Characterization of Carboxylic Acid Reductases for Biocatalytic Synthesis of Industrial Chemicals

Carboxylic acid reductases (CARs) catalyze the reduction of a broad range of carboxylic acids into aldehydes, which can serve as common biosynthetic precursors to many industrial chemicals. This work presents the systematic biochemical characterization of five carboxylic acid reductases from different microorganisms, including two known and three new ones, by using a panel of short‐chain dicarboxylic acids and hydroxy acids, which are common cellular metabolites. All enzymes displayed broad substrate specificities. Higher catalytic efficiencies were observed when the carbon chain length, either of the dicarboxylates or of the terminal hydroxy acids, was increased from C₂ to C₆. In addition, when substrates of the same carbon chain length are compared, carboxylic acid reductases favor hydroxy acids over dicarboxylates as their substrates. Whole‐cell bioconversions of eleven carboxylic acid substrates into the corresponding alcohols were investigated by coupling the CAR activity with that of an aldehyde reductase in Escherichia coli hosts. Alcohol products were obtained in yields ranging from 0.5 % to 71 %. The de novo stereospecific biosynthesis of propane‐1,2‐diol enantiomer was successfully demonstrated with use of CARs as the key pathway enzymes. E. coli strains accumulated 7.0 mm (R)‐1,2‐PDO (1.0 % yield) or 9.6 mm (S)‐1,2‐PDO (1.4 % yield) from glucose. This study consolidates carboxylic acid reductases as promising enzymes for sustainable synthesis of industrial chemicals.     

The Activation of O2 at Ruthenium Complexes: Catalytic Chlorination of Unsaturated Organic Substrates within the System O2/HCl/H2O

A solution of RuCl₃ in concentrated hydrochloric acid is capable of chlorinating aromatic compounds catalytically, if O₂is present as the oxidizing agent (brominations can be achieved in hydrobromic acid). When olefins are chosen as substrates, two Cl atoms are added to the double bond in an anti fashion, while hydrochlorination products are formed simultaneously. According to the products observed in both types of reactions for various substrates in this 3‐phase system, the halogenations proceed via electrophilic pathways.     

Product Distributions of Cytochrome P450 OleTJE with Phenyl-Substituted Fatty Acids: A Computational Study

There are two types of cytochrome P450 enzymes in nature, namely, the monooxygenases and the peroxygenases. Both enzyme classes participate in substrate biodegradation or biosynthesis reactions in nature, but the P450 monooxygenases use dioxygen, while the peroxygenases take H₂O₂ in their catalytic cycle instead. By contrast to the P450 monooxygenases, the P450 peroxygenases do not require an external redox partner to deliver electrons during the catalytic cycle, and also no external proton source is needed. Therefore, they are fully self-sufficient, which affords them opportunities in biotechnological applications. One specific P450 peroxygenase, namely, P450 OleT_JE, reacts with long-chain linear fatty acids through oxidative decarboxylation to form hydrocarbons and, as such, has been implicated as a suitable source for the biosynthesis of biofuels. Unfortunately, the reactions were shown to produce a considerable amount of side products originating from C^α and C^β hydroxylation and desaturation. These product distributions were found to be strongly dependent on whether the substrate had substituents on the C^α and/or C^β atoms. To understand the bifurcation pathways of substrate activation by P450 OleT_JE leading to decarboxylation, C^α hydroxylation, C^β hydroxylation and C^α–C^β desaturation, we performed a computational study using 3-phenylpropionate and 2-phenylbutyrate as substrates. We set up large cluster models containing the heme, the substrate and the key features of the substrate binding pocket and calculated (using density functional theory) the pathways leading to the four possible products. This work predicts that the two substrates will react with different reaction rates due to accessibility differences of the substrates to the active oxidant, and, as a consequence, these two substrates will also generate different products. This work explains how the substrate binding pocket of P450 OleT_JE guides a reaction to a chemoselectivity.     

