Presence Of Reductant Research Articles

ChemInform Abstract: Convenient Reductive Methylation of Amines with Carbonates at Room Temperature.

Methylation of amines is a fundamental and commonly used reaction in organic synthesis. Many methods are known including various reductive methylations using formaldehyde, formic acid, or carbon dioxide in the presence of reductants. However, several of these methods suffer from limited substrate scope and chemoselectivity because of the different nucleophilicities of substrates. In this respect, the combination of carbonates and hydrosilanes is a valuable methylation source in the presence of Pt-based catalysts. This highly tunable method allows for methylation of both aromatic and aliphatic amines, and chemoselective methylation of aminoalcohols and diamines. Notably, the in situ-formed catalyst can also be used for the reduction of carbonates to methanol at room temperature. Mechanistic insights on intermediates formed during the reaction pathway were obtained by using ESI mass spectrometry.

Carbothermal reduction nitridation of slag, glass and minerals: Formation process of SiAlON powders with different morphology

The phase and morphological evolutions during carbothermal reduction nitridation (CRN) process were comparatively investigated using three kinds of raw material (slag, glass and minerals) providing different Si and Al sources. The results showed that the CRN processes using different raw materials presented the same phase transformation steps. In SiAlON family, O-SiAlON was firstly formed then transformed to nitrogen-rich SiAlONs, including β-, α-SiAlON and 15R polytypoid with the increase of temperature or holding time. At the end of CRN, β and 15R were converted to α-SiAlON. In addition, gehlenite phase was also formed at the initial stage of CRN, which could be further converted to SiAlON in the presence of reductant C. The morphology of nitridation specimens was highly dependent on the liquid or gas phases formed via CRN reactions, which contributed to the formation of prismatic, hollow spherical and needle-like SiAlON powders via three CRN processes.

Substrate inhibition of transketolase

We studied the influence of the acceptor substrate of transketolase on the activity of the enzyme in the presence of reductants. Ribose-5-phosphate in the presence of cyanoborohydride decreased the transketolase catalytic activity. The inhibition is caused by the loss of catalytic function of the coenzyme–thiamine diphosphate. Similar inhibitory effect was observed in the presence of NADPH. This could indicate its possible regulatory role not only towards transketolase, but also towards the pentose phosphate pathway of carbohydrate metabolism overall, taking into account the fact that it inhibits not only transketolase but also another enzyme of the pentose phosphate pathway — glucose 6-phosphate dehydrogenase [Eggleston L.V., Krebs H.A. Regulation of the pentose phosphate cycle, Biochem. J. 138 (1974) 425–435].

Convenient Reductive Methylation of Amines with Carbonates at Room Temperature.

Methylation of amines is a fundamental and commonly used reaction in organic synthesis. Many methods are known including various reductive methylations using formaldehyde, formic acid, or carbon dioxide in the presence of reductants. However, several of these methods suffer from limited substrate scope and chemoselectivity because of the different nucleophilicities of substrates. In this respect, the combination of carbonates and hydrosilanes is a valuable methylation source in the presence of Pt-based catalysts. This highly tunable method allows for methylation of both aromatic and aliphatic amines, and chemoselective methylation of aminoalcohols and diamines. Notably, the in situ-formed catalyst can also be used for the reduction of carbonates to methanol at room temperature. Mechanistic insights on intermediates formed during the reaction pathway were obtained by using ESI mass spectrometry.

Microwave-assisted synthesis of L-cysteine-capped nickel nanoparticles for catalytic reduction of 4-nitrophenol

A simple and efficient microwave-assisted procedure for synthesis of L-cysteine-capped nickel nanoparticles (cyst-Ni NPs) in ethylene glycol solvent was demonstrated. The as-synthesised NPs were characterised by ultraviolet–visible (UV–Vis) spectrophotometer, Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM) and X-ray diffractometry (XRD). The cyst-Ni NPs are proved to be excellent heterogeneous catalysts for the 100 % reduction of 4-nitrophenol (4-NPh) in the presence of reductant (NaBH4) within reaction time of 40 s. In contrast, Raney nickel in similar sample environments shows only 25.5 % reduction. The kinetic and energetic behaviours of cyst-Ni NPs were also studied, and the reduction reaction is determined to follow pseudo-first-order kinetics with a rate constant value of 0.115 s−1 and activation energy of 36.1 kJ·mol−1. In addition to its high catalytic competence, cyst-Ni NPs catalyst exhibits excellent recyclability with negligible catalytic poisoning.

