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Activation Parameters (activation + parameter)
Selected AbstractsActivation Parameters for the Epoxidation of Substituted cis/trans Pairs of 1,2-Dialkylalkenes by DimethyldioxiraneEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 20 2006Brian S. Crow Abstract The first activation parameter data for the reaction of dimethyldioxirane (1) with five cis/trans pairs of alkenes are reported. The epoxidation of cis -1,2-dialkylalkenes (2cis: R1 = Me, R2 = iPr; 3cis: R1 = Me, R2 = tBu; 4cis: R1 = R2 = Et; 5cis: R1 = Et, R2 = iPr; 6cis: R1 = Et, R2 = tBu) and trans -1,2-dialkylalkenes (2trans: R1 = Me, R2 = iPr; 3trans: R1 = Me, R2 = tBu; 4trans: R1 = R2 = Et; 5trans: R1 = Et, R2 = iPr; 6trans: R1 = Et, R2 = tBu) by 1 produced the corresponding epoxides, quantitatively and stereospecifically, as the sole observable products. Activation parameters of the epoxidation of the five pairs of alkenes, 2cis,6cis and 2trans,6trans, by 1 were determined using the Arrhenius method. Enhanced selectivity for cis - vs. trans -alkene epoxidation was observed at lower temperatures. In general, the ,G, terms were larger and showed more variability for the reaction of 1 with trans -alkenes as compared to those for the corresponding cis isomers. The ,H, terms mirrored trends observed in ,G, because ,S, terms for all ten of the compounds were roughly identical. The ,,G, values, a comparison of the trans to the cis isomer data, yielded positive values of 1.2 to 1.8 kcal/mol for the five sets of data and appeared to be dependent on relative steric interactions. The experimental activation parameter data, consistent with predictions from ab initio calculations based on a spiro transition-state model, showed that the lower reactivity of trans -alkenes is due to enthalpy factors. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Analysis of Linear Free-Energy Relationships Combined with Activation Parameters Assigns a Concerted Mechanism to Alkaline Hydrolysis of X-Substituted Phenyl DiphenylphosphinatesCHEMISTRY - A EUROPEAN JOURNAL, Issue 24 2008Ik-Hwan Um Prof. Abstract A kinetic study is reported for alkaline hydrolysis of X-substituted phenyl diphenylphosphinates (1,a,i). The Brønsted-type plot for the reactions of 1,a,i is linear over 4.5 pKa units with ,lg=,0.49, a typical ,lg value for reactions which proceed through a concerted mechanism. The Hammett plots correlated with ,o and ,, constants are linear but exhibit many scattered points, while the corresponding Yukawa,Tsuno plot results in excellent linear correlation with ,=1.42 and r=0.35. The r value of 0.35 implies that leaving-group departure is partially advanced at the rate-determining step (RDS). A stepwise mechanism, in which departure of the leaving group from an addition intermediate occurs in the RDS, is excluded since the incoming HO, ion is much more basic and a poorer nucleofuge than the leaving aryloxide. A dissociative (DN + AN) mechanism is also ruled out on the basis of the small ,lg value. As the substituent X in the leaving group changes from H to 4-NO2 and 3,4-(NO2)2, ,H,, decreases from 11.3,kcal,mol,1 to 9.7 and 8.7,kcal,mol,1, respectively, while ,S,, varies from ,22.6,cal,mol,1,K,1 to ,21.4 and ,20.2,cal,mol,1,K,1, respectively. Analysis of LFERs combined with the activation parameters assigns a concerted mechanism to the current alkaline hydrolysis of 1,a,i. [source] Activation Parameters for the Epoxidation of Substituted cis/trans Pairs of 1,2-Dialkylalkenes by DimethyldioxiraneEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 20 2006Brian S. Crow Abstract The first activation parameter data for the reaction of dimethyldioxirane (1) with five cis/trans pairs of alkenes are reported. The epoxidation of cis -1,2-dialkylalkenes (2cis: R1 = Me, R2 = iPr; 3cis: R1 = Me, R2 = tBu; 4cis: R1 = R2 = Et; 5cis: R1 = Et, R2 = iPr; 6cis: R1 = Et, R2 = tBu) and trans -1,2-dialkylalkenes (2trans: R1 = Me, R2 = iPr; 3trans: R1 = Me, R2 = tBu; 4trans: R1 = R2 = Et; 5trans: R1 = Et, R2 = iPr; 6trans: R1 = Et, R2 = tBu) by 1 produced the corresponding epoxides, quantitatively and stereospecifically, as the sole observable products. Activation parameters of the epoxidation of the five pairs of alkenes, 2cis,6cis and 2trans,6trans, by 1 were determined using the Arrhenius method. Enhanced selectivity for cis - vs. trans -alkene epoxidation was observed at lower temperatures. In general, the ,G, terms were larger and showed more variability for the reaction of 1 with trans -alkenes as compared to those for the corresponding cis isomers. The ,H, terms mirrored trends observed in ,G, because ,S, terms for all ten of the compounds were roughly identical. The ,,G, values, a comparison of the trans to the cis isomer data, yielded positive values of 1.2 to 1.8 kcal/mol for the five sets of data and appeared to be dependent on relative steric interactions. The experimental activation parameter data, consistent with predictions from ab initio calculations based on a spiro transition-state model, showed that the lower reactivity of trans -alkenes is due to enthalpy factors. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Mechanism of oxidation of alanine by chloroaurate(III) complexes in acid medium: Kinetics of the rate processesINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 7 2009Pratik K. Sen The kinetics of the oxidation of alanine by chloroaurate(III) complexes in acetate buffer medium has been investigated. The major oxidation product of alanine has been identified as acetaldehyde by 1H NMR spectroscopy. Under the experimental conditions, AuCl and AuCl3(OH), are the effective oxidizing species of gold(III). The reaction is first order with respect to Au(III) as well as alanine. The effects of H+ and Cl, on the second-order rate constant k2, have been analyzed, and accordingly the rate law has been deduced: k2, = (k1[H+][Cl,] + k3K4K5)/(K4K5 + [H+][Cl,]). Increasing dielectric constant of the medium has an accelerating effect on the reaction rate. Activation parameters associated with the overall reaction have been calculated. A mechanism involving the two effective oxidizing species of gold(III) and zwitterionic species of alanine, consistent with the rate law, has been proposed. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 473,482, 2009 [source] Kinetic investigation on the reactions of p -toluenesulfonyl chloride with p -substituted benzoic acid(s) in the presence of triethylamine in aprotic solventsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 5 2009Subbiah Ananthalakshmi Second-order rate constants of the reactions of p -toluenesulfonyl chloride with p -substituted benzoic acids in the presence of triethylamine in acetonitrile/acetone under equimolar and pseudo-first-order conditions have been determined by the conductometric method using the Guggenheim principle at 25, 30, 35, and 40°C. The reactions follow second order with respect to the whole and first order with respect to each of the reactants. The order of reactivity of the substituents in benzoic acid is rationalized. Activation parameters are obtained by applying the usual methods. The Hammett plot has been found nonlinear, whereas the Bronsted plot shows good correlation. This may be explained on the basis of electronic effects of substituents on the reaction center. Kinetic data and the product analyses indicate that the reaction proceeds through direct nucleophilic attack on the sulfur center. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 303,308, 2009 [source] Dediazoniation of 1-naphthalenediazonium tetrafluoroborate in aqueous acid and in micellar solutionsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 6 2008Carlos Bravo-Diaz We have measured the rates and product yields of dediazoniation of 1-naphthalenediazonium (1ND) tetrafluoroborate in the presence and absence of sodium dodecyl sulfate (SDS) micellar aggregates by employing a combination of UV,vis spectroscopy and high-performance liquid chromatography (HPLC) measurements. Kinetic data were obtained by a derivatization procedure with product yields were determined by HPLC. HPLC chromatograms show that in aqueous acid and in micellar solutions only one dediazoniation product is formed in significant quantities, 1-naphthol (NOH), and the observed rate constants (kobs) are the same when 1ND loss is monitored spectrometrically and when NOH formation is monitored by HPLC. Activation parameters were obtained both in the presence and absence of SDS micellar aggregates. In both the systems, the enthalpies of activation are high and the entropies of activation are positive. The enthalpy of activation in the absence of SDS is very similar to that in the presence of SDS micelles, but the entropy of activation is lower by a factor of 4. As a consequence, SDS micelles speed up the thermal decomposition of 1ND and increase kobs by a factor of 1.5 when [SDS] = 0.02 M. In contrast, results obtained in the presence of complexing systems such as crown ethers and polyethers show significant stabilization of the parent arenediazonium ions. Kinetic and HPLC data are consistent with the heterolytic DN + AN mechanism that involves the rate-determining fragmentation of the arenediazonium ion into a very reactive phenyl cation that reacts competitively with available nucleophiles. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 301,309, 2008 [source] Kinetics and mechanism of oxidation of the drug mephenesin by bis(hydrogenperiodato)argentate(III) complex anionINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 8 2007Shigang Shen Mephenesin is being used as a central-acting skeletal muscle relaxant. Oxidation of mephenesin by bis(hydrogenperiodato)argentate(III) complex anion, [Ag(HIO6)2]5,, has been studied in aqueous alkaline medium. The major oxidation product of mephenesin has been identified as 3-(2-methylphenoxy)-2-ketone-1-propanol by mass spectrometry. An overall second-order kinetics has been observed with first order in [Ag(III)] and [mephenesin]. The effects of [OH,] and periodate concentration on the observed second-order rate constants k, have been analyzed, and accordingly