Oxidation of N,N‐Disubstituted Hydroxylamines to Nitrones with Hydrogen Peroxide Catalyzed by Polymer‐Supported Methylrhenium Trioxide Systems

Poly(4‐vinylpyridine)/methylrhenium trioxide (MTO) compounds I III and microencapsulated polystyrene/MTO systems IV V are efficient catalysts for the oxidation of secondary hydroxylamines to the corresponding nitrones with H₂O₂. Complete conversions of substrates and quantitative yields of products are obtained under environmentally friendly experimental conditions and with the use of simple work‐up procedures. Symmetrically substituted hydroxylamines, and non‐symmetrical 3‐substituted and 2‐substituted hydroxypyrrolidines, precursors of nitrones applied in the synthesis of alkaloids and biologically active congeners, have been considered as substrates. The heterogeneous catalysts are stable under the reaction conditions and can be recovered and recycled for at least five times without any appreciable loss in efficiency.     

Estimating Heats of Detonation and Detonation Velocities of Aromatic Energetic Compounds

A new method is introduced to predict reliable estimation of heats of detonation of aromatic energetic compounds. At first step, this procedure assumes that the heat of detonation of an explosive compound of composition C_aH_bN_cO_d can be approximated as the difference between the heat of formation of all H₂O CO₂ arbitrary (H₂O, CO₂, N₂) detonation products and that of the explosive, divided by the formula weight of the explosive. Overestimated results based on (H₂O CO₂ arbitrary) can be corrected in the next step. Predicted heats of detonation of pure energetic compounds with the product H₂O in the liquid state for 31 aromatic energetic compounds have a root mean square (rms) deviation of 2.08 and 0.34 kJ g⁻¹ from experiment for (H₂O CO₂ arbitrary) and new method, respectively. The new method also gives good results as compared to the second sets of decomposition products, which consider H₂, N₂, H₂O, CO, and CO₂ as major gaseous products. It is shown here how the predicted heats of detonation by the new method can be used to obtain reliable estimation of detonation velocity over a wide range of loading densities.     

Combustion Reactions of Poly(Carbon Monofluoride), (CF)n,with Different Reductants and Characterization of the Products

Mixtures of poly(carbon monofluoride) also known as graphite fluoride, (CF)_n, with different reductants, such as NaN₃, B, Ti, ZrTi, Si, CaSi₂, AlSi, TiSi, MoSi₂ and CrSi₂ were investigated. Reactions between these substrates are fast and exothermic enough to proceed in a high temperature, self‐sustaining regime. Heat effects accompanying the reactions were measured and the solid reaction products were analyzed. SEM observation revealed the presence of exfoliated graphite in each case. Nanostructures, like nanofibers, nanorods, nanospheres and nanosized crystals, were also present in most of the products. Phase (XRD) and elemental composition as well as porous structure (N₂ adsorption) of chosen reaction products were also determined.     

Synthesis,Enzymatic Evaluation,and Docking Studies of Fluorogenic Caspase 8 Tetrapeptide Substrates

The synthesis, enzymatic evaluation, and molecular modeling studies of new fluorogenic tetrapeptide‐based substrates selective for caspase 8, having the general structure Ac‐IETD–AXX, are described. Various fluorescent reporter groups (AXX), i.e., 3‐ and 4‐substituted coumarins and quinolin‐2(1H)‐ones were synthesized by von Pechmann condensation. They were subsequently coupled with the caspase‐8‐selective tetrapeptide Ac‐IETD‐OH under newly developed synthetic conditions to give the desired substrates in good yields and in high enantiomeric purity. Based on K_M and V_max values, the new compounds proved to be excellent substrates for recombinant human caspase 8. In contrast, the K_M values for the same compounds as substrates for human caspase 3 were approximately 10–20‐fold higher. Molecular modeling studies based on the X‐ray crystal structures of both human caspases 3 and 8 revealed that there is sufficient room within both active sites to accommodate substrates with moderately bulky substituents in the 3‐ and 4‐positions of the fluorogenic coumarins and quinolin‐2(1H)‐ones. Automated docking of the substrates into the active sites of both human caspases 3 and 8 with the program AutoDock 3 gave structures similar to the published crystallographic structures for the same tetrapeptide bound to caspase 8 in the form of an irreversible inhibitor. The calculated binding energies for the new substrates to either caspase 3 or 8 showed little difference between the substrates, consistent with the K_M data. In addition, the calculated binding energies (ΔG) to caspase 8 were considerably more negative than those to caspase 3, also consistent with the K_M data. A possible molecular interaction that might explain the selectivity of the IETD tetrapeptide motif for caspase 8 over caspase 3 is discussed.     