Synergistic effect of reductive and ligand-promoted dissolution of goethite.

Ligand-promoted dissolution and reductive dissolution of iron (hydr)oxide minerals control the bioavailability of iron in many environmental systems and have been recognized as biological iron acquisition strategies. This study investigated the potential synergism between ligands (desferrioxamine B (DFOB) or N,N'-Di(2-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid (HBED)) and a reductant (ascorbate) in goethite dissolution. Batch experiments were performed at pH 6 with ligand or reductant alone and in combination, and under both oxic and anoxic conditions. Goethite dissolution in the presence of reductant or ligand alone followed classic surface-controlled dissolution kinetics. Ascorbate alone does not promote goethite dissolution under oxic conditions due to rapid reoxidation of Fe(II). The rate coefficients for goethite dissolution by ligands are closely correlated with the stability constants of the aqueous Fe(III)-ligand complexes. A synergistic effect of DFOB and ascorbate on the rate of goethite dissolution was observed (total rates greater than the sum of the individual rates), and this effect was most pronounced under oxic conditions. For HBED, macroscopically the synergistic effect was hidden due to the inhibitory effect of ascorbate on HBED adsorption. After accounting for the concentrations of adsorbed ascorbate and HBED, a synergistic effect could still be identified. The potential synergism between ligand and reductant for iron (hydr)oxide dissolution may have important implications for iron bioavailability in soil environments.

Mechanistic aspects of the release and the reduction of NOx stored on Pt–Ba/Al2O3

The release and reduction of NOx stored on Pt–Ba/Al2O3 is here investigated under nearly isothermal conditions. It is shown that the release driven by adsorption–desorption equilibrium of stored NOx is negligible. It is also shown that NO isotopic exchange between stored 14NOx and 15NO in absence of O2, which implies the release of stored NOx and has the potential to decouple NOx release from its further reduction, is catalyzed by Pt and is observed from temperatures well below that of the NOx thermal desorption. It is suggested that reduced Pt sites activate stored NOx that are destabilized and eventually released as NOx in the gas phase or adsorbed onto Pt. Therefore, under nearly isothermal conditions, the NOx release is a chemical reaction catalyzed by Pt occurring at the Pt/Ba interface. In the presence of reductants, the release of stored NOx and its further reduction cannot be decoupled. The higher is the reducing power the more effective is the reduction of Pt and activation of stored NOx by metal Pt that represent the first step of release and reduction of stored NOx.

Glutathione-garlic sulfur conjugates: slow hydrogen sulfide releasing agents for therapeutic applications.

Natural organosulfur compounds (OSCs) from Allium sativum L. display antioxidant and chemo-sensitization properties, including the in vitro inhibition of tumor cell proliferation through the induction of apoptosis. Garlic water- and oil-soluble allyl sulfur compounds show distinct properties and the capability to inhibit the proliferation of tumor cells. In the present study, we optimized a new protocol for the extraction of water-soluble compounds from garlic at low temperatures and the production of glutathionyl-OSC conjugates during the extraction. Spontaneously, Cys/GSH-mixed-disulfide conjugates are produced by in vivo metabolism of OSCs and represent active molecules able to affect cellular metabolism. Water-soluble extracts, with (GSGaWS) or without (GaWS) glutathione conjugates, were here produced and tested for their ability to release hydrogen sulfide (H2S), also in the presence of reductants and of thiosulfate:cyanide sulfurtransferase (TST) enzyme. Thus, the TST catalysis of the H2S-release from garlic OSCs and their conjugates has been investigated by molecular in vitro experiments. The antiproliferative properties of these extracts on the human T-cell lymphoma cell line, HuT 78, were observed and related to histone hyperacetylation and downregulation of GAPDH expression. Altogether, the results presented here pave the way for the production of a GSGaWS as new, slowly-releasing hydrogen sulfide extract for potential therapeutic applications.

