Molecular Investigation of Iron–Sulfur Cluster Assembly Scaffolds under Stress

Fe/S biosynthesis is controlled in <i>Escherichia coli</i> by two machineries, the housekeeping ISC machinery and the SUF system that is functional under stress conditions. Despite many <i>in vivo</i> studies showing that SUF is more adapted for Fe/S assembly under stress, no molecular data supporting this concept have been provided so far. This work focuses on molecular studies of key actors in Fe/S assembly, the SufB and IscU scaffolds under oxidative stress and iron limitation. We show that the IscU Fe₂S₂ cluster is less stable than the SufB Fe₂S₂ cluster in the presence of hydrogen peroxide, oxygen, and an iron chelator

Activation of a Unique Flavin-Dependent tRNA-Methylating Agent

TrmFO is a tRNA methyltransferase that uses methylenetetrahydrofolate (CH₂THF) and flavin adenine dinucleotide hydroquinone as cofactors. We have recently shown that TrmFO from Bacillus subtilis stabilizes a TrmFO–CH₂–FADH adduct and an ill-defined neutral flavin radical. The adduct contains a unique N–CH₂–S moiety, with a methylene group bridging N⁵ of the isoalloxazine ring and the sulfur of an active-site cysteine (Cys53). In the absence of tRNA substrate, this species is remarkably stable but becomes catalytically competent for tRNA methylation following tRNA addition using the methylene group as the source of methyl. Here, we demonstrate that this dormant methylating agent can be activated at low pH, and we propose that this process is triggered upon tRNA addition. The reaction proceeds via protonation of Cys53, cleavage of the C–S bond, and generation of a highly reactive [FADH­(N⁵)CH₂]⁺ iminium intermediate, which is proposed to be the actual tRNA-methylating agent. This mechanism is fully supported by DFT calculations. The radical present in TrmFO is characterized here by optical and EPR/ENDOR spectroscopy approaches together with DFT calculations and is shown to be the one-electron oxidized product of the TrmFO–CH₂–FADH adduct. It is also relatively stable, and its decomposition is facilitated by high pH. These results provide new insights into the structure and reactivity of the unique flavin-dependent methylating agent used by this class of enzymes

FAD/Folate-Dependent tRNA Methyltransferase: Flavin as a New Methyl-Transfer Agent

RNAs contain structurally and functionally important modified nucleosides. Methylation, the most frequent RNA modification in all living organisms, mostly relies on SAM (<i>S</i>-adenosylmethionine)-dependent methyltransferases. TrmFO was recently discovered as a unique tRNA methyltransferase using instead methylenetetrahydrofolate and reduced flavin adenine dinucleotide (FAD) as essential cofactors, but its mechanism has remained elusive. Here, we report that TrmFO carries an active tRNA-methylating agent and characterize it as an original enzyme-methylene-FAD covalent adduct by mass spectrometry and a combination of spectroscopic and biochemical methods. Our data support a novel tRNA methylating mechanism

Combined Experimental–Theoretical Characterization of the Hydrido-Cobaloxime [HCo(dmgH)₂(P<i>n</i>Bu₃)]

A combined theoretical and experimental approach has been employed to characterize the hydrido-cobaloxime [HCo­(dmgH)₂(P<i>n</i>Bu₃)] compound. This complex was originally investigated by Schrauzer et al. [Schrauzer et al., <i>J. Am. Chem. Soc</i>. <b>1971</b>, <i>93</i>,1505] and has since been referred to as a key, stable analogue of the hydride intermediate involved in hydrogen evolution catalyzed by cobaloxime compounds [Artero, V. et al.<i> Angew. Chem., Int. Ed</i>. <b>2011</b>, <i>50</i>, 7238–7266]. We employed quantum chemical calculations, using density functional theory and correlated RI-SCS-MP2 methods, to characterize the structural and electronic properties of the compound and observed important differences between the calculated ¹H NMR spectrum and that reported in the original study by Schrauzer and Holland. To calibrate the theoretical model, the stable hydrido tetraamine cobalt­(III) complex [HCo­(tmen)₂(OH₂)]²⁺ (tmen = 2,3-dimethyl-butane-2,3-diamine) [Rahman, A. F. M. M. et al.<i> Chem. Commun</i>. <b>2003</b>, 2748–2749] was subjected to a similar analysis, and, in this case, the calculated results agreed well with those obtained experimentally. As a follow-up to the computational work, the title hydrido-cobaloxime compound was synthesized and recharacterized experimentally, together with the Co­(I) derivative, giving results that were in agreement with the theoretical predictions

Phosphine Coordination to a Cobalt Diimine–Dioxime Catalyst Increases Stability during Light-Driven H₂ Production

The combination of cobalt diimine–dioxime complexes with a cyclometalated iridium photosensitizer gives efficient systems for hydrogen generation under visible-light irradiation using triethylamine as a sacrificial electron donor. Interestingly, the addition of triphenylphosphine (PPh₃) to the medium results in a significant improvement of the stability of the system, with up to ∼700 turnovers achieved within 10 h. UV–visible spectroscopic monitoring of the reaction allows identification of a PPh₃-coordinated Co^I intermediate as the active species. Mechanistic issues regarding (i) the photogeneration of the Co^I species, (ii) the nature of the active species, and (iii) the influence of PPh₃ on the H₂-evolution mechanism are discussed

Bioinspired Tungsten Dithiolene Catalysts for Hydrogen Evolution: A Combined Electrochemical, Photochemical, and Computational Study