Formation Mechanism of Alkyl Nitrites,Valuable Intermediates in C1-Upgrading Chemistry and Oxidation Processes

In this contribution we use computational tools to investigate the reaction of alcohol substrates with reactive nitrogen oxide species such as N₂O₃ and N₂O₄, leading to the formation of alkyl nitrites. These nitrites are interesting intermediates which can be processed to various valuable chemicals such as ketones/aldehydes and dimethyl oxalate while regenerating NO _x . As such, NO _x is used as an oxidation mediator, converting alcohol substrates to more reactive nitrites which can be selectively converted to more desired compounds, closing a catalytic cycle in NO _x species.     

Hexachloroethane as an efficient oxidizer in combustion synthesis of carbonaceous and ceramic nanostructures

Combustion of hexachloroethane (C₂Cl₆) mixtures with different reductants — such as NaN₃, B, Al, Mg, Zn, Ti, Si, CaSi₂, AlSi, TiSi, MoSi₂, CrSi₂ — was investigated. Reactions between these substrates are fast and exothermic enough to proceed in a self-sustaining regime. Heat effects accompanying the reactions were measured and the solid reaction products were analyzed. SEM observation revealed the presence of nanocrystallites and nanofibers in the products. Phase compositions of solid products were determined by XRD.     

Graft polymerization on magnetic polymer substrates

Functional monomers were graft polymerized onto substrates consisting of magnetic iron oxides in a crosslinked PVA matrix. Initiation was carried out with H₂O₂ and FeSO₄; other common chemical initiating systems proved unsuccessful. Peroxide treatment of the substrates must be carried out separately from the reaction with Fe²⁺ and monomer. The amount of grafting achieved is highly variable, depending on the nature of the monomer. Strong base and weak acid magnetic shell resins prepared by graft polymerization are useful in water treatment by flocculation and moving-bed ion exchange, respectively. A wider range of products can be obtained by chemical modification of grafted chains. The mechanism of grafting is discussed in an endeavor to explain the highly specific nature of the reaction.     

Novel synthesis method and microstructure evolution of Ti3C2(OH)2/K2Ti8O17 nanocomposites as an effective surface enhanced Raman scattering substrate

In this paper, we have developed a mild and facile approach using KOH solution as etchants to corrode Ti₃SiC₂ at room temperature, fabricating novel Ti₃C₂(OH)₂/K₂Ti₈O₁₇ nanocomposites with open network structures and abundant surface functional groups. The as-produced products obtain the excellent surface enhanced Raman scattering (SERS) performance in the determination of crystal violet (CV), rhodamine 6G (R6G) and methylene blue (MB) dye molecules. The enhancement factors of the three can be calculated to be 3.98 × 10⁵, 1.78 × 10⁵ and 5.27 × 10⁵, and the Raman signals can be detected at the concentrations as low as 10^?6 M, 10^?8 M and 10^?6 M, respectively, suggesting the potential for reliable SERS substrates. In this regard, this work provides a new strategy for exploring and synthesizing MAX phase-derived nanomaterials as promising SERS substrates for high-sensitive molecular detection.     

High temperature wettability and corrosion of ZrO2, Al2O3, Al2O3-C,MgO and MgAlON ceramic substrates by an AZ91 magnesium alloy melt

Wettability and interfacial reactivity of various ceramics (ZrO₂, Al₂O₃, Al₂O₃-C, MgO, MgAlON) in contact with molten AZ91 were investigated in this paper. MgAlON and Al₂O₃-C are new materials to be used in magnesium melt filtration, a comparative study of its wettability and corrosion resistance is performed to evaluate their applicability. Modified sessile-drop tests were conducted, measuring contact angles between the ceramic substrates and molten AZ91 droplets at 680 °C; contact surfaces were analyzed by means of scanning electron microscopy. MgAlON showed the largest contact angles, indicating lowest wettability. Al₂O₃, Al₂O₃-C, and MgO proved non-wettable as well. Al₂O₃- and ZrO₂-containing substrates underwent noticeable interface reactions with molten AZ91.     