Kinetic studies on the oxidation of semiquinone and hydroquinone forms of Arabidopsis cryptochrome by molecular oxygen

Cryptochromes (crys) are flavoprotein photoreceptors present throughout the biological kingdom that play important roles in plant development and entrainment of the circadian clock in several organisms. Crys non-covalently bind flavin adenine dinucleotide (FAD) which undergoes photoreduction from the oxidised state to a radical form suggested to be active in signalling in vivo. Although the photoreduction reactions have been well characterised by a number of approaches, little is known of the oxidation reactions of crys and their mechanisms. In this work, a stopped-flow kinetics approach is used to investigate the mechanism of cry oxidation in the presence and absence of an external electron donor. This in vitro study extends earlier investigations of the oxidation of Arabidopsis cryptochrome1 by molecular oxygen and demonstrates that, under some conditions, a more complex model for oxidation of the flavin than was previously proposed is required to accommodate the spectral evidence (see P. Müller and M. Ahmad (2011) J. Biol. Chem. 286, 21033–21040 [1]). In the absence of an electron donor, photoreduction leads predominantly to the formation of the radical FADH. Dark recovery most likely forms flavin hydroperoxide (FADHOOH) requiring superoxide. In the presence of reductant (DTT), illumination yields the fully reduced flavin species (FADH−). Reaction of this with dioxygen leads to transient radical (FADH) and simultaneous accumulation of oxidised species (FAD), possibly governed by interplay between different cryptochrome molecules or cooperativity effects within the cry homodimer.

A novel cryptochrome in the diatom Phaeodactylum tricornutum influences the regulation of light‐harvesting protein levels

Diatoms possess several genes for proteins of the cryptochrome/photolyase family. A typical sequence for a plant cryptochrome was not found in our analysis of the Phaeodactylum tricornutum genome, but one protein grouped with higher plant and green algal cryptochromes. This protein, CryP, binds FAD and 5,10-methenyltetrahydrofolate, according to our spectroscopic studies on heterologously expressed protein. 5,10-Methenyltetrahydrofolate binding is a feature common to both cyclobutane pyrimidine dimer photolyases and DASH cryptochromes. In recombinant CryP, however, the FAD chromophore was present in its neutral radical state and had a red-shifted absorption maximum at 637 nm, which is more characteristic for a DASH cryptochrome than a cyclobutane pyrimidine dimer photolyase. Upon illumination with blue light, the fully reduced state of FAD was formed in the presence of reductant. Expression of CryP was silenced by antisense approaches, and the resulting cell lines showed increased levels of proteins of light-harvesting complexes, the Lhcf proteins, in vivo. In contrast, the levels of proteins active in light protection, the Lhcx proteins, were reduced. Thus, CryP cannot be directly grouped with known members of the cryptochrome/photolyase family. Of all P. tricornutum proteins, it is the most similar in sequence to a plant cryptochrome, and is involved in the regulation of light-harvesting protein expression, but shows spectroscopic features and a chromophore composition that are most typical of a DASH cryptochrome.

Production of four Neurospora crassa lytic polysaccharide monooxygenases in Pichia pastoris monitored by a fluorimetric assay

BackgroundRecent studies demonstrate that enzymes from the glycosyl hydrolase family 61 (GH61) show lytic polysaccharide monooxygenase (PMO) activity. Together with cellobiose dehydrogenase (CDH) an enzymatic system capable of oxidative cellulose cleavage is formed, which increases the efficiency of cellulases and put PMOs at focus of biofuel research. Large amounts of purified PMOs, which are difficult to obtain from the native fungal producers, are needed to study their reaction kinetics, structure and industrial application. In addition, a fast and robust enzymatic assay is necessary to monitor enzyme production and purification.ResultsFour pmo genes from Neurospora crassa were expressed in P. pastoris under control of the AOX1 promoter. High yields were obtained for the glycosylated gene products PMO-01867, PMO-02916 and PMO-08760 (>300 mg L-1), whereas the yield of non-glycosylated PMO-03328 was moderate (~45 mg L-1). The production and purification of all four enzymes was specifically followed by a newly developed, fast assay based on a side reaction of PMO: the production of H2O2 in the presence of reductants. While ascorbate is a suitable reductant for homogeneous PMO preparations, fermentation samples require the specific electron donor CDH.ConclusionsP. pastoris is a high performing expression host for N. crassa PMOs. The pmo genes under control of the native signal sequence are correctly processed and active. The novel CDH-based enzyme assay allows fast determination of PMO activity in fermentation samples and is robust against interfering matrix components.

