Cooperative Self-Assembly Process Involving Giant Toroidal Polyoxometalate as a Membrane Building Block in Nanoscale Vesicles.

The self-assembly of organic amphiphilic species into various aggregates such as spherical or elongated micelles and cylinders up to the formation of lyotropic hexagonal or lamellar phases results from cooperative processes orchestrated by the hydrophobic effect, while those involving ionic inorganic polynuclear entities and nonionic organic components are still intriguing. Herein, we report on the supramolecular behavior of giant toroidal molybdenum blue-type polyoxometalate, namely, the {Mo154} species in the presence of n-octyl-ß-glucoside (C8G1), widely used as a surfactant in biochemistry. Structural investigations were carried out using a set of complementary multiscale methods including single-crystal X-ray diffraction analysis supported by molecular modeling, small-angle X-ray scattering and cryo-TEM observations. In addition, liquid NMR, viscosimetry, surface tension measurement, and isothermal titration calorimetry provided further information to decipher the complex aggregation pathway. Elucidation of the assembly process reveals a rich scenario where the presence of the large {Mo154} anion disrupts the self-assembly of the C8G1, well-known to produce micelles, and induces striking successive phase transitions from fluid-to-gel and from gel-to-fluid. Herein, intimate organic-inorganic primary interactions arising from the superchaotropic nature of the {Mo154} lead to versatile nanoscopic hybrid C8G1-{Mo154} aggregates including crystalline discrete assemblies, smectic lamellar liquid crystals, and large uni- or multilamellar vesicles where the large torus {Mo154} acts a trans-membrane component.

Unravelling the Molecular Structure and Confining Environment of an Organometallic Catalyst Heterogenized within Amorphous Porous Polymers.

The catalytic activity of multifunctional, microporous materials is directly linked to the spatial arrangement of their structural building blocks. Despite great achievements in the design and incorporation of isolated catalytically active metal complexes within such materials, a detailed understanding of their atomic-level structure and the local environment of the active species remains a fundamental challenge, especially when these latter are hosted in non-crystalline organic polymers. Here, we show that by combining computational chemistry with pair distribution function analysis, 129 Xe NMR, and Dynamic Nuclear Polarization enhanced NMR spectroscopy, a very accurate description of the molecular structure and confining surroundings of a catalytically active Rh-based organometallic complex incorporated inside the cavity of amorphous bipyridine-based porous polymers is obtained. Small, but significant, differences in the structural properties of the polymers are highlighted depending on their backbone motifs.

Impact of Organic Templates on the Selective Formation of Zeolite Oligomers.

Zeolites are essential materials to industry due to their adsorption and catalytic properties. The best current approach to prepare a targeted zeolite still relies on trial and error's synthetic procedures since a rational understanding of the impact of synthesis variables on the final structures is still missing. To discern the role of a variety of organic templates, we perform simulations of the early stages of condensation of silica oligomers by combining DFT, Brønsted-Evans-Polanyi relationships and kinetic Monte Carlo simulations. We investigate an extended reaction path mechanism including 258 equilibrium reactions and 242 chemical species up to silica octamers, comparing the computed concentrations of Si oligomers with 29 SI NMR experimental data. The effect of the templating agent is linked to the modification of the intramolecular H-bond network in the growing oligomer, which produces higher concentration of 4-membered ring intermediates, precursors of the key double-four ring building blocks present on more than 39 known zeolite topologies.

Co-immobilization of a Rh Catalyst and a Keggin Polyoxometalate in the UiO-67 Zr-Based Metal-Organic Framework: In Depth Structural Characterization and Photocatalytic Properties for CO2 Reduction.

