Mechanism Of Chain Initiation Research Articles

Thermal Stimulation as a Prevailing Mechanism of Methane Conversion in Barrier Discharge

The effect of energy deposition on the composition of the outlet mixture was studied in a plasmachemical reactor with barrier discharge. The composition of the conversion products was analyzed using a gas chromatograph and a quadrupole mass spectrometer. The process mainly occurred by the thermal mechanism of chain initiation and propagation. At an energy input of 3.25 eV/molecule, the selectivity for the main reaction products was 11.56% for acetylene, 14.76% for ethylene, 30.51% for ethane, 16.84% for propylene, 15.01% for propane, and 7.02% for hydrogen.

Preparation of epoxy monoliths via chemically induced phase separation

Epoxy porous monoliths were prepared from a commercial epoxy resin, D.E.R. 331, that cured with a tertiary amine, 2,4,6-tris-(dimethylaminomethyl) phenol, in the presence of a solvent, diisobutyl ketone (DIBK). During the curing process, polymers were formed and a decrease in its solubility in DIBK; the solution thus phase-separated, usually referred to as chemically induced phase separation. The phase separation formed interconnected polymer-poor phase that then became interconnected pores after the removal of DIBK. By varying the content of DIBK from 32 to 40 vol.%, epoxy monoliths with interconnected pores were prepared, with surface pore size ranging from 0.20 to 2.33 μm, overall porosity from 0.41 to 0.60, and ethanol permeability from 10 to 4,717 L/(m2 h−1 bar−1). The glass transition temperatures of the epoxy monoliths, measured with differential scanning calorimetry, were all higher than 100 °C, and temperatures of 5 % weight loss, analyzed by thermal gravimetry, were higher than 350 °C, evidencing the monoliths’ high thermal stability. Also, the monolith morphology was found to be strongly related to the reaction mechanism of polymerization. The results indicate that the mechanism of chain initiation and propagation associated with the tertiary amine can effectively form monoliths with interconnected pores, which cannot be easily prepared with a stepwise polymerization mechanism associated with using primary amine as the curing agent.

Selective copolymerization of carbon dioxide with propylene oxide catalyzed by a nanolamellar double metal cyanide complex catalyst at low polymerization temperatures

Traditional cobalt-zinc double metal cyanide complex [Zn–Co(III)DMCC] catalysts could catalyze the copolymerization of carbon dioxide (CO 2) with propylene oxide (PO) to afford poly (propylene carbonate) (PPC) with high productivity. But the molecular weight (MW) of PPC and the polycarbonate selectivity were not satisfied. In this work, by using a nanolamellar Zn–Co(III) DMCC catalyst, the CO 2–PO copolymerization was successfully performed to yield PPC with high molecular weight ( M n : 36.5 kg/mol) and high molar fraction of CO 2 in the copolymer ( F CO 2 : 74.2%) at low polymerization temperatures (40∼80 °C). Improved selectivity ( F CO 2 : 72.6%) and productivity of the catalyst (6050 g polymer/g Zn) could be achieved at 60 °C within 10 h. The influences of water content on CO 2–PO copolymerization were quantitatively investigated for the first time. It was proposed that trace water in the reaction system not only acted as an efficient chain transfer agent, which decreased MW of the resultant copolymer, but also strongly interacted with zinc site of the catalyst, which led to low productivity of PPC and more amounts of cyclic propylene carbonate (cPC). These conclusions were also supported by the apparent kinetics of CO 2–PO copolymerization. ESI-MS results showed that all polymers have two end alkylhydroxyl groups. It was thus proposed that the alkylhydroxyl groups came from the initiation reaction of Zn–OH in the catalyst and the chain transfer reaction by H 2O. The proposed mechanism of chain initiation, propagation and chain transfer reaction were proved by the experimental results.