Attempts to synthesize the framework nitrogen bearing zeolites

The attempts to synthesize the nitrogen bearing zeolite analogs have been conducted under conditions resembling hydrothermal synthesis of zeolite with SiCl₄, AlCl₃, AlN, or metallic Al, and NaNH₂, as principal substrates. Ammonolysis of the substrates (mostly SiCl₄) was always the first step of synthesis and yielded the amorphous, porous products showing very high surface area and well ordered mesopore system. Their further treatment with NaNH₂ in liquid NH₃ or in other aprotic solvents (i.e., crystallization) usually did not lead to crystalline, porous materials, whereas the porosity of the products declined markedly. The crystalline products have been obtained by ammonolysis of Al₂(SiF₆)₃, but the product showed a low thermal stability.     

Cyclic ablation of high-emissivity Sm-doped ZrB2/SiC coatings on alumina substrates

Samarium-doped ZrB₂/SiC (ZBS) coatings possess properties of high emissivity and excellent ablation performance suitable for hypersonic applications. Of interest in the current study is how cyclic ablation affects the scale development on alumina substrates. ZBS coatings with 3, 5 and 8 mol% of samarium (Sm) dopant were prepared via shrouded plasma spray onto alumina substrates and subjected to two 60-s ablation cycles with temperatures reaching up to 1700 °C. Blisters were observed on the Sm-doped coatings after the 1st cycle as a result of a local eutectic reaction between the ablation products and alumina substrate. A Sm-stabilized t-ZrO₂ phase was identified through X-ray diffraction after the ablation of the Sm-doped coatings. The ZBS with 5 mol% of Sm dopant produced a flower-like microstructure after the 2nd cycle due to the formation of convection cells.     

Heterogeneous sulphochlorination of polyethylene. The effect of polymer morphology on the reaction rates and properties of reaction products

Polyethylenes of low, middle and high density have been submitted to heterogeneous sulphochlorination in the presence of hydroperoxides as initiators. Approximately equal amounts of SO₂Cl and chlorine could be attached to the polyethylenic precursors. Although the reaction kinetics are strongly affected by the initial morphology of the substrates, it has been found that maximum amounts of the SO₂Cl groups which can be introduced under such conditions are determined by the fraction of the amorphous material. The reaction kinetics are also strongly affected by the presence of antioxidants.X-ray diffraction analysis, birefringence and polarized i.r. measurements indicated that the morphology of the sulphochlorinated materials and of the products of their hydrolysis is strongly affected by the initial morphology of the polyethylenic precursors. Fluorescence probe measurements revealed that in highly oriented materials, the orientation of the amorphous regions is retained even upon water swelling of the final ion exchange membranes. It has been found that the initial morphology of the polyethylenic substrates together with the degree of the chemical modification (concentration of the introduced chlorine and SO₂Cl groups) determine the properties of the products.     

Elaboration of aflatoxin on cottonseed products byAspergillus flavus

In controlled laboratory experiments heat sterilized and unautoclaved glanded and glandless whole cottonseed or decorticated kernels and sterilized cottonseed meals were found to be utilized as substrates by an aflatoxin elaborating strain ofA. flavus with the production of high levels of aflatoxins B₁, B₂, G₁ and G₂. Gossypol pigments in cottonseed products are apparently not a barrier to either mold invasion or aflatoxin production. Cottonseed hulls, lint cotton, and cottonseed linters were found to be poorly utilized as substrates for either mold growth or aflatoxin production. So. Utiliz. Res. Dev. Div., ARS. USDA.     

Wettability and interfacial reactions of a low Hf-containing nickel-based superalloy on Al2O3-based,SiO2-based,ZrSiO4, and CoAl2O4 substrates