The Keggin-type polyoxometalate (POM) PW12O403- and the catalytic complex Cp*Rh(bpydc)Cl2 (bpydc = 2,2'-bipyridine-5,5'-dicarboxylic acid) were coimmobilized in the Zr(IV) based metal organic framework UiO-67. The POM is encapsulated within the cavities of the MOF by in situ synthesis, and then, the Rh catalytic complex is introduced by postsynthetic linker exchange. Infrared and Raman spectroscopies, 31P and 13C MAS NMR, N2 adsorption isotherms, and X-ray diffraction indicate the structural integrity of all components (POM, Rh-complex and MOF) within the composite of interest (PW12,Cp*Rh)@UiO-67. DFT calculations identified two possible locations of the POM in the octahedral cavities of the MOF: one at the center of a UiO-67 pore with the Cp*Rh complex pointing toward an empty pore and one off-centered with the Cp*Rh pointing toward the POM. 31P-1H heteronuclear (HETCOR) experiments ascertained the two environments of the POM, equally distributed, with the POM in interaction either with the Cp* fragment or with the organic linker. In addition, Pair Distribution Function (PDF) data were collected on the POM@MOF composite and provided key evidence of the structural integrity of the POM once immobilized into the MOF. The photocatalytic activity of the (PW12,Cp*Rh)@UiO-67 composite for CO2 reduction into formate and hydrogen were evaluated. The formate production was doubled when compared with that observed with the POM-free Cp*Rh@UiO-67 catalyst and reached TONs as high as 175 when prepared as thin films, showing the beneficial influence of the POM. Finally, the stability of the composite was assessed by means of recyclability tests. The combination of XRD, IR, ICP, and PDF experiments was essential in confirming the integrity of the POM, the catalyst, and the MOF after catalysis.

Molecular Porous Photosystems Tailored for Long-Term Photocatalytic CO2 Reduction.

The molecular-level structuration of two full photosystems into conjugated porous organic polymers is reported. The strategy of heterogenization gives rise to photosystems which are still fully active after 4 days of continuous illumination. Those materials catalyze the carbon dioxide photoreduction driven by visible light to produce up to three grams of formate per gram of catalyst. The covalent tethering of the two active sites into a single framework is shown to play a key role in the visible light activation of the catalyst. The unprecedented long-term efficiency arises from an optimal photoinduced electron transfer from the light harvesting moiety to the catalytic site as anticipated by quantum mechanical calculations and evidenced by in situ ultrafast time-resolved spectroscopy.

The UbiK protein is an accessory factor necessary for bacterial ubiquinone (UQ) biosynthesis and forms a complex with the UQ biogenesis factor UbiJ.

Ubiquinone (UQ), also referred to as coenzyme Q, is a widespread lipophilic molecule in both prokaryotes and eukaryotes in which it primarily acts as an electron carrier. Eleven proteins are known to participate in UQ biosynthesis in Escherichia coli, and we recently demonstrated that UQ biosynthesis requires additional, nonenzymatic factors, some of which are still unknown. Here, we report on the identification of a bacterial gene, yqiC, which is required for efficient UQ biosynthesis, and which we have renamed ubiK Using several methods, we demonstrated that the UbiK protein forms a complex with the C-terminal part of UbiJ, another UQ biogenesis factor we previously identified. We found that both proteins are likely to contribute to global UQ biosynthesis rather than to a specific biosynthetic step, because both ubiK and ubiJ mutants accumulated octaprenylphenol, an early intermediate of the UQ biosynthetic pathway. Interestingly, we found that both proteins are dispensable for UQ biosynthesis under anaerobiosis, even though they were expressed in the absence of oxygen. We also provide evidence that the UbiK-UbiJ complex interacts with palmitoleic acid, a major lipid in E. coli Last, in Salmonella enterica, ubiK was required for proliferation in macrophages and virulence in mice. We conclude that although the role of the UbiK-UbiJ complex remains unknown, our results support the hypothesis that UbiK is an accessory factor of Ubi enzymes and facilitates UQ biosynthesis by acting as an assembly factor, a targeting factor, or both.

A Fully Noble Metal-Free Photosystem Based on Cobalt-Polyoxometalates Immobilized in a Porphyrinic Metal-Organic Framework for Water Oxidation.

The sandwich-type polyoxometalate (POM) [(PW9O34)2Co4(H2O)2]10- was immobilized in the hexagonal channels of the Zr(IV) porphyrinic MOF-545 hybrid framework. The resulting composite was fully characterized by a panel of physicochemical techniques. Calculations allowed identifying the localization of the POM in the vicinity of the Zr6 clusters and porphyrin linkers constituting the MOF. The material exhibits a high photocatalytic activity and good stability for visible-light-driven water oxidation. It thus represents a rare example of an all-in-one fully noble metal-free supramolecular heterogeneous photocatalytic system, with the catalyst and the photosensitizer within the same porous solid material.

