Axial Hydroxyl Research Articles

Catalytic Hydrogenolysis by Atomically Dispersed Iron Sites Embedded in Chemically and Redox Non-innocent N-Doped Carbon.

Atomically dispersed first-row transition metals embedded in nitrogen-doped carbon materials (M-N-C) show promising performance in catalytic hydrogenation but are less well-studied for reactions with more complex mechanisms, such as hydrogenolysis. Their ability to catalyze selective C-O bond cleavage of oxygenated hydrocarbons such as aryl alcohols and ethers is enhanced with the participation of ligands directly bound to the metal ion as well as longer-range contributions from the support. In this article, we describe how Fe-N-C catalysts with well-defined local structures for the Fe sites catalyze C-O bond hydrogenolysis. The reaction is facilitated by the N-C support. According to spectroscopic analyses, the as-synthesized catalysts contain mostly pentacoordinated FeIII sites, with four in-plane nitrogen donor ligands and one axial hydroxyl ligand. In the presence of 20 bar of H2 at 170-230 °C, the hydroxyl ligand is lost when N4FeIIIOH is reduced to N4FeII, assisted by the H2 chemisorbed on the support. When an alcohol binds to the tetracoordinated FeII sites, homolytic cleavage of the O-H bond is accompanied by reoxidation to FeIII and H atom transfer to the support. The role of the N-C support in catalytic hydrogenolysis is analogous to the behavior of chemically and redox-non-innocent ligands in molecular catalysts based on first-row transition metal ions and enhances the ability of M-N-Cs to achieve the types of multistep activations of strong bonds needed to upgrade renewable and recycled feedstocks.

FeN3S1─OH Single-Atom Sites Anchored on Hollow Porous Carbon for Highly Efficient pH-Universal Oxygen Reduction Reaction.

Regulating the asymmetric active center of a single-atom catalyst to optimize the binding energy is critical but challenging to improve the overall efficiency of the electrocatalysts. Herein, an effective strategy is developed by introducing an axial hydroxyl (OH) group to the Fe─N4 center, simultaneously assisting with the further construction of asymmetric configurations by replacing one N atom with one S atom, forming FeN3S1─OH configuration. This novel structure can optimize the electronic structure and d-band center shift to reduce the reaction energy barrier, thereby promoting oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic activities. The optimal catalyst, FeSA-S/N-C (FeN3S1─OH anchored on hollow porous carbon) displays remarkable ORR performance with a half-wave potential of 0.92, 0.78, and 0.64V versus RHE in 0.1m KOH, 0.5m H2SO4, and 0.1m PBS, respectively. The rechargeable liquid Zn-air batteries (LZABs) equipped with FeSA-S/N-C display a higher power density of 128.35mWcm-2, long-term operational stability of over 500h, and outstanding reversibility. More importantly, the corresponding flexible solid-state ZABs (FSZABs@FeSA-S/N-C) display negligible voltage changes at different bending angles during the charging and discharging processes. This work provides a new perspective for the design and optimization of asymmetric configuration for single-atom catalysts applied to the area of energy conversion and storage.

Coordination Engineering Induced d-Band Center Shift on Single-Atom Fe Electrocatalysts for Enhanced Oxygen Reduction.

Part of the performance of single-atom catalysts (SACs) is greatly influenced by the microenvironment around a single metal site, of which the oxygen reduction reaction (ORR) is counted as one of them. However, an in-depth understanding of the catalytic activity regulation by the coordination environment is still lacking. In this study, a single Fe active center with axial fifth hydroxyl (OH) and asymmetric N,S coordination embedded in a hierarchically porous carbon material (Fe-SNC) are prepared. Compared to Pt/C and most of the reported SACs, as-prepared Fe-SNC has certain advantages in terms of ORR activity and maintains sufficient stability. Furthermore, the assembled rechargeable Zn-air battery exhibits impressive performance. The combination of multiple findings revealed that the introduction of S atoms not only facilitates the formation of porous structures but also facilitates the desorption and adsorption of oxygen intermediates. On the other hand, with the introduction of axial OH groups, the bonding strength of the ORR intermediate is reduced, and even the central position of the Fe d-band is optimized. The developed catalyst is expected to lead to further research on the multiscale design of the electrocatalyst microenvironment.