Polymerization of Naturally Renewable Methylene Butyrolactones by Half-Sandwich Indenyl Rare Earth Metal Dialkyls with Exceptional Activity

Four discrete half-sandwich dialkyl rare earth metal (REM) complexes incorporating a disilylated indenyl ligand, (1,3-(SiMe3)2C9H5)M(CH2SiMe3)2(THF) (M = Sc, Y, Dy, Lu), have been investigated for the coordination−addition polymerization of naturally renewable methylene butyrolactones, α-methylene-γ-butyrolactone (MBL) and γ-methyl-α-methylene-γ-butyrolactone (MMBL). Initial screening for the polymerization of methyl methacrylate highlighted several differences in catalytic behavior between these half-sandwich REM catalysts and well-studied sandwich REM catalysts in terms of reactivity trend, polymer tacticity, and solvent dependence. Most significantly, all four catalysts herein exhibit exceptional activity for polymerization of MMBL in DMF, achieving quantitative monomer conversion in <1 min with a 0.20 mol % catalyst loading and giving a high turnover frequency of >30 000 h−1. Slower polymerizations occur in CH2Cl2, allowing for establishment of the activity trend within this REM series, which follows: Dy (largest ion) ≥ Y > Lu > Sc (smallest ion). The most active and effective Dy catalyst has been examined in detail, demonstrating its ability to control the polymerization for producing PMMBL with high Tg (221 °C) and with molecular weight ranging from a medium Mn of 1.89 × 104 Da to a high Mn of 1.63 × 105 Da, programmed by the [MMBL]/[Dy] ratio. Kinetic experiments have revealed a first-order dependence on [monomer] and a second-order dependence on [REM]. These kinetic results, coupled to catalyst efficiencies, NMR studies, as well as with chain-end group analysis by MALDI-TOF mass spectrometry, have yielded a chain initiation mechanism that involves both alkyl groups on each metal center and a bimolecular chain propagation that involves two metal centers in the rate-limiting C−C bond forming step. The Dy catalyst response to enolizable organo acids, externally added as chain-transfer agents, has also been examined.

In Vivo and In Vitro Analysis of the Hedamycin Polyketide Synthase

Hedamycin is an antitumor polyketide antibiotic with unusual biosynthetic features. Earlier sequence analysis of the hedamycin biosynthetic gene cluster implied a role for type I and type II polyketide synthases (PKSs). We demonstrate that the hedamycin minimal PKS can synthesize a dodecaketide backbone. The ketosynthase (KS) subunit of this PKS has specificity for both type I and type II acyl carrier proteins (ACPs) with which it collaborates during chain initiation and chain elongation, respectively. The KS receives a C(6) primer unit from the terminal ACP domain of HedU (a type I PKS protein) directly and subsequently interacts with the ACP domain of HedE (a type II PKS protein) during the process of chain elongation. HedE is a bifunctional protein with both ACP and aromatase activity. Its aromatase domain can modulate the chain length specificity of the minimal PKS. Chain length can also be influenced by HedA, the C-9 ketoreductase. While co-expression of the hedamycin minimal PKS and a chain-initiation module from the R1128 PKS yields an isobutyryl-primed decaketide, the orthologous PKS subunits from the hedamycin gene cluster itself are unable to prime the minimal PKS with a nonacetyl starter unit. Our findings provide new insights into the mechanism of chain initiation and elongation by type II PKSs.

Mycobacterial Phenolic Glycolipid Virulence Factor Biosynthesis: Mechanism and Small-Molecule Inhibition of Polyketide Chain Initiation

Phenolic glycolipids (PGLs) are polyketide-derived virulence factors produced by Mycobacterium tuberculosis, M. leprae, and other mycobacterial pathogens. We have combined bioinformatic, genetic, biochemical, and chemical biology approaches to illuminate the mechanism of chain initiation required for assembly of the p-hydroxyphenyl-polyketide moiety of PGLs. Our studies have led to the identification of a stand-alone, didomain initiation module, FadD22, comprised of a p-hydroxybenzoic acid adenylation domain and an aroyl carrier protein domain. FadD22 forms an acyl-S-enzyme covalent intermediate in the p-hydroxyphenyl-polyketide chain assembly line. We also used this information to develop a small-molecule inhibitor of PGL biosynthesis. Overall, these studies provide insights into the biosynthesis of an important group of small-molecule mycobacterial virulence factors and support the feasibility of targeting PGL biosynthesis to develop new drugs to treat mycobacterial infections.