Maximizing the Photocatalytic Activity of Metal-Organic Frameworks with Aminated-Functionalized Linkers: Substoichiometric Effects in MIL-125-NH2.

Despite the promise of utilizing metal-organic frameworks (MOFs) as highly tunable photocatalytic materials, systematic studies that interrogate the relationship between their catalytic performances and the amount of functionalized linkers are lacking. Aminated linkers are known to enhance the absorption of light and afford photocatalysis with MOFs under visible-light irradiation. However, the manner in which the photocatalytic performances are impacted by the amount of such linkers is poorly understood. Here, we assess the photocatalytic activity of MIL-125, a TiO2/1,4-benzenedicarboxylate (bdc) MOF for the oxidation of benzyl alcohol to benzaldehyde when increasing amounts of bdc-NH2 linkers (0%, 20%, 46%, 70%, and 100%) are incorporated in the framework. Analytical TEM allowed assessing the homogeneous localization of bdc-NH2 in these mixed-linker MOFs. Steady state reaction rates reveal two regimes of catalytic performances: a first linear regime up to ∼50% bdc-NH2 into the hybrid framework whereby increased amounts of bdc-NH2 yielded increased photocatalytic rates, followed by a plateau up to 100% bdc-NH2. This unexpected "saturation" of the catalytic activity above ∼50% bdc-NH2 content in the framework whatever the wavelength filters used demonstrates that amination of all linkers of the MOF is not required to obtain the maximum photocatalytic activity. This is rationalized on the basis of mixed-valence Ti3+/Ti4+ intermediate catalytic centers revealed by electron spin resonance (ESR) measurements and recent knowledge of lifetime excited states in MIL-125-type of solids.

Coenzyme Q Biosynthesis: Evidence for a Substrate Access Channel in the FAD-Dependent Monooxygenase Coq6.

Coq6 is an enzyme involved in the biosynthesis of coenzyme Q, a polyisoprenylated benzoquinone lipid essential to the function of the mitochondrial respiratory chain. In the yeast Saccharomyces cerevisiae, this putative flavin-dependent monooxygenase is proposed to hydroxylate the benzene ring of coenzyme Q (ubiquinone) precursor at position C5. We show here through biochemical studies that Coq6 is a flavoprotein using FAD as a cofactor. Homology models of the Coq6-FAD complex are constructed and studied through molecular dynamics and substrate docking calculations of 3-hexaprenyl-4-hydroxyphenol (4-HP6), a bulky hydrophobic model substrate. We identify a putative access channel for Coq6 in a wild type model and propose in silico mutations positioned at its entrance capable of partially (G248R and L382E single mutations) or completely (a G248R-L382E double-mutation) blocking access to the channel for the substrate. Further in vivo assays support the computational predictions, thus explaining the decreased activities or inactivation of the mutated enzymes. This work provides the first detailed structural information of an important and highly conserved enzyme of ubiquinone biosynthesis.

Coq6 is responsible for the C4-deamination reaction in coenzyme Q biosynthesis in Saccharomyces cerevisiae.

The yeast Saccharomyces cerevisiae is able to use para-aminobenzoic acid (pABA) in addition to 4-hydroxybenzoic acid as a precursor of coenzyme Q, a redox lipid essential to the function of the mitochondrial respiratory chain. The biosynthesis of coenzyme Q from pABA requires a deamination reaction at position C4 of the benzene ring to substitute the amino group with an hydroxyl group. We show here that the FAD-dependent monooxygenase Coq6, which is known to hydroxylate position C5, also deaminates position C4 in a reaction implicating molecular oxygen, as demonstrated with labeling experiments. We identify mutations in Coq6 that abrogate the C4-deamination activity, whereas preserving the C5-hydroxylation activity. Several results support that the deletion of Coq9 impacts Coq6, thus explaining the C4-deamination defect observed in Δcoq9 cells. The vast majority of flavin monooxygenases catalyze hydroxylation reactions on a single position of their substrate. Coq6 is thus a rare example of a flavin monooxygenase that is able to act on two different carbon atoms of its C4-aminated substrate, allowing its deamination and ultimately its conversion into coenzyme Q by the other proteins constituting the coenzyme Q biosynthetic pathway.