TsOH-catalyzed acyl migration reaction of the Bz-group: innovative assembly of various building blocks for the synthesis of saccharides.

We developed an efficient method to achieve the regioselective acyl migration of benzoyl ester. In all the cases, the reactions required only the commercially available organic acid catalyst TsOH·H2O. This method enables the benzoyl group to migrate from secondary groups to primary hydroxyl groups, or from equatorial secondary hydroxyl groups to axial hydroxyl groups. The 1,2 or 1,3 acyl migration would potentially occur via five- and six-membered cyclic ortho acid intermediates. A wide range of orthogonally protected monosaccharides, which are useful intermediates for the synthesis of natural oligosaccharides, were synthesized. Finally, to demonstrate the utility of the method, a tetrasaccharide portion from a mycobacterial cell wall polysaccharide was assembled.

Selectivity of Oxidizing Agents toward Axial and Equatorial Hydroxyl Groups.

A total of 16 oxidizing reagents were screened to compare their oxidation selectivities for axial and equatorial hydroxyl moieties using steroidal methyl chenodeoxycholate, methyl deoxycholate, and 4-tert-butylcyclohexanol (cis/trans 1:1 mixture). These compounds were selected for their stable chair conformations. The results of our study demonstrated that, for the oxidation of a scaffold bearing both axial and equatorial hydroxyl groups, nitroxide-radical-based reagents should be the first choice if oxidation of the equatorial hydroxyl group is needed and Stevens or Dess-Martin reagents should be the first choice for the preferential oxidation of the axial hydroxyl group.

Pattern of Specific Oxidation of Konjac Glucomannan with TEMPO/NaBr/NaClO system

The TEMPO/NaBr/NaClO system was used to modify konjac glucomannan and prepare β-1,4-linked D-mannuronic/glucuronic acid oligosaccharides. The impact of oxidant amount on the degree of oxidation, molecular weight, terminal structure and the ratio of sugar units were systematically investigated. Mannoses were transformed to mannuronic acids quantitatively, while some glucose were transformed to glucuronic acids, some degraded. The unspecific oxidation causes strong degradation of konjac glucomannan sugar chains. Multiple terminal structures were observed in the oxidized KGM products. According to those observations, the oxidation pattern of konjac glucomannan with the TEMPO/NaBr/NaClO system was speculated. It is suggested that the axial hydroxyl group at position 2 in mannose might form a hydrogen bond with the oxygen on the sugar ring. It would against the unspecific oxidation. While, both specific and unspecific oxidations were observed at glucose units with an equatorial hydroxyl group at position 2, and the degradation occurs mostly on glucose during the oxidation.

Penta -hexa coordination behaviour of ABA-P(V) corrole

• Phosphorous(V) corroles exist in penta-hexa coordination in non-coordinating solvents. • Hexa-coordinated geometries were obtained in coordinating solvents. • Solid state geometries prefer to be in hexa-coordinated systems. • 31 P, 1 H and 19 F NMR provide proof for penta-hexa behavior. ABA (where A = nitrophenyl, and B = pentafluorophenyl group) corrole was isolated as hexacoordinate P(V) complexes that contained two axial hydroxyl groups. NMR spectroscopy ( 1 H, 19 F and 31 P) in CDCl 3 indicated that the hexacoordinated P(V) complexes were prone to axial-ligand dissociation to form pentacoordinated P(V) complexes. Although, in the presence of strong coordinating solvents, such as CH 3 OH and CD 3 OD, the P(V) corrole complexes were observed to exist in a hexacoordinated geometry in which the strong solvent molecules acted as the ligands at the axial position. Also, we report here the existence of the hexacoordinated P(V) complexes in non-coordinating solvents such as CHCl 3, CH 2 Cl 2 by using fluoride and methyl substituted silyloxy groups acting as the ligands at the axial position. Moderately bulky nature and electron-donation effect of silyloxy groups proved to be of great aid to prevent the axial ligand dissociation of silyloxy substituted hexacoordinated P(V) complex and strong P-F bond in non-coordinating solvents. The structure of hexacoordinate P(V) complex containing two axial -OH groups and hexacoordinate P(V) complex containing two axial -OCH 3 groups were confirmed by X-ray crystallography.

