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Recombination Reaction (recombination + reaction)

Distribution by Scientific Domains
Chemistry37%
Polymers and Materials Science25%
Life Sciences25%

Selected Abstracts

Gas-phase radical,radical recombination reactions of nitroxides with substituted phenyl radicals

INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 4 2004
J. L. Heidbrink
Fourier-transform ion cyclotron resonance mass spectrometry has been used to examine gas-phase reactions of four different nitroxide free radicals with eight positively charged pyridyl and phenyl radicals (some containing a Cl, F, or CF3 substituent). All the radicals reacted rapidly (near collision rate) with nitroxides by radical,radical recombination. However, some of the radicals were also able to abstract a hydrogen atom from the nitroxide. The results establish that the efficiency (kreaction/kcollision) of hydrogen atom abstraction varies with the electrophilicity of the radical, and hence is attributable to polar effects (a lowering of the transition-state energy by an increase in its polar character). The efficiency of the recombination reaction is not sensitive to substituents, presumably due to a very low reaction barrier. Even so, after radical,radical recombination has occurred, the nitroxide adduct was found to fragment in different ways depending on the structure of the radical. For example, a cationic fragment was eliminated from the adducts of the more electrophilic radicals via oxygen anion abstraction by the radical (i.e., the nitroxide adduct cleaves heterolytically), whereas adducts of the less electrophilic radicals predominantly fragmented via homolytic cleavage (oxygen atom abstraction). Therefore, differences in the product branching ratios were found to be attributable to polar factors. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 216,229 2004 [source]

Nitroxide-Mediated Polymerization of Methyl Methacrylate Using an SG1-Based Alkoxyamine: How the Penultimate Effect Could Lead to Uncontrolled and Unliving Polymerization

MACROMOLECULAR CHEMISTRY AND PHYSICS, Issue 14 2006
Yohann Guillaneuf
Abstract Summary: The nitroxide-mediated polymerization (NMP) of MMA initiated with a new crowded SG1-based alkoxyamine was performed. Contrary to the results expected after a kinetic analysis (Fischer's diagram), the polymerization of MMA at 45,°C with SG1 showed only partial control and livingness during the first 15% of conversion. Simulations using PREDICI highlighted that the kinetic rate constants currently in use had not been correctly estimated and that a strong penultimate effect drastically increased the equilibrium constant K (7,×,10,7), preventing a well-controlled polymerization. Experimental determination of the kc value (1.4,×,104 L,·,mol,1,·,s,1) confirmed a strong penultimate effect on the recombination reaction, whereas for the dissociation reaction this effect is lower (kd,=,10,2,·,s,1). Nitroxide-mediated polymerization of MMA at 45,°C initiated with a new crowded SG1-based alkoxyamine. [source]

Topological analysis of Hin-catalysed DNA recombination in vivo and in vitro

MOLECULAR MICROBIOLOGY, Issue 4 2004
Stacy K. Merickel
Summary In vitro studies have demonstrated that Hin-catalysed site-specific DNA inversion occurs within a tripartite invertasome complex assembled at a branch on a supercoiled DNA molecule. Multiple DNA exchanges within a recombination complex (processive recombination) have been found to occur with particular substrates or reaction conditions. To investigate the mechanistic properties of the Hin recombination reaction in vivo , we have analysed the topology of recombination products generated by Hin catalysis in growing cells. Recombination between wild-type recombination sites in vivo is primarily limited to one exchange. However, processive recombination leading to knotted DNA products is efficient on substrates containing recombination sites with non-identical core nucleotides. Multiple exchanges are limited by a short DNA segment between the Fis-bound enhancer and closest recombination site and by the strength of Fis,Hin interactions, implying that the enhancer normally remains associated with the recombining complex throughout a single exchange reaction, but that release of the enhancer leads to multiple exchanges. This work confirms salient mechanistic aspects of the reaction in vivo and provides strong evidence for the propensity of plectonemically branched DNA in prokaryotic cells. We also demonstrated that a single DNA exchange resulting in inversion in vitro is accompanied by a loss of four negative supercoils. [source]

An accumulative site-specific gene integration system using cre recombinase-mediated cassette exchange

BIOTECHNOLOGY & BIOENGINEERING, Issue 6 2010
Yujiro Kameyama
Abstract The Cre- loxP system is frequently used for site-specific recombination in animal cells. The equilibrium and specificity of the recombination reaction can be controlled using mutated loxPs. In the present study, we designed an accumulative site-specific gene integration system using Cre recombinase and mutated loxPs in which the Cre-mediated cassette exchange reaction is infinitely repeatable for target gene integration into loxP target sites. To evaluate the feasibility and usefulness of this system, a series of integration reactions were repeated and confirmed in vitro using Cre recombinase protein and plasmids. Accumulative gene integration was also performed on the genome of Chinese hamster ovary (CHO) cells. The results indicated that the system was applicable for repeated gene integration of multiple genes to the target sites on both plasmids and CHO cell genomes. This gene integration system provides a novel strategy for gene amplification and for biological analyses of gene function through the genetic modification of cells and organisms. Biotechnol. Bioeng. 2010;105: 1106,1114. © 2009 Wiley Periodicals, Inc. [source]