an empirical expression has been deduced: k, = (ka + kb[OH,])K1/{f([OH,])[IO,4]tot + K1}, where [IO,4]tot denotes the total concentration of periodate, ka = (1.35 ± 0.14) × 10,2M,1s,1 and kb = 1.06 ± 0.01 M,2s,1 at 25.0°C, and ionic strength 0.30 M. Activation parameters associated with ka and kb have been calculated. A mechanism has been proposed to involve two pre-equilibria, leading to formation of a periodato-Ag(III)-mephenesin complex. In the subsequent rate-determining steps, this complex undergoes inner-sphere electron transfer from the coordinated drug to the metal center by two paths: one path is independent of OH, whereas the other is facilitated by a hydroxide ion. In the appendix, detailed discussion on the structure of the Ag(III) complex, reactive species, as well as pre-equilibrium regarding the oxidant is provided. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 440,446, 2007 [source] N -bromosuccinimide oxidation of dipeptides and their amino acids: Synthesis, kinetics and mechanistic studiesINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 6 2006N. S. Linge Gowda Dipeptides (DP), namely valyl,glycine (Val,Gly), alanyl,proline (Ala,Pro), and valyl,proline (Val,Pro) were synthesized by classical solution phase methods and characterized. The kinetics of oxidation of amino acids (AA) and DP by N -bromosuccinimide (NBS) was studied in the presence of perchlorate ions in acidic medium at 28°C. The reaction was followed spectrophotometrically at ,max = 240 nm. The reactions follow identical kinetics, being first order each in [NBS], [AA], and [DP]. No effect on [H+], reduction product [succinimide], and ionic strength was observed. Effects of varying dielectric constant of the medium and addition of anions such as chloride and perchlorate were studied. Activation parameters have been computed. The oxidation products of the reaction were isolated and characterized. The proposed mechanism is consistent with the experimental results. An apparent correlation was noted between the rate of oxidation of AA and DP. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 376,385, 2006 [source] Kinetics and mechanism of sodium N -halo- p -toluenesulfonamides oxidation of diclofenac in alkaline mediumAICHE JOURNAL, Issue 12 2009Puttaswamy Abstract Diclofenac belongs to a class of drugs called nonsteroidal antiinflammatory drugs. The kinetics and mechanism of oxidation of diclofenac by sodium N -halo- p -toluenesulfonamides viz., chloramine-T and bromamine-T in NaOH medium have been studied at 293 K. Under comparable experimental conditions, reactions with both the oxidants follow identical kinetics with a first-order dependence on each [oxidant]o and a fractional-order dependence on each [diclofenac]o and [NaOH]. Activation parameters have been computed. N -hydroxyldiclofenac is identified as the oxidation product of diclofenac. Michaelis-Menten type of mechanism has been suggested. The rate of oxidation of diclofenac is about four-fold faster with bromamine-T when compared with chloramine-T. This may be attributed to the difference in electrophilicities of Cl+ and Br+ ions and also the van der Waal's radii of chlorine and bromine. Plausible mechanism and related rate law have been designed for the observed kinetics. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Primary kinetic hydrogen isotope effects in deprotonations of a nitroalkane by intramolecular phenolate groupsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8 2010Nicholas Backstrom Abstract Rate constants and kinetic isotope effects have been determined for the formation of nitronate anions from the ethers 1-(2-methoxyphenyl)-2-nitropropane, 7(X,=,H, L,=,H and D) and 1-(2-methoxy-5-nitrophenyl)-2-nitropropane, 7(X,=,NO2, L,=,H and D), and from the corresponding phenols, 1-(2-hydroxyphenyl)-2-nitropropane, 3(X,=,H, L,=,H and D), and 1-(2-hydroxy-5-nitrophenyl)-2-nitropropane, 3(X,=,NO2, L,=,H and D), in aqueous basic medium. For the ethers 7, rates of deprotonation by hydroxide are comparable with those found for deprotonations of 2-nitropropane, with kH/kD (25,°C),=,7.7 and 7.8, respectively. In both the cases, the isotope effects are conventionally temperature dependent. For the corresponding phenols 3, conditions have been established under which the deprotonations of the nitroalkane are dominated by intramolecular deprotonation by the kinetically first-formed phenolate anion, with an estimated effective molarity EM,,,250. For 3 (X,=,H, L,=,H or D), kH/kD (25,°C),=,7.8, with E,,,E,=,6.9,kJ,mol,1 and AH/AD,=,0.5. For 3(X,=,NO2, L,=,H or D), rates of intramolecular deprotonation are reduced 30-fold, and an elevated kinetic isotope effect is found (kH/kD (25,°C),=,10.7). Activation parameters (E,,,E,=,17.8,kJ,mol,1 and AH/AD,=,0.008) are compatible with an enhanced tunnelling contribution to reactivity in the H-isotopomer. Copyright © 2009 John Wiley & Sons, Ltd. [source] Kinetics of the ring-opening metathesis polymerization of a 7-oxanorbornene derivative by Grubbs' catalystJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 13 2003Marium G. Holland Abstract The kinetics of the initiation and propagation of the ring-opening metathesis polymerization of exo,exo -5,6-bis(methoxycarbonyl)-7-oxabicyclo[2.2.1]hept-2-ene catalyzed by Grubbs' catalyst (Cl2(PCy3)2RuCHPh) were measured by ultraviolet,visible and 1H NMR spectroscopy, respectively. Activation parameters for these processes were also determined. Although the ratio of the rate constant of initiation to the rate constant of propagation was determined to be less than 1 for this system, this polymerization showed many of the characteristics of a living system, including low polydispersities. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2125,2131, 2003 [source] Investigation of the stereodynamics of tris-(, -diimine),transition metal complexes by enantioselective dynamic MEKCELECTROPHORESIS, Issue 2 2009Sabrina Bremer Abstract Enantiomerization of octahedral tris(, -diimine),transition metal complexes was investigated by enantioselective dynamic MEKC. Varying both the transition metal ion (Fe2+, Fe3+, and Ni2+) and the bidentate diimine ligand (1,10-phenanthroline and 2,2,-bipyridyl), the enantiomer separations were performed either in a 100,mM sodium tetraborate buffer (pH 9.3) or in a 100,mM sodium tetraborate/sodium dihydrogenphosphate buffer (pH 8.0) both containing sodium cholate as chiral surfactant. The unified equation of dynamic chromatography was employed to determine apparent reaction rate constants from the electropherograms showing distinct plateau formation. Apparent activation parameters ,H, and ,S, were calculated from temperature-dependent measurements between 10.0 and 35.0°C in 2.5,K steps. It was found that the nature of the central metal ion and the ligand strongly influence the enantiomerization barrier. Surprisingly, complexes containing the 2,2,-bipyridyl ligand show highly negative activation entropies between ,103 and ,116,J (K,mol),1 while the activation entropy of tris(1,10-phenanthroline) complexes is positive indicating a different mechanism of interconversion. Furthermore, it was found that the Ni2+ complexes are stereostable under the conditions investigated here making them a lucent target as enantioselective catalysts. [source] Determination of the cis-trans isomerization barrier of enalaprilat by dynamic capillary electrophoresis and computer simulationELECTROPHORESIS, Issue 2 2004Oliver Trapp Abstract Dynamic capillary electrophoresis (DCE) and computer simulation of the elution profiles with the stochastic model has been applied to determine the isomerization barriers of the angiotensin converting enzyme inhibitor enalaprilat. The separation of the rotational cis-trans isomeric drug has been performed in an aqueous 20 mM borate buffer at pH 9.3. Interconversion profiles featuring plateau formation and peak broadening were observed. To evaluate the rate constants kcis,trans and ktrans,cis of the cis-trans isomerization from the experimental electropherograms obtained by dynamic capillary electrophoresis, elution profiles were analyzed by a simulation with iterative convergence to the experimental data using the ChromWin software which requires the total migration times of the individual isomers tR, the electroosmotic break-through time t0, the plateau height hplateau, the peak widths at half height of the individual isomers wh, as well as the peak ratio of the isomers as experimental data input. From temperature-dependent measurements between 0° and 15°C the thermodynamic parameters ,G, ,H and ,S, the rate constants kcis,trans and ktrans,cis and the kinetic activation parameters ,G,, ,H,, and ,S, of the cis-trans isomerization of enalaprilat were obtained. From the activation parameters the isomerization barriers at 37°C were calculated to be ,G,trans,cis = 87.2 kJ·mol,1 and ,G,cis,trans = 91.9 kJ·mol,1. [source] Kinetic Studies of the Oxidative Addition and Transmetallation Steps Involved in the Cross-Coupling of Alkynyl Stannanes with Aryl Iodides Catalysed by ,2 -(Dimethyl fumarate)(iminophosphane)palladium(0) ComplexesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 4 2004Bruno Crociani Abstract The complexes [Pd(,2 -dmfu)(P,N)] {dmfu = dimethyl fumarate; P,N = 2-(PPh2)C6H4,1-CH=NR, R = C6H4OMe-4 (1a), CHMe2 (2a), C6H3Me2 -2,6 (3a), C6H3(CHMe2)2 -2,6 (4a)} undergo dynamic processes in solution which consist of a P,N ligand site exchange through initial rupture of the Pd,N bond at lower energy and an olefin dissociation-association at higher energy. According to equilibrium constant values for olefin replacement, the complex [Pd(,2 -fn)(P,N)] (fn = fumaronitrile, 1b) has a greater thermodynamic stability than its dmfu analogue 1a. The kinetics of the oxidative addition of ArI (Ar = C6H4CF3 -4) to 1a and 2a lead to the products [PdI(Ar)(P,N)] (1c, 2c) and obey the rate law, kobs = k1A + k2A[ArI]. The k1A step involves oxidative addition to a reactive species [Pd(solvent)(P,N)] formed from dmfu dissociation. The k2A step is better interpreted in terms of oxidative addition to a species [Pd(,2 -dmfu)(solvent)(,1 -P,N)] formed in a pre-equilibrium