Progesterone receptor membrane component 1 (PGRMC1) binds and stabilizes cytochromes P450 through a heme-independent mechanism

Progesterone receptor membrane component 1 (PGRMC1) is a heme-binding protein implicated in a wide range of cellular functions. We previously showed that PGRMC1 binds to cytochromes P450 in yeast and mammalian cells and supports their activity. Recently, the paralog PGRMC2 was shown to function as a heme chaperone. The extent of PGRMC1 function in cytochrome P450 biology and whether PGRMC1 is also a heme chaperone are unknown. Here, we examined the function of Pgrmc1 in mouse liver using a knockout model and found that Pgrmc1 binds and stabilizes a broad range of cytochromes P450 in a heme-independent manner. Proteomic and transcriptomic studies demonstrated that Pgrmc1 binds more than 13 cytochromes P450 and supports maintenance of cytochrome P450 protein levels posttranscriptionally. In vitro assays confirmed that Pgrmc1 KO livers exhibit reduced cytochrome P450 activity consistent with reduced enzyme levels. Mechanistic studies in cultured cells demonstrated that PGRMC1 stabilizes cytochromes P450 and that binding and stabilization do not require PGRMC1 binding to heme. Importantly, Pgrmc1-dependent stabilization of cytochromes P450 is physiologically relevant, as Pgrmc1 deletion protected mice from acetaminophen-induced liver injury. Finally, evaluation of Y113F mutant Pgrmc1, which lacks the axial heme iron-coordinating hydroxyl group, revealed that proper iron coordination is not required for heme binding, but is required for binding to ferrochelatase, the final enzyme in heme biosynthesis. PGRMC1 was recently identified as the causative mutation in X-linked isolated pediatric cataract formation. Together, these results demonstrate a heme-independent function for PGRMC1 in cytochrome P450 stability that may underlie clinical phenotypes.

Understanding carbon dioxide capture on metal-organic frameworks from first-principles theory: The case of MIL-53(X), with X = Fe3+, Al3+, and Cu2.

Metal-organic frameworks (MOFs) constitute a class of three-dimensional porous materials that have shown applicability for carbon dioxide capture at low pressures, which is particularly advantageous in dealing with the well-known environmental problem related to the carbon dioxide emissions into the atmosphere. In this work, the effect of changing the metallic center in the inorganic counterpart of MIL-53 (X), where X = Fe3+, Al3+, and Cu2+, has been assessed over the ability of the porous material to adsorb carbon dioxide by means of first-principles theory. In general, the non-spin polarized computational method has led to adsorption energies in fair agreement with the experimental outcomes, where the carbon dioxide stabilizes at the pore center through long-range interactions via oxygen atoms with the axial hydroxyl groups in the inorganic counterpart. However, spin-polarization effects in connection with the Hubbard corrections, on Fe 3d and Cu 3d states, were needed to properly describe the metal orbital occupancy in the open-shell systems (Fe- and Cu-based MOFs). This methodology gave rise to a coherent high-spin configuration, with five unpaired electrons, for Fe atoms leading to a better agreement with the experimental results. Within the GGA+U level of theory, the binding energy for the Cu-based MOF is found to be Eb = -35.85 kJ/mol, which is within the desirable values for gas capture applications. Moreover, it has been verified that the adsorption energetics is dominated by the gas-framework and internal weak interactions.

A Synthetic Galectin Mimic

AbstractGalectins are a galactoside specific subclass of carbohydrate binding proteins (lectins) involved in various cellular activities, certain cancers, infections, inflammations, and many other biological processes. The molecular basis for the selectivity of galectins is well‐documented and revolves around appropriate interaction complementarity: an aromatic residue for C−H⋅⋅⋅π interactions and polar residues for (charge assisted) hydrogen bonds with the axial hydroxyl group of a galactoside. However, no synthetic mimics are currently available. We now report on the design and synthesis of the first galectin mimic (6), and show that it has a higher than 65‐fold preference for n‐octyl‐β‐galactoside (8) over n‐octyl‐β‐glucoside (7) in CD2Cl2 containing 5 % [D6]DMSO (with Ka≥4500 M−1 for 6:8). Molecular modeling informed by nOe studies reveal a high degree of interaction complementarity between 6 and galactoside 8, which is very similar to the interaction complementarity found in natural galectins.

