Initiation Of Chain Reactions Research Articles

CO2-laser-induced breakdown in mono- and diatomic gases

Several monoatomic and homonuclear diatomic gases absorb energy from a focused CO2-laser photon field. It has been established that the pressure threshold for the energy absorption correlates qualitatively with the known ionization potentials of those gases. The simplified phenomenological theory of the CO2-laser-induced dielectric breakdown of gases is invoked to explain this phenomenon. In the H2−D2 system, the formation of HD is observed under these conditions. The examination of the reaction yields for HD formation demonstrates that the system studied does not reach equilibrium under our experimental conditions. Considerations regarding kinetics of primary processes reveal that ionic species, created originally via an inverse bremsstrahlung mechanism, are converted into atomic transients in fast ionic association processes. The latter species initiate chain reactions with surrounding molecules of substrates leading to the formation of HD. Simple kinetic analysis based on a non-steady-state assumption permitted the derivation of an expression for the yield of HD formation. This equation was fitted to the experimental data assuming that the temperature of the reaction rises with an increase of the amount of D2 in the mixture. Some other aspects regarding the behavior of this system in a focused CO2 laser beam are also discussed.

Generation of hydrogen peroxide by incidental metal ion-catalyzed autooxidation of glutathione

Autooxidation of reduced glutathione in 50 mM buffer at pH 7.9 is indetectably slow in the presence of 1 mM DETAPAC, EDTA, TET, or tripyridine, but passing buffer through Chelex resin was insufficient to remove traces of catalytically active metals. Production of hydrogen peroxide during glutathione autooxidation was catalyzed by traces of Fe +2 or Cu +2, and to a much lesser extent by Cu +1 and Ni +2, but not to a detectable extent by Na +1, K +1, Fe +3, Al +3, Cd +2, Zn +2, Ca +2, Mg +2, Mn +2, or Hg +2. Cysteine was a much better precursor for hydrogen peroxide production than were cysteine sulfinic or sulfonic acids. The chelators EGTA, NTA, bipyridine, dimethylglyoxime, salicylate, and Desferal were ineffective at preventing autooxidation. EDDA and 8-hydroxyquinoline were partially effective. Catalase could completely prevent the accumulation of detectable H 2O 2, but Superoxide dismutase was only slightly inhibitory. Hydroxyl radical and singlet oxygen quenching agents (mannitol and histidine) stimulated. A mechanism for the production of H 2O 2 during trace metal catalyzed oxidation of glutathione is proposed, involving glutathione-complexed metal and dissolved oxygen. Although a radical intermediate can not be ruled out, no radical initiated chain reaction is necessary.

Identification of ribosome-bound eukaryotic initiation factor 2.GDP binary complex as an intermediate in polypeptide chain initiation reaction.

Studies on the formation and release of the eukaryotic initiation factor (eIF)-2.GDP binary complex formed during eIF-5-mediated assembly of an 80 S initiation complex have been carried out. Incubation of a 40 S initiation complex with eIF-5, in the presence or absence of 60 S ribosomal subunits at 25 degrees C, causes rapid and quantitative hydrolysis of ribosome-bound GTP to form an eIF-2.GDP binary complex and Pi. Analysis of both reaction products by Sephadex G-200 gel filtration reveals that while Pi is released from ribosomes, the eIF-2.GDP complex remains bound to the ribosomal initiation complex. The eIF-2.GDP binary complex can however be released from ribosome by subjecting the eIF-5-catalyzed reaction products to either longer periods of incubation at 37 degrees C or sucrose gradient centrifugation. Furthermore, addition of a high molar excess of isolated eIF-2.GDP binary complex to a 40 S initiation reaction mixture does not cause exchange of ribosome-bound eIF-2.GDP complex formed by eIF-5-catalyzed hydrolysis of GTP. These results indicate that eIF-2.GDP complex is directly formed on the surface of ribosomes following hydrolysis of GTP bound to a 40 S initiation complex, and that ribosome-bound eIF-2 X GDP complex is an intermediate in polypeptide chain initiation reaction.

Purification and characterization of a guanosine diphosphatase activity from calf liver microsomal salt wash proteins.

A potent guanosine diphosphatase activity that hydrolyzes GDP to 5'-GMP + Pi has been isolated and purified from the salt wash proteins of calf liver microsomes. The purified enzyme, a monomeric protein of approximate Mr 46,000, possesses nucleotide substrate specificity since, among the nucleoside diphosphates and triphosphates tested, only GDP and UDP are hydrolyzed by the enzyme. The relative affinity of the enzyme for GDP is, however, much higher than for UDP. The effect of the enzyme on the binary complex formed between eukaryotic initiation factor 2 (eIF-2) and GDP has also been investigated. The enzyme neither hydrolyzes GDP bound to eIF-2 nor catalyzes the exchange of eIF-2-bound GDP with GTP even in the presence of Met-tRNAf. The enzyme, therefore, is presumably not involved in recycling of eIF-2 in eukaryotic polypeptide chain initiation reaction. The possible biological function of the enzyme in maintaining the cellular pool of GTP-GDP is discussed.