step from Pd,N bond breaking. The complexes 1c and 2c react with PhC,CSnBu3 in the presence of an activated olefin (ol = dmfu, fn) to yield the palladium(0) derivatives [Pd(,2 -ol)(P,N)] along with ISnBu3 and PhC,CAr. The kinetics of the transmetallation step, which is rate-determining for the overall reaction, obey the rate law: kobs = k2T[PhC,CSnBu3]. The k2T values are markedly enhanced in more polar solvents such as CH3CN and DMF. The solvent effect and the activation parameters suggest an associative SE2 mechanism with substantial charge separation in the transition state. The kinetic data of the above reactions in various solvents indicate that, for the cross-coupling of PhC,CSnBu3 with ArI catalysed by 1a or 2a, the rate-determining step is represented by the oxidative addition and that CH3CN is the solvent in which the highest rates are observed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Coenzyme B12 Model Studies , Kinetics of Axial Ligation of Alkylcobalt Complexes Containing a Tridentate Amino-Oximate LigandEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 20 2003Basam M. Alzoubi Abstract The ligand-substitution reaction of [RCo(LNHpy)(HLNHpy)]+ by cyanide was studied in methanol as a solvent, where R = Et (1), Me (2), CF3CH2 (3), and bridging CH2 (4), HLNHpy = 2-(2-pyridylethyl)amino-3-butanone oxime and LNHpy, is its conjugate base. The second-order rate constants for the substitution of the 2-pyridylethyl moiety in [RCo(LNHpy)(HLNHpy)]+ by cyanide were found to be 19.1, 0.25, 2.2·10,2, and 1.7·10,2M,1·s,1 for R = Et, Me, CF3CH2, and bridging CH2, respectively. From the temperature- and pressure-dependence studies of the substitution reaction, the activation parameters (,H,, ,S,, ,V,) for the reaction of [RCo(LNHpy)(HLNHpy)]+ with cyanide in methanol were found to be, for R = Et: 69±2 kJ·mol,1, +11±7 J·K,1 mol,1, +9.6±0.3 cm3·mol,1; R = Me: 86±4 kJ·mol,1, +31±15 J·K,1·mol,1, +4.3±0.2 cm3·mol,1; R = CF3CH2: 73±3 kJ·mol,1, ,33±10 J·K,1·mol,1, +8.7±0.2 cm3·mol,1; R = bridging CH2: 80±1 kJ·mol,1, ,13±3 J·K,1·mol,1, +5.4±0.1 cm3·mol,1, respectively. Based on the reported rate and activation parameters, the mechanism of the ligand-substitution reaction varies between a limiting dissociative and a dissociative interchange type of mechanism depending on the ,-donor and steric effects of the alkyl group. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Cortical control of thermoregulatory sympathetic activationEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 11 2010M. Fechir Abstract Thermoregulation enables adaptation to different ambient temperatures. A complex network of central autonomic centres may be involved. In contrast to the brainstem, the role of the cortex has not been clearly evaluated. This study was therefore designed to address cerebral function during a whole thermoregulatory cycle (cold, neutral and warm stimulation) using 18-fluordeoxyglucose-PET (FDG-PET). Sympathetic activation parameters were co-registered. Ten healthy male volunteers were examined three times on three different days in a water-perfused whole-body suit. After a baseline period (32°C), temperature was either decreased to 7°C (cold), increased to 50°C (warm) or kept constant (32°C, neutral), thereafter the PET examination was performed. Cerebral glucose metabolism was increased in infrapontine brainstem and cerebellar hemispheres during cooling and warming, each compared with neutral temperature. Simultaneously, FDG uptake decreased in the bilateral anterior/mid-cingulate cortex during warming, and in the right insula during cooling and warming. Conjunction analyses revealed that right insular deactivation and brainstem activation appeared both during cold and warm stimulation. Metabolic connectivity analyses revealed positive correlations between the cortical activations, and negative correlations between these cortical areas and brainstem/cerebellar regions. Heart rate changes negatively correlated with glucose metabolism in the anterior cingulate cortex and in the middle frontal gyrus/dorsolateral prefrontal cortex, and changes of sweating with glucose metabolism in the posterior cingulate cortex. In summary, these results suggest that the cerebral cortex exerts an inhibitory control on autonomic centres located in the brainstem or cerebellum. These findings may represent reasonable explanations for sympathetic hyperactivity, which occurs, for example, after hemispheric stroke. [source] Cycloadditions and Methylene Transfer in Reactions of Substituted Thiocarbonyl S -Methylides with Thiobenzophenone: A Computational StudyEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 8 2005Reiner Sustmann Abstract Regiochemistry and methylene transfer reactions in cycloadditions of aliphatic thiocarbonyl S -methylides and thiobenzophenone are analyzed by ab initio [(U)HF/3-21G*] and DFT calculations [(U)B3LYP/6-31G*//(U)HF/3-21G* and (U)B3LYP/6-31G*]. The formation of regioisomeric 1,3-dithiolanes is explained by the competition of concerted (2,4-substituted 1,3-dithiolane) and stepwise cycloaddition via C,C -biradicals (4,5-substituted 1,3-dithiolane). Aliphatic thiocarbonyl S -methylides with sterically demanding substituents undergo substantial methylene transfer in the reaction with thiobenzophenone. This process involves dissociation of the C,C -biradical intermediate with liberation of thiobenzophenone S -methylide which, in turn, combines with a second molecule of thiobenzophenone. Calculated activation parameters for the different processes are in agreement with the experimental observations. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Comparison of the specificity, stability and individual rate constants with respective activation parameters for the peptidase activity of cruzipain and its recombinant form, cruzain, from Trypanosoma cruziFEBS JOURNAL, Issue 24 2001Wagner A. S. Judice The Trypanosoma cruzi cysteine protease cruzipain contains a 130-amino-acid C-terminal extension, in addition to the catalytic domain. Natural cruzipain is a complex of isoforms, because of the simultaneous expression of several genes, and the presence of either high mannose-type, hybrid monoantennary-type or complex biantenary-type oligosacharide chains at Asn255 of the C-terminal extension. Cruzipain and its recombinant form without this extension (cruzain) were studied comparatively in this work. S2 to S2, subsite specificities of these enzymes were examined using four series of substrates derived from the internally quenched fluorescent peptide Abz-KLRFSKQ-EDDnp (Abz, ortho -aminobenzoic acid; EDDnp, N -(2,4-dinitrophenyl)-ethylenediamine). Large differences in the kinetic parameters were not observed between the enzymes; however, Km values were consistently lower for the hydrolysis of most of the substrates by cruzain. No difference in the pH,activity profile between the two enzymes was found, but in 1 m NaCl cruzipain presented a kcat value significantly higher than that of cruzain. The activation energy of denaturation for the enzymes did not differ significantly; however, a negative entropy value was observed for cruzipain denaturation whereas the value for cruzain was positive. We determined the individual rate constants (k1, substrate diffusion; k,1, substrate dissociation; k2, acylation; k3, deacylation) and the respective activation energies and entropies for hydrolysis of Abz-KLRFSKQ-EDDnp determining the temperature dependence of the Michaelis,Menten parameters kcat/Km and kcat as previously described [Ayala, Y.M. & Di Cera, E. (2000) Protein Sci.9, 1589,1593]. Differences between the two enzymes were clearly detected in the activation energies E1 and E,1, which are significantly higher for cruzipain. The corresponding ,S1 and ,S,1 were positive and significantly higher for cruzipain than for cruzain. These results indicate the presence of a larger energy barrier for cruzipain relating to substrate diffusion and dissociation, which could be related to the C-terminal extension and/or glycosylation state of cruzipain. [source] Lipopolysaccharide is a frequent and significant contaminant in microglia-activating factorsGLIA, Issue 1 2008Jonathan R. Weinstein Abstract Lipopolysaccharide (LPS/endotoxin) is a potent immunologic stimulant. Many commercial-grade reagents used in research are not screened for LPS contamination. LPS induces a wide spectrum of proinflammatory responses in microglia, the immune cells of the brain. Recent studies have demonstrated that a broad range of endogenous factors including plasma-derived proteins and bioactive phospholipids can also activate microglia. However, few of these studies have reported either the LPS levels found in the preparations used or the effect of LPS inhibitors such as polymyxin B (PMX) on factor-induced responses. Here, we used the Limulus amoebocyte lysate assay to screen a broad range of commercial- and pharmaceutical-grade proteins, peptides, lipids, and inhibitors commonly used in microglia research for contamination with LPS. We then characterized the ability of PMX to alter a representative set of factor-induced microglial activation parameters including surface antigen expression, metabolic activity/proliferation, and NO/cytokine/chemokine release in both the N9 microglial cell line and primary microglia. Significant levels of LPS contamination were detected in a number of commercial-grade plasma/serum- and nonplasma/serum-derived proteins, phospholipids, and synthetic peptide preparations, but not in pharmaceutical-grade recombinant proteins or pharmacological inhibitors. PMX had a significant inhibitory effect on the microglia-activating potential of a number of commercial-, but not pharmaceutical-grade, protein preparations. Novel PMX-resistant responses to ,2 -macroglobulin and albumin were incidentally observed. Our results indicate that LPS is a frequent and significant contaminant in commercial-grade preparations of previously reported microglia-activating factors. Careful attention to LPS levels and appropriate controls are necessary for future studies in the neuroinflammation field. © 2007 Wiley-Liss, Inc. [source] The silver(I)-catalyzed