A Simple and Non-Destructive Method for Assessing the Incorporation of Bipyridine Dicarboxylates as Linkers within Metal-Organic Frameworks.

As a novel avenue for applications, metal-organic frameworks (MOFs) are increasingly used for heterogenizing catalytic molecular species as linkers into their crystalline framework. These multifunctional compounds can be accessed with mixed linkers synthesis or postsynthetic-exchange strategies. Major limitations still reside in their challenging characterization; in particular, to provide evidence of the genuine incorporation of the functionalized linkers into the framework and their quantification. Herein, we demonstrate that a combination of computational chemistry, spectroscopy and X-ray diffraction allows access to a non-destructive analysis of mixed-linker UiO-67-type materials featuring biphenyl- and bipyridine-dicarboxylates. Our UV/Vis-based methodology has been further applied to characterize a series of Rh-functionalized UiO-67-type catalysts. The proposed approach allows a recurrent key issue in the characterization of similar supported organometallic systems to be solved.

Molecular Level Characterization of the Structure and Interactions in Peptide-Functionalized Metal-Organic Frameworks.

We use density functional theory, newly parameterized molecular dynamics simulations, and last generation 15 N dynamic nuclear polarization surface enhanced solid-state NMR spectroscopy (DNP SENS) to understand graft-host interactions and effects imposed by the metal-organic framework (MOF) host on peptide conformations in a peptide-functionalized MOF. Focusing on two grafts typified by MIL-68-proline (-Pro) and MIL-68-glycine-proline (-Gly-Pro), we identified the most likely peptide conformations adopted in the functionalized hybrid frameworks. We found that hydrogen bond interactions between the graft and the surface hydroxyl groups of the MOF are essential in determining the peptides conformation(s). DNP SENS methodology shows unprecedented signal enhancements when applied to these peptide-functionalized MOFs. The calculated chemical shifts of selected MIL-68-NH-Pro and MIL-68-NH-Gly-Pro conformations are in a good agreement with the experimentally obtained 15 N NMR signals. The study shows that the conformations of peptides when grafted in a MOF host are unlikely to be freely distributed, and conformational selection is directed by strong host-guest interactions.

Connecting defects and amorphization in UiO-66 and MIL-140 metal­organic frameworks: a combined experimental and computational study.

The mechanism and products of the structural collapse of the metal­organic frameworks (MOFs) UiO-66, MIL-140B and MIL-140C upon ball-milling are investigated through solid state 13C NMR and pair distribution function (PDF) studies, finding amorphization to proceed by the breaking of a fraction of metal­ligand bonding in each case. The amorphous products contain inorganic­organic bonding motifs reminiscent of the crystalline phases. Whilst the inorganic Zr6O4(OH)4 clusters of UiO-66 remain intact upon structural collapse, the ZrO backbone of the MIL-140 frameworks undergoes substantial distortion. Density functional theory calculations have been performed to investigate defective models of MIL-140B and show, through comparison of calculated and experimental 13C NMR spectra, that amorphization and defects in the materials are linked.

Combined experimental and computational NMR study of crystalline and amorphous zeolitic imidazolate frameworks.

Zeolitic imidazolate frameworks (ZIFs) have attracted great interest in recent years due to their high chemical and thermal stability with promising applications in gas storage and separations. We investigate the structures of three different crystalline ZIFs - ZIF-4, ZIF-8, ZIF-zni - and their amorphous counterparts using high field (13)C and (15)N CP MAS NMR. The high field (20 T) allows for the observation of all crystallographically independent carbon and nitrogen atoms in the crystalline ZIFs. Combining our experimental results with density functional theory calculations enabled the assignment of all chemical shifts. The crystalline spectra reveal the potential of high field NMR to distinguish between two ZIF polymorphs, ZIF-4 and ZIF-zni, with identical [Zn(C3H3N2)2] chemical compositions. (13)C and (15)N CP MAS NMR data obtained for the amorphous ZIFs clearly showed signal broadening upon amorphization, confirming the retention of chemical composition and the structural similarity of amorphous ZIF-4 and ZIF-zni. In the case of amorphous ZIF-8, we present evidence for the partial de-coordination of the 2-methyl imidazole linker.