Rapid Equilibrium Kinetic Analysis of Arsenite Methylation Catalyzed by Recombinant Human Arsenic (+3 Oxidation State) Methyltransferase (hAS3MT)

In the human body, arsenic is metabolized by methylation. Understanding this process is important and provides insight into the relationship between arsenic and its related diseases. We used the rapid equilibrium kinetic model to study the reaction sequence of arsenite methylation. The results suggest that the mechanism for arsenite methylation is a completely ordered mechanism that is also of general interest in reaction systems with different reductants, such as tris(2-carboxyethyl)phosphine, cysteine, and glutathione. In the reaction, cysteine residues of recombinant human arsenic (+3 oxidation state) methyltransferase (hAS3MT) coordinate with arsenicals and involve the methyl transfer step. S-Adenosyl-l-methionine (AdoMet) is the first-order reactant, which modulates the conformation of hAS3MT to a best matched state by hydrophobic interaction. As the second-order reactant, reductant reduces the disulfide bond, most likely between Cys-250 and another cysteine residue of hAS3MT, and exposes the active site cysteine residues for binding trivalent inorganic arsenic (iAs(3+)) to give monomethylarsonic dicysteine (MADC(3+)). In addition, the reaction can be extended to further methylate MADC(3+) to dimethylarsinic cysteine (DAMC(3+)). In the methylation reaction, the β-pleated sheet content of hAS3MT is increased, and the hydrophobicity of the microenvironment around the active sites is decreased. Similarly, we confirm that both the high β-pleated sheet content of hAS3MT and the high dissociation ability of the enzyme-AdoMet-reductant improve the yield of dimethylated arsenicals.

Redox alters yellow dragonflies into red

Body color change associated with sexual maturation--so-called nuptial coloration--is commonly found in diverse vertebrates and invertebrates, and plays important roles for their reproductive success. In some dragonflies, whereas females and young males are yellowish in color, aged males turn vivid red upon sexual maturation. The male-specific coloration plays pivotal roles in, for example, mating and territoriality, but molecular basis of the sex-related transition in body coloration of the dragonflies has been poorly understood. Here we demonstrate that yellow/red color changes in the dragonflies are regulated by redox states of epidermal ommochrome pigments. Ratios of reduced-form pigments to oxidized-form pigments were significantly higher in red mature males than yellow females and immature males. The ommochrome pigments extracted from the dragonflies changed color according to redox conditions in vitro: from red to yellow in the presence of oxidant and from yellow to red in the presence of reductant. By injecting the reductant solution into live insects, the yellow-to-red color change was experimentally reproduced in vivo in immature males and mature females. Discontinuous yellow/red mosaicism was observed in body coloration of gynandromorphic dragonflies, suggesting a cell-autonomous regulation over the redox states of the ommochrome pigments. Our finding extends the mechanical repertoire of pigment-based body color change in animals, and highlights an impressively simple molecular mechanism that regulates an ecologically important color trait.

Kinetics of the Reductive Dissolution of Lead(IV) Oxide by Iodide

Lead(IV) oxide (PbO(2)) is a corrosion product found in lead service lines used to convey drinking water. The presence of reductants can accelerate PbO(2) dissolution and enhance lead release to drinking water. The dissolution rate rather than the equilibrium solubility of PbO(2) can control the dissolved lead concentrations in water distributed through pipes containing PbO(2). Iodide, a known reductant for PbO(2), was selected as a model reductant for investigating the kinetics and mechanisms of the reductive dissolution of PbO(2). The dissolution rate of plattnerite (β-PbO(2)) was determined as a function of pH, iodide concentration, and dissolved inorganic carbon (DIC) concentration using continuously stirred tank reactors. The dissolution rate of plattnerite increased with decreasing pH and increasing iodide concentrations. The presence of 10 mg C/L DIC accelerated plattnerite dissolution, but further increases in DIC concentration did not affect the dissolution rate. The reductive dissolution of PbO(2) can be interpreted as a coupled process involving chemical reduction of Pb(IV) to Pb(II) at the PbO(2) surface followed by detachment of Pb(II) to solution. The data suggest that chemical reduction is the rate-limiting step for PbO(2) dissolution in the presence of iodide.