Inhibition of eukaryotic protein chain initiation by vanadate.

Vanadate inhibits protein chain initiation in rabbit reticulocyte lysates. The evidence that supports this conclusion is as follows: (i) the biphasic kinetics of inhibition in which protein synthesis is maintained at the control rate for 1-2 min is followed by an abrupt decline in the rate of synthesis; (ii) inhibition is associated with a marked disaggregation of polyribosomes and a concomitant increase in 80S ribosomes; and (iii) vanadate concentrations that inhibit protein chain initiation do not inhibit polypeptide chain elongation or the aminoacylation of tRNA. In partial reactions of protein chain initiation, vanadate concentrations that inhibit protein synthesis have no detectable effect on the formation of eukaryotic initiation factor eIF-2-promoted ternary complex with Met-tRNAf and GTP and on the assembly of 40S ribosomal subunit-Met-tRNAf complexes. On the addition of mRNA, the 40S ribosomal subunit-Met-tRNAf complexes also are transformed into 80S ribosome-mRNA-Met-tRNAf complexes, termed 80S initiation complexes. In vanadate-treated samples, however, these 80S initiation complexes are defective and unable to proceed beyond this step.

Laser initiated chain reactions: A generalized extension to complex chemical chain systems

We present a generalized extension of the detailed kinetic and experimental analysis for studying rapid gas phase chemical chain reactions initiated by short pulse laser photolysis. The key ingredients of the technique are pulsed laser initiation of the chain by dissociation of a small fraction of a precursor molecule to produce radicals (Cl2→hνCl⋅+Cl⋅), followed by time resolved, infrared chemiluminescence detection of product HCl(v) emission from the propagation steps (Cl⋅+RH→k1 R⋅+HCl(v), R⋅+Cl2→k2Cl⋅+RCl). This laser initiated chain reaction technique is now extended to encompass a far wider range of chain kinetic rates, mechanisms, and product state distributions. Three prototypical systems are investigated which demonstrate distinctly different facets of the extended technique: (1) Cl2/HBr—a slow chain reaction system, (2) Cl2/CH3SH—a multiple chain reaction system, and (3) Cl2/H2—a slow starting chain reaction system in which the product emission appears after an intital induction period. Quantitative rate data are obtained for several chain propagation steps: Br⋅+Cl2→Cl⋅+BrCl, k = 2.4(±0.2)×10−15 cm3 molecule−1 s−1; Cl⋅+CH3SH→⋅CH2SH+HCl(v), k = 4.3(±1)×10−12 cm3 molecule−1 s−1; ⋅CH2SH+Cl2→Cl⋅+ClCH2SH, k = 2.6(±1)×10−13 cm3 molecule−1 s−1. The results illuminate a more general approach to precise kinetic and mechanistic studies of complex chain reactions under nearly ideal psuedo-first-order laboratory conditions.

Artifacts due to the lifetimes of radicals in chain photoreactions at high absorbance

In this article we define the mean lifetime τ of a chain propagating radical and show its influence on the kinetics of photochemically initiated chain reactions when the distribution of the light in the reactor is not homogeneous.

A mutation of the B220 subunit gene affects the structural and functional properties of yeast RNA polymerase B in vitro.

The Saccharomyces cerevisiae mutant rpo B1 produces a DNA-dependent RNA polymerase B defective in RNA synthesis in vitro. RNA polymerase B purified from the mutant is altered both structurally and functionally. The enzyme is defective in the RNA chain initiation and elongation reactions. Enzyme-DNA binding is comparatively much less affected. These enzymological defects in the mutant enzyme are enhanced at elevated ionic strengths. Purified rpo B1 RNA polymerase B is lacking B32 and B16.5 subunits. However, the low activity of the mutant enzyme cannot be accounted for only by the loss of these two polypeptides. Wild type enzyme devoid of B32 and B16.5 subunits can be obtained after a mild urea treatment. This enzyme variant, called RNA polymerase B, does not share the enzymological properties of the mutant RNA polymerase. Immunoprecipitation of the enzyme from crude extracts shows that, in the rpo B1 mutant, a normal amount of RNA polymerase B is synthesized which contains the full complement of subunits. The polypeptide chain altered by the rpo B1 mutation was identified by partial proteolysis with proteinase K in the presence of sodium dodecyl sulfate. The 35S-labeled peptide pattern generated from the B220 subunit of the mutant enzyme differs markedly from the peptide pattern of the wild type subunit. The rpo B1 mutation therefore alters the B220 subunit, suggesting a role for this subunit in RNA chain elongation and in the association of the B32 and B16.5 subunits to the RNA polymerase molecule.