Engineering of complex polyketide biosynthesis--insights from sequencing of the monensin biosynthetic gene cluster.

The biosynthesis of complex reduced polyketides is catalysed in actinomycetes by large multifunctional enzymes, the modular Type I polyketide synthases (PKSs). Most of our current knowledge of such systems stems from the study of a restricted number of macrolide-synthesising enzymes. The sequencing of the genes for the biosynthesis of monensin A, a typical polyether ionophore polyketide, provided the first genetic evidence for the mechanism of oxidative cyclisation through which polyethers such as monensin are formed from the uncyclised products of the PKS. Two intriguing genes associated with the monensin PKS cluster code for proteins, which show strong homology with enzymes that trigger double bond migrations in steroid biosynthesis by generation of an extended enolate of an unsaturated ketone residue. A similar mechanism operating at the stage of an enoyl ester intermediate during chain extension on a PKS could allow isomerisation of an E double bond to the Z isomer. This process, together with epoxidations and cyclisations, form the basis of a revised proposal for monensin formation. The monensin PKS has also provided fresh insight into general features of catalysis by modular PKSs, in particular into the mechanism of chain initiation.

In vitro reconstitution and analysis of the chain initiating enzymes of the R1128 polyketide synthase.

Biosynthesis of the carbon chain backbone of the R1128 substances is believed to involve the activity of a ketosynthase/chain length factor (ZhuB/ZhuA), an additional ketosynthase (ZhuH), an acyl transferase (ZhuC), and two acyl carrier proteins (ACPs; ZhuG and ZhuN). A subset of these proteins initiate chain synthesis via decarboxylative condensation between an acetyl-, propionyl-, isobutyryl-, or butyryl-CoA derived primer unit and a malonyl-CoA derived extender unit to yield an acetoacetyl-, beta-ketopentanoyl-, 3-oxo-4-methylpentanoyl-, or beta-ketohexanoyl-ACP product, respectively. To investigate the precise roles of ZhuH, ZhuC, ZhuG, and ZhuN, each protein was expressed in Escherichia coli and purified to homogeneity. Although earlier reports had proposed that ZhuC and its homologues played a role in primer unit selection, direct in vitro analysis of ZhuC showed that it was in fact a malonyl-CoA:ACP malonyltransferase (MAT). The enzyme could catalyze malonyl transfer but not acetyl- or propionyl-transfer onto R1128 ACPs or onto ACPs from other biosynthetic pathways, suggesting that ZhuC has broad substrate specificity with respect to the holo-ACP substrate but is specific for malonyl-CoA. Thus, ZhuC supplies extender units to both the initiating and elongating ketosynthases from this pathway. To interrogate the primer unit specificity of ZhuH, the kinetics of beta-ketoacyl-ACP formation in the presence of various acyl-CoAs and malonyl-ZhuG were measured. Propionyl-CoA and isobutyryl-CoA were the two most preferred substrates of ZhuH, although acetyl-CoA and butyryl-CoA could also be accepted and elongated. This specificity is not only consistent with earlier reports demonstrating that R1128B and R1128C are the major products of the R1128 pathway in vivo, but is also in good agreement with the properties of the ZhuH substrate binding pocket, as deduced from a recently solved crystal structure of the enzyme. Finally, to investigate the molecular logic for the occurrence of not one but two ACP genes within the R1128 gene cluster, the inhibition of ZhuH-catalyzed formation of beta-ketopentanoyl-ACP was quantified in the presence of apo-ZhuG or apo-ZhuN. Both apo-proteins were comparable inhibitors of the ZhuH catalyzed reaction, suggesting that the corresponding apo-proteins can be used interchangeably during chain initiation. Together, these results provide direct biochemical insights into the mechanism of chain initiation of an unusual bacterial aromatic PKS.

Photoxidation of ethylbenzene with air catalyzed by a polymer supported Rose Bengal photosensitizer

This paper deals with the liquid-phase photoxidation of ethylbenzene with air to give ethylbenzene hydroperoxide, irradiating with a 500 W tungsten halogen lamp and using a polymer supported Rose Bengal photosensitizer, in a stirred tank reactor at atmospheric pressure. The effects of the temperature and the amount of supported photosensitizer on the ethylbenzene conversion and the ethylbenzene hydroperoxide selectivity and formation kinetic rate have been explored. Photochemically generated singlet oxygen is probably the active oxygen species in this oxidation reaction, although a thermal cleavage of the –O–O– bond of the ethylbenzene hydroperoxide present as a trace in the starting ethylbenzene should be also taken into account as a radical chain initiation mechanism.