Bis­(dithiolene)­tungsten complexes, W^VIO₂ (L = dithiolene)₂ and W^IVO (L = dithiolene)₂, which mimic the active site of formate dehydrogenases, have been characterized by cyclic voltammetry and controlled potential electrolysis in acetonitrile. They are shown to be able to catalyze the electroreduction of protons into hydrogen in acidic organic media, with good Faradaic yields (75–95%) and good activity (rate constants of 100 s^–1), with relatively high overpotentials (700 mV). They also catalyze proton reduction into hydrogen upon visible light irradiation, in combination with [Ru­(bipyridine)₃]²⁺ as a photosensitizer and ascorbic acid as a sacrificial electron donor. On the basis of detailed DFT calculations, a reaction mechanism is proposed in which the starting W^VIO₂ (L = dithiolene)₂ complex acts as a precatalyst and hydrogen is further formed from a key reduced W–hydroxo–hydride intermediate

A Bioinspired Nickel(bis-dithiolene) Complex as a Homogeneous Catalyst for Carbon Dioxide Electroreduction

Inspired by the metal active sites of formate dehydrogenase and CO-dehydrogenase, a nickel complex containing a NiS₄ motif with two dithiolene ligands mimicking molybdopterin has been prepared and structurally characterized. During electroreduction, it converts to a good catalyst for the reduction of CO₂ into formate as the major product, together with minor amounts of carbon monoxide and hydrogen, with reasonable overpotential requirement, good faradaic yield, and notable stability. Catalysis operates on a mercury electrode and dramatically less on a carbon electrode, as observed in the case of [Ni­(cyclam)]²⁺ complexes. Density functional theory (DFT) computations indicate the key role of a Ni­(III)-hydride intermediate and provide insights into the different reaction pathways leading to HCOOH, CO, and H₂. This study opens the route toward a new, yet unexplored, class of mononuclear sulfur-coordinated Ni catalysts for CO₂ reduction

Engineering the Optical Response of the Titanium-MIL-125 Metal–Organic Framework through Ligand Functionalization

Herein we discuss band gap modification of MIL-125, a TiO₂/1,4-benzenedicarboxylate (<b>bdc</b>) metal–organic framework (MOF). Through a combination of synthesis and computation, we elucidated the electronic structure of MIL-125 with aminated linkers. The band gap decrease observed when the monoaminated <b>bdc-NH</b>_<b>2</b> linker was used arises from donation of the N 2p electrons to the aromatic linking unit, resulting in a red-shifted band above the valence-band edge of MIL-125. We further explored in silico MIL-125 with the diaminated linker <b>bdc-(NH</b>_<b>2</b><b>)</b>_<b>2</b> and other functional groups (−OH, −CH₃, −Cl) as alternative substitutions to control the optical response. The <b>bdc-(NH</b>_<b>2</b><b>)</b>_<b>2</b> linking unit was predicted to lower the band gap of MIL-125 to 1.28 eV, and this was confirmed through the targeted synthesis of the <b>bdc-(NH</b>_<b>2</b><b>)</b>_<b>2</b>-based MIL-125. This study illustrates the possibility of tuning the optical response of MOFs through rational functionalization of the linking unit, and the strength of combined synthetic/computational approaches for targeting functionalized hybrid materials

Porous–Hybrid Polymers as Platforms for Heterogeneous Photochemical Catalysis

A number of permanently porous polymers containing Ru­(bpy)_<i>n</i> photosensitizer or a cobaloxime complex, as a proton-reduction catalyst, were constructed via one-pot Sonogashira–Hagihara (SH) cross-coupling reactions. This process required minimal workup to access porous platforms with control over the apparent surface area, pore volume, and chemical functionality from suitable molecular building blocks (MBBs) containing the Ru or Co complexes, as rigid and multitopic nodes. The cobaloxime molecular building block, generated through <i>in situ</i> metalation, afforded a microporous solid that demonstrated noticeable catalytic activity toward hydrogen-evolution reaction (HER) with remarkable recyclability. We further demonstrated, in two cases, the ability to affect the excited-state lifetime of the covalently immobilized Ru­(bpy)₃ complex attained through deliberate utilization of the organic linkers of variable dimensions. Overall, this approach facilitates construction of tunable porous solids, with hybrid composition and pronounced chemical and physical stability, based on the well-known Ru­(bpy)_<i>n</i>or the cobaloxime complexes

Rhenium Complexes Based on 2‑Pyridyl-1,2,3-triazole Ligands: A New Class of CO₂ Reduction Catalysts

A series of [Re­(N^N)­(CO)₃(X)] (N^N = diimine and X = halide) complexes based on 4-(2-pyridyl)-1,2,3-triazole (pyta) and 1-(2-pyridyl)-1,2,3-triazole (tapy) diimine ligands have been prepared and electrochemically characterized. The first ligand-based reduction process is shown to be highly sensitive to the nature of the isomer as well as to the substituents on the pyridyl ring, with the peak potential changing by up to 700 mV. The abilities of this class of complexes to catalyze the electroreduction and photoreduction of CO₂ were assessed for the first time. It is found that only Re pyta complexes that have a first reduction wave with a peak potential at ca. −1.7 V vs SCE are active, producing CO as the major product, together with small amounts of H₂ and formic acid. The catalytic wave that is observed in the CVs is enhanced by the addition of water or trifluoroethanol as a proton source. Long-term controlled potential electrolysis experiments gave total Faradaic yield close to 100%. In particular, functionalization of the triazolyl ring with a 2,4,6-tri-<i>tert</i>-butylphenyl group provided the catalyst with a remarkable stability