To understand the wetting behavior and interfacial phenomena between molten superalloys and ceramic materials, the wettability and interfacial reactions of a low Hf-containing Nickel-based superalloy on the Al₂O₃-based, SiO₂-based, ZrSiO₄, and CoAl₂O₄ substrates were studied using the sessile drop method at 1773 K. The wetting angles of the alloy on the Al₂O₃-based, SiO₂-based, ZrSiO₄, and CoAl₂O₄ substrates were 141.4°, 143.5°, 135.7°, and 128.4°, respectively. This indicated that the wettability of the alloy on the Al₂O₃-based substrate was comparable to that on the SiO₂-based substrate, and the wettability of the CoAl₂O₄ system was the best among the four systems. The microstructure characteristics of the interface implied that Hf has a strong tendency to react with ceramic substrates, even at low contents. Additionally, the interfacial reactions transformed the Al₂O₃-based, SiO₂-based, ZrSiO₄, and CoAl₂O₄ ceramic substrates into (Al₂O₃ + HfO₂), (Al₂O₃ + HfO₂), (Al₂O₃ + HfO₂ + ZrO₂), and (Al₂O₃ + HfO₂ + Co), respectively, which were in contact with the alloys. The experimental results demonstrated that the wettability of the system was governed by the properties of the reaction products.     

Hydrosilylation Synthesis of New Ferrocenylene Copolymers Containing Carbosilylene,Carbosiloxy and Cyclopentadienyliron Dicarbonyl Bridges

Bifunctional organometallic silicon precursor monomers and substrates FC(SiMe₂H)₂ (1) [FC = (η⁵-C₅H₄)Fe(η⁵-C₅H₄)]; FC(SiMe₂(CH₂)_xCH=CH₂)₂ [x = 0 (2), 1 (3)], [η⁵-C₅H₄-SiMe₂(CH₂)_xCH=CH₂)]Fe(CO)₂SiMe₂(CH₂)_xCH=CH₂ x = 0 (4), 1 (5) and (η⁵-C₅H₄-SiMe₂H)Fe(CO)₂SiMe₂H (6) have been used to make a series of new iron containing polymers via hydrosilylation reactions. In addition to the vinyl- and allyl-containing substrates 2, 3, 4 and 5 the organosilicon compounds [CH₂=CHSiMe₂]₂O, 1,4-(H₂C=CH-SiMe₂)₂C₆H₄ and (HC≡CH–SiMe₂)₂O were also used as substrates for the hydrosilylation reaction. The reactions between the various SiH and CH=CH₂ and C≡C functionalities were performed in the presence of Pt(0) catalyst and resulted in regioselective (β-isomer and β-(E) isomer) products as determined by NMR spectroscopy. Molecular weights of all the polymers were determined by Gel Permeation Chromatography, which revealed oligomeric materials with narrow polydispersity. Cyclic voltammetric studies of exhibited single reversible redox processes due to the Fe(II)/Fe(III) couple when present, and irreversible oxidation for the presence of any Fp Fe atom. This article is dedicated to Professor Astruc.     

Structural colors from TiO2/SiO2 multilayer flakes prepared by sol-gel process

Preparation of TiO₂/SiO₂ multilayer flakes and their application to decorative powders were investigated. In contrast to conventional products prepared through the multicoating of core platelets, the coreless TiO₂/SiO₂ multilayer flakes were prepared by detaching multilayer films from their substrates. These flakes exhibited structural colors, when the optical path length of both the TiO₂ and SiO₂ layers are adjusted to be one fourth of the wavelength of visible light. A multicoating of more than five layers resulted in the propagation of cracks, which prevented the preparation of thick flakes. Paint films fabricated using the multilayer flakes and acrylic resins showed reflectance spectra that were comparable with those obtained for multicoatings on substrates.     

Comparative Studies of Chlorine Dioxide Reactions with Muconic Acid Derivatives and Lignin Model Compounds

Abstract

The reaction of chlorine dioxide with different types of lignin model compounds was investigated in order to compare the kinetics and to evaluate the amount of oxidant consumed by the different substrates. Complete reaction of lignin model compounds was observed at ClO₂‐to‐substrate molar ratios of 0.9–1.2, which corresponds to an electron transfer varying between 5–6 equivalents per mole of substrate. Muconic acid derivatives also fully reacted, at a ClO₂‐to‐substrate molar ratio of 1.2, with the oxidant consumption being about 4 equivalents per mole of substrate. The reaction of mixtures of phenolic, non‐phenolic, and muconic acid type substrates showed that the reaction rates of non‐phenolic and muconic acid type substrates were rather similar. This study suggests that further reaction between ClO₂ and the primary lignin oxidation products, such as muconic acid type structures could be the cause of overconsumption of oxidant in a D stage.