Laser amplifier based on a photon-branched chain reaction in an aerosol-evaporation reactor cavity

A new design is proposed and the characteristics are calculated of a pulsed chemical HF laser with a two-phase active medium and a photon-branched chain initiation mechanism. This laser includes an aerosol-evaporation reactor coupled structurally to an unstable telescopic cavity. Such a closed system reduces considerably (down to 0.217 ms) the time needed for the formation of a two-phase active medium with the required properties. When the working volume of the laser is ~7 litres, the following high output characteristics can be achieved: the maximum power per pulse ~2×1010 W, the pulse duration ~740 ns, the total output energy ~1.5 kJ, the specific output energy ~200 J litre-1 , and the energy gain 105 .

Mechanism of Chain Initiation and Propagation Stages in the Cationic Polymerization of Olefins

Abstract A discussion is presented of the mechanism of chain initiation and propagation stages in the cationic polymerization of olefins based on the example of catalytic systems of the type “aluminum chlorides-weak proton donor” (water, alcohols).

Interaction of chromocene with the silica surface, and structure of the active species for ethene polymerization

The anchoring process of Cr(Cp)2 on silica hydroxyl groups occurs by elimination of C5H6 and formation of Si—O—CrCp mononuclear species. These anchored species are then able to adsorb incoming Cr(Cp)2 to give catalytically inactive dimeric species. These reactions are heavily diffusion-controlled. Interaction of CO and NO with mononuclear anchored species gives well defined dicarbonylic and dinitrosylic compounds, while the reaction of CO with dinuclear (catalytically inactive) species gives much more complex polycarbonylic compounds. The Si—O—CrCp mononuclear surface species are the active sites for ethene polymerization. The chain-initiation mechanism probably consists of the formation of a metallocyclic structure.

Preferred sites of recombination in poliovirus RNA: an analysis of 40 intertypic cross-over sequences

The genome of poliovirus consists of a single strand of RNA approximately 7.5 kb long. Analysis of the sequences around 40 unique recombination sites reveals several features that differ significantly from those expected of randomly located sites. These features, which include a broad zone of elevated homology on the 3' side of the cross-over, support the theory that RNA recombination occurs by a template-switching mechanism during synthesis of the complementary strand, and that sites are chosen to minimise the adverse free energy change involved in switching to a heterotypic template. There is also a strong sequence bias, almost two-thirds of cross-overs, according to a computer simulation, occurring immediately after synthesis of UU. These features shed new light on the extent of base-pairing in replicative intermediate RNA, and on the mechanism of chain initiation.

Mechanism of chain initiation by dextransucrase: Absence of terminal ‘sucrose’ linkage in newly synthesized dextran chains

Dextran was synthesized using dextransucrase from Streptococus sanguis 10558 and (F)-[ 14C]sucrose as substrate to test the possibility that sucrose may be the initial acceptor for glucose. If sucrose is the initial acceptor, then dextran chains should have [ 14C] fructose in a terminal ‘sucrose’ linkage which can be cleaved under mild conditions. Although incorporation of [ 14C]fructose into dextran was observed, the label was not released by mild hydrolysis, indicating that sucrose is not the initiator for dextran synthesis. Incorporation of [ 14C]fructose into dextran might represent its ability to act as an acceptor, as suggested by the isolation of leucrose as a by-product in the reaction.

Heterogeneity of the small ribosomal subunit and mechanism of chain initiation in eukaryotes

[ 35S]Met-tRNA F has been incubated with a crude rabbit reticulocyte ribosomal preparation in the presence of cycloheximide. Binding to a heavy component of the small ribosomal subunit has been observed. Addition of the AUG codon leads to a shift of the [ 35S]Met-tRNA F into an 80-S complex. These results suggest that the heterogeneity of small ribosomal subunits in sucrose gradient sedimentation previously reported may be explained by the binding to some of them of initiator tRNA. A model of chain initiation is proposed, in which one of the first steps is the binding of initiator tRNA to the small ribosomal subunit; this binding does not require the presence of messenger RNA.