Extreme Flexibility in a Zeolitic Imidazolate Framework: Porous to Dense Phase Transition in Desolvated ZIF-4.

Desolvated zeolitic imidazolate framework ZIF-4(Zn) undergoes a discontinuous porous to dense phase transition on cooling through 140 K, with a 23 % contraction in unit cell volume. The structure of the non-porous, low temperature phase was determined from synchrotron X-ray powder diffraction data and its density was found to be slightly less than that of the densest ZIF phase, ZIF-zni. The mechanism of the phase transition involves a cooperative rotation of imidazolate linkers resulting in isotropic framework contraction and pore space minimization. DFT calculations established the energy of the new structure relative to those of the room temperature phase and ZIF-zni, while DSC measurements indicate the entropic stabilization of the porous room temperature phase at temperatures above 140 K.

ubiI, a new gene in Escherichia coli coenzyme Q biosynthesis, is involved in aerobic C5-hydroxylation.

Coenzyme Q (ubiquinone or Q) is a redox-active lipid found in organisms ranging from bacteria to mammals in which it plays a crucial role in energy-generating processes. Q biosynthesis is a complex pathway that involves multiple proteins. In this work, we show that the uncharacterized conserved visC gene is involved in Q biosynthesis in Escherichia coli, and we have renamed it ubiI. Based on genetic and biochemical experiments, we establish that the UbiI protein functions in the C5-hydroxylation reaction. A strain deficient in ubiI has a low level of Q and accumulates a compound derived from the Q biosynthetic pathway, which we purified and characterized. We also demonstrate that UbiI is only implicated in aerobic Q biosynthesis and that an alternative enzyme catalyzes the C5-hydroxylation reaction in the absence of oxygen. We have solved the crystal structure of a truncated form of UbiI. This structure shares many features with the canonical FAD-dependent para-hydroxybenzoate hydroxylase and represents the first structural characterization of a monooxygenase involved in Q biosynthesis. Site-directed mutagenesis confirms that residues of the flavin binding pocket of UbiI are important for activity. With our identification of UbiI, the three monooxygenases necessary for aerobic Q biosynthesis in E. coli are known.

Zr-Based MOF-545 Metal-Organic Framework Loaded with Highly Dispersed Small Size Ni Nanoparticles for CO2 Methanation.

We report the use of Zr-based metal-organic frameworks (MOFs) MOF-545 and MOF-545(Cu) as supports to prepare catalysts with uniformly and highly dispersed Ni nanoparticles (NPs) for CO2 hydrogenation into CH4. In the first step, we studied the MOF support under catalytic conditions using operando diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, ex situ characterizations (PXRD, XPS, TEM, and EDX-element mapping), and DFT calculations. We showed that the high-temperature conditions undoubtedly confer a potential for catalytic functionality to the solids toward CH4 production, while no role of the Cu could be evidenced. The MOF was shown to be transformed into a catalytically active material, amorphized but still structured with dehydroxylated Zr-oxoclusters, in line with DFT calculations. In the second step, Ni@MOF-545 catalysts were prepared using either impregnation (IM) or double solvent (DS) methods, followed by a dry reduction (R) route under H2 to immobilize Ni NPs. The highest catalytic activity was obtained with the Ni@MOF-545 DS R catalyst (595 mmolCH4 gNi-1 h-1) with 100% CH4 selectivity and 60% CO2 conversion after ∼3 h. The higher catalytic activity of Ni@MOF-545 DS R is a result of much smaller (∼5 nm) and better dispersed Ni NPs than in the IM sample (20-40 nm), the latter exhibiting sintering. The advantages of the encapsulation of Ni NPs by the DS method and of the use of a MOF-545-based support are discussed, highlighting the interest of designing yet-unexplored Zr-based MOFs loaded with Ni NPs for CO2 hydrogenation.

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 (bdc) 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 bdc-NH2 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 bdc-(NH2)2 and other functional groups (-OH, -CH3, -Cl) as alternative substitutions to control the optical response. The bdc-(NH2)2 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 bdc-(NH2)2-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.