Determination of the rate constants of chain initiation and propagation in the decomposition of nitrogen trichloride

1. By the method of ignition delay, the rate constant of the chain-initiation reaction is determined in the process of branching-chain decomposition of NCl3 on various surfaces. Chain initiation is a heterogeneous process. 2. The rate constant of the reaction Cl + NCl3 — NCl2 + Cl2 is measured. 3. The stoichiometry of the branching stage is determined. When an NCl2 radical reacts with an NCl3 molecule, three chlorine atoms are formed.

New developments and future of synthetic polymer chemistry

Polymer chemistry is oriented towards improvement of existing materials through better understanding reaction mechanism and kinetics and towards the creation of specialty polymers,i.e. materials for special uses.This statement is illustrated by examples intentionally selected in different domains of basic research as chain initiation reaction, copolymerization of heterocyclic monomers,block polymerization and polymer transformation reactions.As specialty polymers special attention is given to polymers as organic reagent,as models for biopolymers and enzymes and as biological carriers.On the basis of the actual achievements future orientations of research are outlined.

Protein synthesis in rabbit reticulocytes. Demonstration of the requirements for eIF-2 and Co-eIF-2A for peptide chain initiation using immune sera.

Antisera to eIF-2 and Co-eIF-2A have been prepared by immunizing two chickens separately with homogeneous preparations of either eIF-2 or Co-eIF-2A. Addition of anti-eIF-2 or anti-Co-eIF-2A to reticulocyte lysates strongly inhibits protein synthesis and, in each case, protein synthesis inhibition is reversed by the addition of the corresponding homogeneous factor. Protein synthesis inhibition by anti-Co-eIF-2A is not reversed by eIF-2 at any concentration tested indicating an absolute requirement for Co-eIF-2A in protein synthesis. Also, in partial peptide chain initiation reactions, the addition of anti-Co-eIF-2A inhibits Co-eIF-2A stimulation of ternary complex formation by eIF-2 and Co-eIF-2A protection of ternary complexes in the presence of aurintricarboxylic acid.

New initiation systems for cationic polymerisation

Experimental results are presented to show that suitable free radical sources, in combination with aryldiazonium, diaryliodonium, and triarylsulphonium salts, may be used to promote cationic polymerisation of alkyl vinyl ethers and cyclic ethers and cyclic ethers such as tetrahydrofuran. Mechanistic features vary between the different classes as monomer but a general feature is a free radically initiated chain reaction leading to formation of initiating or propagating cationic centres. The process is especially weil suited to use of photochemically active free radical sources and, for polymerisation of tetrahydrofuran, it is shown that at least thirty propagating cations are produced for each photon of light absorbed byu the free radical initiator β,β-dimethoxy-β-phenylacetophenone. Product studies confirm that the initiating free radicals undergo electron transfer oxidation.

Kinetics and mechanism of dimethylcadmium auto-oxidation in decane

The liquid-phase auto-oxidation of dimethylcadmium in n-decane has been investigated at 45–60°C. The process occurs by a chain free-radical mechanism, to give the auto-oxidation products MeOOCdMe and (MeOO) 2Cd. The chain-initiation reaction has been studied by the inhibitor method. The rate of radical generation decreases with increasing oxygen concentration in the reaction mixture. The role of peroxide products in the auto-oxidation process has been clarified, and the probable mechanism of dimethylcadmium auto-oxidation is discussed.

DNA Replication by Bacteriophage T4 Proteins: Role of the DNA-delay Gene 61 in the Chain-initiation Reaction

DNA Replication by Bacteriophage T4 Proteins: Role of the DNA-delay Gene 61 in the Chain-initiation Reaction

Regulation of protein synthesis in rabbit reticulocyte lysates by the heme-regulated protein kinase: Inhibition of interaction of Met-tRNA f Met binding factor with another initiation factor in formation of Met-tRNA f Met ·40S ribosomal subunit complexes