The mechanism of chain initiation, stereoregulation and propagation during mechanical-chemical polymerization in the presence of sodium chloride

The mechanism of chain initiation, stereoregulation and propagation during mechanical-chemical polymerization in the presence of sodium chloride

Peptide chain initiation in homologous and heterologous systems from mitochondria and bacteria.

Native ribosomal subunits from mitochondria of Neurospora crassa bind fMet‐tRNA and synthesize fMet‐puromycin at 5 mM Mg 2+. The reactions are stimulated by the trinucleotide ApUpG and by GTP. After washing the subunits with 1 M NH4CI the binding capacity is lost but can be restored by adding initiation factors from Escherichia coliUnwashed mitochondrial 73 S ribosomes require E. coli initiation factors for binding fMet‐tRNA and forming fMet‐puromycin. Cytoplasmic 77 S ribosomes from Neurospora are inactive in these chain initiation reactions, even in the presence of bacterial initiation factors.The requirement of the initiation reactions for AUG, GTP and initiation factors is similar in systems from E. coli, Bacillus subtilis and Neurospora mitochondria.Ribosomes and initiation factors from E. coli and B. subtilis are completely interchangeable.The results suggest that mitochondria from Neurospora use a bacteria‐like chain initiation mechanism.

Initiation of Hemoglobin Synthesis with Rabbit and E. coli tRNA in a Reticulocyte Cell‐Free System

The mechanism of chain initiation has been investigated in experiments with intact reticulocytes and with a cell‐free system prepared from these cells. Polyribosomes were isolated from reticulocytes incubated with labeled amino acids. The NH2‐terminal amino acid of polyribosome‐bound peptide chains has been analyzed by different methods. No evidence has been obtained for the presence of a blocked NH2‐terminal in polyribosome‐bound peptide chains. Moreover, only valine could be detected at the NH2‐terminal. Globin chains were labeled in cell‐free synthesis with [14C]valyl‐tRNA prepared from rabbit liver or E. coli. Valine was incorporated into the NH2‐terminal as well as into internal positions with both tRNAs. This indicated that a valine codon corresponds to the NH2‐terminal amino acids of rabbit globin since this codon can be recognized by E. coli tRNA. Acetyl‐valyl‐tRNA was not utilized by the cell‐free system nor had it any stimulatory activity on the incorporation of valine. These results are discussed in view of the observation that acetyl‐valyl‐tRNA seems to be required for the translation of hemoglobin messenger RNA in a cell‐free system prepared from E. coli.

Initiation of the alpha chain of rabbit hemoglobin.

The first step in the synthesis of all proteins in E. coli is the binding of N-formyl-methionine tRNA to a complex formed between a 30 S ribosome and a mRNA molecule (Nomura and Lowry, 1967). The N-formyl-methionine initiates the synthesis of the protein, becoming incorporated at the amino terminal end of the growing polypeptide chains (Capecchi, 1966). At some point in the synthesis of each protein either the formyl group or the formyl group and the methionine residue are cleaved off to expose the amino terminal residue of the finished protein. We made the assumption that the mechanism of chain initiation in mammalian cells would also result in the transient incorporation of an initiator molecule at the amino terminal position of the growing polypeptide chains. We decided to check on this assumption by studying the synthesis of the α chain of hemoglobin in rabbit reticulocytes. This system has two advantages:...

The Polymerization of Isoprene with sec-Butyllithium in Hexane

Abstract : The polymerization of isoprene in n-hexane, initiated by sec-butyllithium, has been investigated with particular emphasis on the initiation step. The mechanism of chain initiation appears to be quite different from that previously observed in benzene and involves direct attack of sec-butyllithium tetramers on isoprene. The effect of lithium t-butoxide has been investigated and found to accelerate the initiation rate but to depress the propagation rate. Some experiments were carried out to determine the rate of intermolecular exchange between sec-butyllithium and polyisoprenyllithium. (Author)