Glycopeptide Self-Assembly Modulated by Glycan Stereochemistry through Glycan-Aromatic Interactions.

Carbohydrates are often utilized to provide hydrophilicity and hydroxyl-based hydrogen bonds in self-assembling glycopeptides, affording versatile scaffolds with wide applicability in biomedical research. However, how stereochemistry of carbohydrates impacts the self-assembly process remains unclear. Here we have established a dimeric tyrosine-rich glycopeptide system for probing the corresponding hydrogelating behavior under the influence of site- and stereospecific glycosylations. Comparison of 18 glycoforms bearing monosaccharides at Tyr4 and Tyr4' shows that the glycopeptides with either α- or β-anomers exhibit contrary gelating abilities, when the glycan moieties contain axial hydroxyl groups. A high-resolution X-ray crystallographic structure of the β-galactose-containing gelator, along with other results from spectroscopic, microscopic, and rheological experiments, indicate an unusual carbohydrate-aromatic CH-π bonding that promotes glycopeptide self-assembly. These mechanistic findings, particularly evidence obtained at the angstrom scale, illuminate an unconventional role that carbohydrates can play in building supramolecules. Potential biomaterials exploiting the CH-π bond-based stabilization, as exemplified by an enzyme-resistant hydrogel, may thus be developed.

Unveiling the Axial Hydroxyl Ligand on Fe-N4-C Electrocatalysts and Its Impact on the pH-Dependent Oxygen Reduction Activities and Poisoning Kinetics.

Fe—N—C materials have shown a promising nonprecious oxygen reduction reaction (ORR) electrocatalyst yet their active site structure remains elusive. Several previous works suggest the existence of a mysterious axial ligand on the Fe center, which, however, is still unclarified. In this study, the mysterious axial ligand is identified as a hydroxyl ligand on the Fe centers and selectively promotes the ORR activities depending on different Fe—N4—C configurations, on which the adsorption free energy of the hydroxyl ligand also differs greatly. The selective formation of hydroxyl ligand on specific Fe—N—C configurations can resolve contradictories between previous theoretical and experimental results regarding the ORR activities and associated active configurations of Fe—N—C catalysts. It also explains the pH‐dependent ORR activities and, moreover, a previously unreported pH‐dependent poisoning kinetics of the Fe—N—C catalysts.

Myo-Inositol dehydrogenase and scyllo-inositol dehydrogenase from Lactobacillus casei BL23 bind their substrates in very different orientations

Many bacteria can use myo-inositol as the sole carbon source using enzymes encoded in the iol operon. The first step is catalyzed by the well-characterized myo-inositol dehydrogenase (mIDH), which oxidizes the axial hydroxyl group of the substrate to form scyllo-inosose. Some bacteria, including Lactobacillus casei, contain more than one apparent mIDH-encoding gene in the iol operon, but such redundant enzymes have not been investigated. scyllo-Inositol, a stereoisomer of myo-inositol, is not a substrate for mIDH, but scyllo-inositol dehydrogenase (sIDH) enzymes have been reported, though never observed to be encoded within the iol operon. Sequences indicate these enzymes are related, but the structural basis by which they distinguish their substrates has not been determined. Here we report the substrate selectivity, kinetics, and high-resolution crystal structures of the proteins encoded by iolG1 and iolG2 from L. casei BL23, which we show encode a mIDH and sIDH, respectively. Comparison of the ternary complex of each enzyme with its preferred substrate reveals the key variations allowing for oxidation of an equatorial versus an axial hydroxyl group. Despite the overall similarity of the active site residues, scyllo-inositol is bound in an inverted, tilted orientation by sIDH relative to the orientation of myo-inositol by mIDH.