Protein synthesis in reticulocytes and their lysates is regulated by heme. In heme deficiency a heme-regulated translational inhibitor (HRI) that blocks initiation of polypeptide chains is activated. HRI is a protein kinase (ATP: protein phosphotransferase, EC 2.7.1.37) that specifically phosphorylates the 38,000-dalton subunit of the Met-tRNA(f) (Met) binding factor (IF), which forms a ternary complex with Met-tRNA(f) (Met) and GTP, a finding that suggests that the inhibition by HRI involves the phosphorylation of IF. We have investigated the effect of HRI in the partial reactions of protein chain initiation in which the IF-promoted binding of Met-tRNA(f) (Met) to 40S ribosomal subunits is enhanced by another initiation factor [ternary complex dissociation factor (TDF)] and AUG. The results show that HRI at very low concentrations markedly inhibits the binding of Met-tRNA(f) (Met) to 40S subunits. The inhibitory effect of HRI requires ATP. Under these conditions HRI phosphorylates only the 38,000-dalton subunit of IF. The TDF preparations not only promote the binding of the ternary complex to 40S subunits but also promote the dissociation of the ternary complex in the presence of 5 mM Mg(2+) at 0 degrees . The preincubation of purified IF alone with low concentrations of HRI and ATP does not significantly affect its capacity to form the ternary complex; however, the TDF-promoted dissociation of the ternary complex is inhibited. The nonhydrolyzable analog adenosine 5'-[beta,gamma-imido]triphosphate does not substitute for ATP. These findings suggest that phosphorylation causes a conformational modification in IF, which results in inhibition of the interaction between the ternary complex and TDF that is required for the binding of the ternary complex to 40S subunits.

Comment on the radiation initiation of chain reactions

Comment on the radiation initiation of chain reactions

Reactive silica: X. Ethylene sorption and polymerization

A variety of data indicates that unusual adsorption centers containing silicon radicals are formed when high surface area silicas are subjected to an activation procedure consisting of methoxylation, pyrolysis of the methoxy layer, and high temperature degassing. The reactive silica (RS) adsorbent can chemisorb oxygen, the oxygenated reactive silica (ORS) containing SiO radicals. To test the capability of the radical centers of RS and ORS to act as initiators for free radical initiated chain reactions, the sorption of ethylene by RS and ORS was studied using infrared spectroscopic techniques. The data indicate that several tightly bonded surface structures were formed, including polyethylene.

Solid phase radical reactions and the mechanism of carbon chain polymer oxidations

The oxidation of polymers in the solid state, like the liquid phase oxidation of hydrocarbons, has a spontaneously initiated chain reaction mechanism with a bimolecular chain termination. The following are the characteristic differences between the solid and liquid phase oxidations. The radical reactions are slower in the polymer matrix than in the liquid phase; the lower the molecular mobility in the former, the more difficult will be the elementary act of segmental orientation. The free valency travels in the polymer sample in the shape of alkyl and peroxide radicals by a relay mechanisms, ile. due to a segmental shift and the elementary acts: P + PH → PH + P, and PO 2 + PH → POOH + P. In constrast to the liquid, the solid polymer is non-homogeneous and therefore oxidizes irregularly; the less dense amorphous phase in PE and PP oxidizes earlier than the crystalline phase. The alkyl radicals react more slowly with oxygen in the polymer and various reactions take place in addition to P + O 2 → PO 2 ., especially an isomerization; the latter results in PP in the conversion of the absorbed O 2 into alcohol groups, with the exception of the hydroperoxide which reacts with ROOH and causes its induced decomposition and a reaction with the CH bonds of adjacent segments. This can be seen in PP on the ratio of single to block hydroperoxide groups. the decomposition of the macro-radicals, which still has a relatively unexplored mechanism, plays an important part in the oxidation of polymers.

A General Model for the Shock Initiation of Explosives

AbstractA general model for the shock initiation of explosive reaction in chemical explosives is proposed. The model is based on the concepts of: (1) the kinetics of decomposition in which ions and free radicals produced by the shock wave shear forces initiate chain reactions that contribute to and accelerate the decomposition produced by the thermally activated molecules; (2) the formation of statistically random reaction sites whose number in a specific explosive is a direct function of the shock pressure as the shock transits the explosive; and (3) a critical‐ energy‐fluence requirement for initiation. This model appears to apply to explosive reaction in gases, liquids, and solids.

Chain initiation reaction in the liquid phase oxidation of sebacic acid diesters

The rates of free radical formation in the system oxygen — sebacic acid diesters have been determined at 140–170°C in o-dichlorobenzene solution. The rate constants for chain initiation are: dimethyl sebacate k0=8.5×1013 exp(−39300/Rt)1/mol sec, diethyl sebacate k0=5.1×1011 exp(−33900/RT)1/mol sec, diisopropyl sebacate k0=1.0×1012 exp(−34700/RT)1/mol sec.