Light-Driven Charge Separation in Isoxazolidine,Perylene Bisimide Dyads

CHEMISTRY - A EUROPEAN JOURNAL, Issue 46 2009
Heinz Langhals Prof.
Abstract A series of arrays for light-driven charge separation is presented, in which perylene tetracarboxylic bisimide is the light-absorbing chromophore and electron acceptor, whereas isoxazolidines are colourless electron donors, the electron-releasing properties of which are increased with respect to the amino group by means of the ,-effect. Charge separation (CS) in toluene over a distance ranging from ,10 to ,16,Å, with efficiencies of ,95 to ,50,% and CS lifetimes from 300,ps to 15,ns, are demonstrated. In dichloromethane the charge recombination reaction is faster than charge separation, preventing accumulation of the CS state. The effects of solvent polarity and molecular structure are discussed in the frame of current theories. [source]

High Density Cascaded Arc Produced Plasma Expanding in a Low Pressure Region

CONTRIBUTIONS TO PLASMA PHYSICS, Issue 5-6 2004
R. P. Dahiya
Abstract Experimental measurements made in thermal expanding argon, nitrogen and hydrogen plasmas with particular reference to molecular kinetics, surface nitriding and intense flux in magnetic field are discussed. The plasma is generated in a cascaded arc source. In the presence of molecular species (H2 / N2) dissociative recombination reactions involving rovibrationally excited molecules contribute to a rapid decay of the plasma species, especially for hydrogen system. A combination of nitrogen and hydrogen plasma gives an efficient plasma nitriding process, which has been applied for case hardening of machinery components. In another setup a strong axial magnetic field (0.4 - 1.6 T) contains and substantially prolongs the plasma beam in the chamber. In the presence of the magnetic field, an additional current drawn through the plasma beam using a biased substrate and a ring creates dense low temperature plasma giving a new unexplored plasma regime. The plasma kinetics are modified in this regime from the recombining to the ionising mode. When the additional current in the argon plasma beam exceeds 30 A, its light emission is predominantly in the blue region. With the additional current and magnetic field, the emission intensity of H, and other lines arising from higher energy levels in the hydrogen Balmer series is enhanced. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Gas-phase radical,radical recombination reactions of nitroxides with substituted phenyl radicals

INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 4 2004
J. L. Heidbrink
Fourier-transform ion cyclotron resonance mass spectrometry has been used to examine gas-phase reactions of four different nitroxide free radicals with eight positively charged pyridyl and phenyl radicals (some containing a Cl, F, or CF3 substituent). All the radicals reacted rapidly (near collision rate) with nitroxides by radical,radical recombination. However, some of the radicals were also able to abstract a hydrogen atom from the nitroxide. The results establish that the efficiency (kreaction/kcollision) of hydrogen atom abstraction varies with the electrophilicity of the radical, and hence is attributable to polar effects (a lowering of the transition-state energy by an increase in its polar character). The efficiency of the recombination reaction is not sensitive to substituents, presumably due to a very low reaction barrier. Even so, after radical,radical recombination has occurred, the nitroxide adduct was found to fragment in different ways depending on the structure of the radical. For example, a cationic fragment was eliminated from the adducts of the more electrophilic radicals via oxygen anion abstraction by the radical (i.e., the nitroxide adduct cleaves heterolytically), whereas adducts of the less electrophilic radicals predominantly fragmented via homolytic cleavage (oxygen atom abstraction). Therefore, differences in the product branching ratios were found to be attributable to polar factors. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 216,229 2004 [source]

Electrical Discharges in Mixtures of Light and Heavy Water

PLASMA PROCESSES AND POLYMERS, Issue 1 2008
Selma Mededovic
Abstract Pulsed electrical discharges in pure solutions and mixtures of light and heavy water are examined. The concentration of H2O2 formed by electrical discharge in 100% H2O was found to be 1.4 times smaller than that of D2O2 produced by electrical discharge in 100% D2O. This difference in peroxide formation was due to the high reactivity of OH radicals with other species in the plasma channel, thereby reducing OH recombination to form H2O2, while the lower reactivity of OD radicals with other species allowed more OD recombination reactions to form D2O2. A new method based on Raman spectroscopy was also developed to quantify H2O2, D2O2, and HDO2 in mixtures containing all three species. [source]