Radical polymerizations of two stereoisomeric trimethacrylates with rigid adamantane‐like cores from naturally occurring myo‐inositol

ABSTRACTTwo stereoisomeric trimethacrylates, T1 and T2, which share a common adamantane‐like rigid core, were synthesized from naturally occurring myo‐inositol, and their radical polymerization behaviors were investigated. For the synthesis of T1, myo‐inositol was converted to triol 1, bearing one equatorial hydroxyl group and two axial hydroxyl groups, by orthoesterification, which was used as a precursor. For the synthesis of T2, 1 was converted to triol 2, bearing three axial hydroxyl groups, which was used as a precursor. Investigations on the radical polymerization of T1 and T2, which potentially accompanies the cyclopolymerization of the axially oriented methacrylate moieties, revealed significant differences between the two. (1) The polymerization of T1 affords networked and thus insoluble polymers PT1, while that of T2 affords less crosslinked and thus soluble polymers PT2. (2) The amount of residual methacrylate moieties was larger in PT2 than in PT1. (3) PT2 had higher thermal stability than PT1, though PT2 contained a larger amount of unreacted methacrylate moieties. These tendencies were successfully correlated with the difference in cyclopolymerization efficiency between the polymerizations of the two monomers. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 1743–1748

Syntheses of the plant auxin conjugate 2-O-(indole-3-acetyl)-myo-inositol IAInos.

The plant hormone conjugate 2-O-(indole-3-acetyl)-myo-inositol (IAInos) has been selectively prepared for the first time by two routes from myo-inositol. One of the syntheses depended upon the construction of the 3-indoleacetyl group by a Fischer indole synthesis on an unreactive axial hydroxyl group, while the other via a direct acylation of the equatorially orientated hydroxy group created by conformational constraint of the cyclohexane ring. The latter synthesis produced IAInos in 5 steps and 29% overall yield.

Site-Divergent Delivery of Terminal Propargyls to Carbohydrates by Synergistic Catalysis

Summary Carbohydrate-containing molecules are ubiquitous in nature and essential to sustaining lives. Selective modification of carbohydrates provides tremendous opportunities to interrogate the biological roles of this fundamental class of molecules. However, to functionalize a specific hydroxyl group out of numerous others in a carbohydrate scaffold has been a historical challenge. Here, we report a strategy that allows for the site-selective and site-divergent delivery of terminal propargyls to various monosaccharides, the basic units of carbohydrates. This strategy is based on synergistic catalysis that combines copper and borinic acid catalysis. With a pair of antipodal ligands, this method can be tuned to propargylate either the equatorial or the axial hydroxyl with high selectivities. The generality of this transformation is manifest in the direct propargylation of complex, unprotected glycosylated natural products. Equipped with terminal alkyne groups, the products obtained by this method are primed for further manipulations, including copper-catalyzed azide-alkyne coupling chemistry.

Simple synthesis of 32P-labelled inositol hexakisphosphates for study of phosphate transformations

Background and aimsIn many soils inositol hexakisphosphate in its various forms is as abundant as inorganic phosphate. The organismal and geochemical processes that exchange phosphate between inositol hexakisphosphate and other pools of soil phosphate are poorly defined, as are the organisms and enzymes involved. We rationalized that simple enzymic synthesis of inositol hexakisphosphate labeled with 32P would greatly enable study of transformation of soil inositol phosphates when combined with robust HPLC separations of different inositol phosphates.MethodsWe employed the enzyme inositol pentakisphosphate 2-kinase, IP5 2-K, to transfer phosphate from [γ-32P]ATP to axial hydroxyl(s) of myo-, neo- and 1D-chiro-inositol phosphate substrates.Results32P-labeled inositol phosphates were separated by anion exchange HPLC with phosphate eluents. Additional HPLC methods were developed to allow facile separation of myo-, neo-, 1D-chiro- and scyllo-inositol hexakisphosphate on acid gradients.ConclusionsWe developed enzymic approaches that allow the synthesis of labeled myo-inositol 1,[32P]2,3,4,5,6-hexakisphosphate; neo-inositol 1,[32P]2,3,4,[32P]5,6–hexakisphosphate and 1D-chiro-inositol [32P]1,2,3,4,5,[32P]6-hexakisphosphate. Additionally, we describe HPLC separations of all inositol hexakisphosphates yet identified in soils, using a collection of soil inositol phosphates described in the seminal historic studies of Cosgrove, Tate and coworkers. Our study will enable others to perform radiotracer experiments to analyze fluxes of phosphate to/from inositol hexakisphosphates in different soils.