Catalytic Reduction of Dioxygen to Water with a Monomeric Manganese Complex at Room Temperature

There have been numerous efforts to incorporate dioxygen into chemical processes because of its economic and environmental benefits. The conversion of dioxygen to water is one such example, having importance in both biology and fuel cell technology. Metals or metal complexes are usually necessary to promote this type of reaction and several systems have been reported. However, mechanistic insights into this conversion are still lacking, especially the detection of intermediates. Reported herein is the first example of a monomeric manganese(II) complex that can catalytically convert dioxygen to water. The complex contains a tripodal ligand with two urea groups and one carboxyamidopyridyl unit; this ligand creates an intramolecular hydrogen-bonding network within the secondary coordination sphere that aids in the observed chemistry. The manganese(II) complex is five-coordinate with an N(4)O primary coordination sphere; the oxygen donor comes from the deprotonated carboxyamido moiety. Two key intermediates were detected and characterized: a peroxo-manganese(III) species and a hybrid oxo/hydroxo-manganese(III) species (1). The formulation of 1 was based on spectroscopic and analytical data, including an X-ray diffraction analysis. Reactivity studies showed dioxygen was catalytically converted to water in the presence of reductants, such as diphenylhydrazine and hydrazine. Water was confirmed as a product in greater than 90% yield. A mechanism was proposed that is consistent with the spectroscopy and product distribution, in which the carboxyamido group switches between a coordinated ligand and a basic site to scavenge protons produced during the catalytic cycle. These results highlight the importance of incorporating intramolecular functional groups within the secondary coordination sphere of metal-containing catalysts.

X-ray diffraction study of detonation nanodiamonds

The paper addresses a structural analysis of detonation nanodiamonds. An analysis procedure is described which has been elaborated on the basis of a comparison between experimental and theoretical diffraction patterns calculated using computer models of nanodiamonds. Detonation nanodiamonds are characterized in terms of their shape, size, crystal lattice parameters, and distribution of atoms in them. The lattice parameters have been determined for new detonation nanodiamonds synthesized by us and other researchers. The quality of detonation nanodiamonds is assessed by the atom distribution which differs from that of natural diamonds and depends on the lattice parameters and size of the detonation nan-odiamonds. The lattice parameters of the detonation nanodiamonds synthesized in the presence of reductants have been found to correspond to that of natural diamond, thus suggesting a high quality of the synthetic nanodiamonds.

Surface-Enhanced Raman Scattering of 4-Aminothiophenol Adsorbed on Silver Nanosheets Deposited onto Cubic Boron Nitride Films

A simple method was found for the fabrication of silver nanosheets (AgNS) by the catalysis of gold nanoparticles (AuNP) on an amine-terminated cubic boron nitride (cBN) surface deposited on a Si(001) substrate in the presence of reductant. The morphology of the AgNS/AuNP/NH(2)-cBN/Si(001) sample was characterized by scanning electron microscopy and X-ray diffraction. The performance of the AgNS/AuNP/NH(2)-cBN/Si(001) sample as surface-enhanced Raman scattering (SERS) active substrate was evaluated by using 4-aminothiophenol (PATP) as the probe molecule. The SERS measurements showed that the maximum intensity was obtained on the AgNS/AuNP/NH(2)-cBN/Si(001) sample for 5 min silver deposition. Compared with the AuNP/NH(2)-cBN/Si(001) sample and a silver film/cBN/Si(001) prepared by the mirror reaction, the SERS signal of PATP was obviously improved on the above AgNS/AuNP/NH(2)-cBN/Si(001) film. The sensitivity and the stability of the AgNS/AuNP/NH(2)-cBN/Si(001) sample were also investigated.

