mechanism of electrochemical response.

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