(Invited) Axially Assembled Photosynthetic Antenna-Reaction Center Mimics

A series of porphyrin based donor-acceptor conjugates in which the arrangement of various components are in perpendicular orientation to the porphyrin plane have been constructed and used to study the pathways of electron and energy transfer processes. The employed aluminum(III) porphyrin in these studies is found to be synthetically more advantageous as it provides reaction site on both faces of the porphyrin plane. The axial hydroxyl group on one side of the aluminum(III) porphyrin plane is able to form a covalent ester or ether bond with an energy harvester or electron acceptor while Lewis bases such as pyridine or an imidazole group form a metal-ligand axial coordination on other side of the plane hosting an electron donor or acceptor. The advantage of this configuration of the conjugates is that the donor and acceptor sides of the complex are located on opposite faces of the porphyrin which minimizes the rate of charge recombination. Moreover, the excited state \U0001d70b* orbital of porphyrin and axially coordinated ligand lie in the same space which causes an increase in the electronic coupling thus facilitating the electron/energy transfer processes. The distance dependence of sequential electron transfer in vertical, linear supramolecular triads constructed using tetrathiafulvalene (TTF), aluminum(III) porphyrin (AlPor) and fullerene (C60) entities will be discussed. Further, results of photoinduced energy transfer followed by electron transfer in antenna-reaction center mimicking BODIPY-AlPor-C60 supramolecular triads will be highlighted. Figure 1

Novel Silicon Phthalocyanines Bearing Triethylene Glycol Groups: Photophysical and Photochemical Properties as well as pH-Induced Spectral Behaviour.

Herein, novel silicon (IV) phthalocyanines peripherally substituted by triethylene glycol groups and bearing axial hydroxyl groups were synthesized and fully characterized by using different analyses techniques. The photophysical and photochemical properties of octa (2a) and tetra (2b) derivatives were investigated in DMF and DMSO. The effect of octa or tetra substitution on fluorescence quantum yield, singlet oxygen generation and photodegradation were examined, and the differences were evaluated regarding their potential efficiency in photodynamic therapy (PDT). Their pH-responses were investigated to determine the influence of protonation of azomethine nitrogen atoms on singlet oxygen generation efficiencies. Dramatic optical changes were observed by protonation of azomethine bridges of 2a and 2b. They exhibited signal decrease from pH4.0 to 1.0 for 2a (pKa=2.6) and pH3.0 to 1.0 for 2b (pKa=1.8). Besides, the compounds exhibited no aggregation tendency, moderate fluorescence quantum yield, solubility in common organic solvents, high singlet oxygen quantum yield and high photostability in DMF and in DMSO, these favorable properties making them good candidates as photosensitizer for PDT.

Role of molecular structure of monosaccharides on the viscosity of aqueous nanometric alumina suspensions

The effect of molecular structure of d-galactose, d-mannose, d-glucose, 2-deoxy-d-glucose and 3-O-methyl-d-glucose on the viscosity of aqueous alumina nanoparticle suspensions was investigated by rheological measurements. The viscosity was found to be dependant on the molecular structure of these hexopyranoses. The research reveals that the orientation of primary and quaternary hydroxyl groups in a pyranose ring plays a crucial role in dispersing properties of monosaccharides. Moreover, the most important is the mutual orientation of these hydroxyl groups. Monosaccharides with axial and equatorial orientation of hydroxyl groups at first (C1) and fourth (C4) carbon atom in pyranose ring decrease the viscosity more than monosaccharides with two axial hydroxyl groups in these positions. Due to the differences in molecular structure of saccharides resulting in different shape of molecules and different compatibility to the water structure, the saccharides differ in ability to disturb water layers around alumina particles.