Heme-copper and Heme O2-derived synthetic (bioinorganic) chemistry toward an understanding of cytochrome c oxidase dioxygen chemistry

Metal-ligand synergy driven functionalisation of alkylene linked bis(aldimine) on a diruthenium(II) platform. Cyclisation versus oxygenation

This article addresses the impact of metal-ligand redox cooperativity on the functionalisation of coordinated ligands. It demonstrates the structure-reactivity correlation of bis(aldimine) derived bis-bidentate L (Py-CHN-(CH2)n-NCH-Py, with n = 2 (L1), 3 (L2), 4 (L3)) as a function of the conformation (syn/anti) of its alkylene linker as well as the overall structural form (cis/trans) of (acac)2RuII(μ-L)RuII(acac)2 complex moieties (1-5) possessing an electron-rich acetylacetonate (acac) co-ligand. A systematic variation of the bridging alkylene unit of L in RuII/RuII-derived 1-5 led to the following reactivity/redox events, which were validated through structural, spectroscopic, electrochemical and theoretical evaluations: (i) Cyclisation of the ethylene linked (syn conformation) bis-aldimine unit of L1 via C-C coupling yielded pyrazine bridged (acac)2RuII(μ-L1')RuII(acac)2, 1a, while the corresponding anti-form (ethylene linker) of the metal-bound L1 in 2 ((acac)2RuII(μ-L1)RuII(acac)2) led to oxygenation at the ligand backbone (bis-aldimine (L) → bis(carboxamido) (L'')) via O2 activation to generate RuIIIRuIII-derived (acac)2RuIII(μ-L1''2-)RuIII(acac)2 (2a). (ii) Consequently, propylene and butylene linked L2 and L3 bridged between two {Ru(acac)2} units in 3 and 4/5 underwent oxygenation of L to L'' to yield diruthenium(III) complexes 3a and 4a/5a, respectively. (iii) In contrast, analogous L bridged oxidised [(acac)2RuIII(μ-L)RuIII(acac)2](ClO4)2 ([2](ClO4)2-[5](ClO4)2) and [{(PPh3)2(CO)(H)RuII}2(μ-L)](ClO4)2 ([6](ClO4)2-[8](ClO4)2) involving electron poor co-ligands failed to undergo the oxygenation of L irrespective of its n value, reemphasising the effective role of redox interplay between RuII and L particularly in the presence of an electron-rich acac co-ligand in the functionalisation of the latter in 1a-5a.

Membrane-Bound Redox Enzyme Cytochrome bd-I Promotes Carbon Monoxide-Resistant Escherichia coli Growth and Respiration

The terminal oxidases of bacterial aerobic respiratory chains are redox-active electrogenic enzymes that catalyze the four-electron reduction of O2 to 2H2O taking out electrons from quinol or cytochrome c. Living bacteria often deal with carbon monoxide (CO) which can act as both a signaling molecule and a poison. Bacterial terminal oxidases contain hemes; therefore, they are potential targets for CO. However, our knowledge of this issue is limited and contradictory. Here, we investigated the effect of CO on the cell growth and aerobic respiration of three different Escherichia coli mutants, each expressing only one terminal quinol oxidase: cytochrome bd-I, cytochrome bd-II, or cytochrome bo3. We found that following the addition of CO to bd-I-only cells, a minimal effect on growth was observed, whereas the growth of both bd-II-only and bo3-only strains was severely impaired. Consistently, the degree of resistance of aerobic respiration of bd-I-only cells to CO is high, as opposed to high CO sensitivity displayed by bd-II-only and bo3-only cells consuming O2. Such a difference between the oxidases in sensitivity to CO was also observed with isolated membranes of the mutants. Accordingly, O2 consumption of wild-type cells showed relatively low CO sensitivity under conditions favoring the expression of a bd-type oxidase.