Promoter Recognition by a Complex of Spx and the C-Terminal Domain of the RNA Polymerase α Subunit

BackgroundSpx, an ArsC (arsenate reductase) family member, is a global transcriptional regulator of the microbial stress response and is highly conserved amongst Gram-positive bacteria. Bacillus subtilis Spx protein exerts positive and negative control of transcription through its interaction with the C-terminal domain of the RNA polymerase (RNAP) α subunit (αCTD). Spx activates trxA (thioredoxin) and trxB (thioredoxin reductase) in response to thiol stress, and bears an N-terminal C10XXC13 redox disulfide center that is oxidized in active Spx.Methodology/Principal FindingsThe structure of mutant SpxC10S showed a change in the conformation of helix α4. Amino acid substitutions R60E and K62E within and adjacent to helix α4 conferred defects in Spx-activated transcription but not Spx-dependent repression. Electrophoretic mobility-shift assays showed αCTD interaction with trxB promoter DNA, but addition of Spx generated a supershifted complex that was disrupted in the presence of reductant (DTT). Interaction of αCTD/Spx complex with promoter DNA required the cis-acting elements -45AGCA-42 and -34AGCG-31 of the trxB promoter. The SpxG52R mutant, defective in αCTD binding, did not interact with the αCTD-trxB complex. SpxR60E not only failed to complex with αCTD-trxB, but also disrupted αCTD-trxB DNA interaction.Conclusions/SignificanceThe results show that Spx and αCTD form a complex that recognizes the promoter DNA of an Spx-controlled gene. A conformational change during oxidation of Spx to the disulfide form likely alters the structure of Spx α helix α4, which contains residues that function in transcriptional activation and αCTD/Spx-promoter interaction. The results suggest that one of these residues, R60 of the α4 region of oxidized Spx, functions in αCTD/Spx-promoter contact but not in αCTD interaction.

Iron-binding and mobilization from ferritin by polypyridyl ligands

Polypyridyl ligands were investigated for their ability to bind and mobilize iron(ii) from ferritin in aqueous solution. Association constants with iron(ii) (pFe(II)) were determined for the pentadentate ligands N4Py (pFe(II) = 14.4) and Bn-TPEN (pFe(II) = 13.7) using a competition method with the hexadentate ligand TPEN (pFe(II) = 14.6, 0.1 M KNO(3)). Ferrous complexes were formed using the polypyridyl ligands and ferritin as the sole iron source in the presence of reductant. The observed rates of iron mobilization from ferritin were dependent on reductant and were higher in the presence of ascorbate than dithiothreitol. TPEN, N4Py and Bn-TPEN demonstrated comparable and in some cases faster rates and higher levels of iron mobilization when compared to the iron(ii) chelator 1,10-phenanthroline, particularly at low concentrations of chelator.

Insight into the Mechanism of Inactivation of Ribonucleotide Reductase by Gemcitabine 5′-Diphosphate in the Presence or Absence of Reductant

Gemcitabine 5'-diphosphate (F(2)CDP) is a potent inhibitor of ribonucleotide reductases (RNRs), enzymes that convert nucleotides (NDPs) to deoxynucleotides and are essential for DNA replication and repair. The Escherichia coli RNR, an alpha2beta2 complex, when incubated with 1 equiv of F(2)CDP catalyzes the release of two fluorides and cytosine concomitant with enzyme inactivation. In the presence of reductant (thioredoxin/thioredoxin reductase/NADPH or DTT), the enzyme inactivation results from its covalent labeling of alpha with the sugar of F(2)CDP (one label/alpha2beta2). SDS-PAGE analysis of the inactivated RNR without boiling of the sample reveals that alpha migrates as an 87 and 110 kDa protein in a ratio of 0.6:0.4. When the reductant is omitted, RNR is inactivated by loss of the essential tyrosyl radical and formation of a new radical. Inactivation studies with C225S-alpha in the presence or absence of reductants, reveal it behaves like wt-RNR in the absence of reductant. Inactivated C225S-alpha migrates as an 87 kDa protein and is not covalently modified. C225 is one of the cysteines in RNR's active site that supplies reducing equivalents to make dNDPs. To identify the new radical formed, [1'-(2)H]-F(2)CDP was studied with wt- and C225S-RNR by 9 and 140 GHz EPR spectroscopy. These studies revealed that the new radical is a nucleotide derived with g values of g(x) 2.00738, g(y) 2.00592, and g(z) 2.00230 and with altered hyperfine interactions (apparent triplet collapsed to a doublet) relative to [1'-(1)H]-F(2)CDP. The EPR features are very similar to those we recently reported for the nucleotide radical generated with CDP and E441Q-RNR.