exchange of coordinated cyanide in hexacyanoferrate(II) by phenylhydrazine in aqueous mediumINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 8 2007R. M. Naik The [Ag]+ -catalyzed exchange of coordinated cyanide in [Fe(CN)6]4, by phenylhydrazine (PhNHNH2) has been studied spectrophotometrically at 488 nm by monitoring increase in the absorbance for the formation of cherry red colored complex [Fe(CN)5PhNHNH2]3,. The other reaction conditions were pH 2.80±,0.02, temperature = 30.0 ± 0.1°C, and ionic strength (I) = 0.02 M (KNO3). The reaction was followed as a function of pH, ionic strength, temperature, [Fe(CN)4,6], [PhNHNH2], [Ag+] by varying one variable at a time. The initial rates were evaluated for each variation using the plane mirror method. The initial rates evaluated as a function of [Fe(CN)4,6] clearly indicate that the initial rate increases with the increase in [Fe(CN)4,6] and finally reaches to a limiting value when [Fe(CN)4,6]/[AgNO3] , 1000. It indicates the formation of a strong adduct between [Fe(CN)6]4, and AgNO3 prior to the abstraction of CN,. The variation in initial rates with [PhNHNH2] also showed limiting values at [Fe(CN)4,6]/[PhNHNH2] , 8.30. The complex behavior due to pH and [Ag+] variations on the rate has been explained in detail. The composition of the final reaction product [Fe(CN)5PhNHNH2] formed during the course of reaction has been found to be 1:1 using the mole ratio method. The evaluated values of activation parameters for the catalyzed reaction are Ea = 53.85 kJ mol,1, , H,, = 51.33 kJ mol,1, and , S, = ,134.63 J K,1 mol,1, which suggest an interchange dissociative mechanism. A most plausible mechanistic scheme has been proposed based on the experimental observations. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 447,456, 2007 [source] Oxidative behavior and relative reactivities of some unsaturated compounds towards hexachloroiridate(IV) in perchloric acid mediumINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 7 2002Kalyan K. Sen Gupta The kinetics of the oxidation of styrene, cinnamic acid, and some of their substituted derivatives by hexachloroiridate(IV) in dimethyl formamide,water mixtures and in the presence of perchloric acid have been investigated. The reactions appear to proceed via the formation of an unstable intermediate 1:1 complex between iridium(IV) and the substrate, followed by the decomposition of the complex in the rate-determining step. Correlation with , yielded , values of ,4.0 and ,3.5 which suggests the formation of a cationic intermediate in the rate-determining step of the reaction. Subsequent cleavage of the carbon,carbon bond yielded the product aldehydes. Thermodynamic and activation parameters associated with the equilibrium and the rate-determining steps were also evaluated. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 411,417, 2002 [source] Preparation of novel ZSM-5 zeolite-filled chitosan membranes for pervaporation separation of dimethyl carbonate/methanol mixturesJOURNAL OF APPLIED POLYMER SCIENCE, Issue 3 2007Bingbing Liu Abstract Novel mixed matrix membranes were prepared by incorporating ZSM-5 zeolite into chitosan polymer for the pervaporative separation of dimethyl carbonate (DMC) from methanol. These membranes were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) to assess their morphology, intermolecular interactions, and crystallinity. Sorption studies indicated that the degree of swelling for zeolite-filled membranes increased with zeolite content in the membrane increasing and the separation selectivity of DMC/methanol was dominated by solubility selectivity rather than diffusivity selectivity. The characteristics of these membranes for separating DMC/methanol mixtures were investigated by varying zeolite content, feed composition, and operating temperature. The pervaporation separation index (PSI) showed that 5 wt % of ZSM-5 zeolite-filled membrane gave the optimum performance in the PV process. From the temperature-dependent permeation values, the Arrhenius activation parameters were estimated. The resulting lower activation energy values obtained for zeolite-filled membranes contribute to the framework of the zeolite. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007 [source] Polyethylene glycol and polyvinyl alcohol as corrosion inhibitors for aluminium in acidic mediumJOURNAL OF APPLIED POLYMER SCIENCE, Issue 6 2007S. A. Umoren Abstract The corrosion inhibition of aluminum in H2SO4 in the presence of polyethylene glycol (PEG) and polyvinyl alcohol (PVA) as inhibitors at 30,60 °C was studied using gravimetric, gasometric, and thermometric techniques. The inhibition efficiency (%I) increased with increase in concentration of the inhibitors. Increase in temperature increased the corrosion rate in the absence and presence of inhibitors but decreased the inhibition efficiency. Both PEG and PVA were found to obey Temkin adsorption isotherm at all concentrations and temperatures studied. Phenomenon of physical adsorption is proposed from the activation parameters obtained. Thermodynamic parameters reveal that the adsorption process is spontaneous. PEG was found to be a better inhibitor than PVA. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007 [source] Selective Blockade of Voltage-Gated Potassium Channels Reduces Inflammatory Bone Resorption in Experimental Periodontal Disease,,JOURNAL OF BONE AND MINERAL RESEARCH, Issue 1 2004Paloma Valverde Abstract The effects of the potassium channel (Kv1.3) blocker kaliotoxin on T-cell-mediated periodontal bone resorption were examined in rats. Systemic administration of kaliotoxin abrogated the bone resorption in conjunction with decreased RANKL mRNA expression by T-cells in gingival tissue. This study suggests a plausible therapeutic approach for inflammatory bone resorption by targeting Kv1.3. Introduction: Kv1.3 is a critical potassium channel to counterbalance calcium influx at T-cell receptor activation. It is not known if Kv1.3 also regulates RANKL expression by antigen-activated T-cells, and consequently affects in vivo bone resorption mediated by activated T-cells. Materials and Methods:Actinobacillus actinomycetemcomitans 29-kDa outer membrane protein-specific Th1-clone cells were used to evaluate the expression of Kv1.3 (using reverse transcriptase-polymerase chain reaction [RT-PCR] and Western blot analyses) and the effects of the potassium channel blocker kaliotoxin (0,100 nM) on T-cell activation parameters ([3H]thymidine incorporation assays and ELISA) and expression of RANKL and osteoprotegerin (OPG; flow cytometry, Western blot, and RT-PCR analyses). A rat periodontal disease model based on the adoptive transfer of activated 29-kDa outer membrane protein-specific Th1 clone cells was used to analyze the effects of kaliotoxin in T-cell-mediated alveolar bone resorption and RANKL and OPG mRNA expression by gingival T-cells. Stimulated 29-kDa outer membrane protein-specific Th1 clone cells were transferred intravenously on day 0 to all animals used in the study (n = 7 animals per group). Ten micrograms of kaliotoxin were injected subcutaneously twice per day on days 0, 1, 2, and 3, after adoptive transfer of the T-cells. The control group of rats was injected with saline as placebo on the same days as injections for the kaliotoxin-treated group. The MOCP-5 osteoclast precursor cell line was used in co-culture studies with fixed 29-kDa outer membrane protein-specific Th1-clone cells to measure T-cell-derived RANKL-mediated effects on osteoclastogenesis and resorption pit formation assays in vitro. Statistical significance was evaluated by Student's t -test. Results: Kaliotoxin decreased T-cell activation parameters of 29-kDa outer membrane protein-specific Th1 clone cells in vitro and in vivo. Most importantly, kaliotoxin administration resulted in an 84% decrease of the bone resorption induced in the saline-treated control group. T-cells recovered from the gingival tissue of kaliotoxin-treated rats displayed lower ratios of RANKL and OPG mRNA expression than those recovered from the control group. The ratio of RANKL and osteoprotegerin protein expression and induction of RANKL-dependent osteoclastogenesis by the activated T-cells were also markedly decreased after kaliotoxin treatments in vitro. Conclusion: The use of kaliotoxin or other means to block Kv1.3 may constitute a potential intervention therapy to prevent alveolar bone loss in periodontal disease. [source] NMR search for polymorphic phase transformations in chlorpropamide form-A at high pressuresJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 4 2009sicki Abstract The high-pressure effects on chlorpropamide (C10H13ClN2O3S) form-A have been studied by 1H NMR spectroscopy at high pressures up to 800 MPa in the temperature range 90,300 K. A study of the NMR second moment and spin-lattice relaxation time has been completed by a calculation of the steric hindrances for molecular reorientations and simulations of the second moment of the NMR line by the Monte,Carlo method, which enabled a precise description of molecular dynamics in the compound studied. Reorientations of the methyl group, oscillations and reorientations of the chlorophenyl ring and reorientations of the propyl group have been revealed and respective activation parameters extracted. No phase transformation of the compound form-A has been detected. © 2008 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 98:1426,1437, 2009 [source] Butanolysis of 2-methylbenzenediazonium ions: product distribution, rate constants of product formation, and activation parametersJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 5 2009M. José Pastoriza-Gallego Abstract We have determined the product distributions, the rate constants of product formation and substrate loss, and the activation parameters for the butanolysis of 2-methylbenzenediazonium, 2MBD, tetrafluoroborate in aqueous 1-Butanol (BuOH) solutions by combining UV,VIS spectroscopy, high performance liquid chromatography (HPLC), and a derivatization protocol that traps unreacted 2MBD as a stable azo dye. BuOH/H2O solutions are miscible over a narrow composition range, but in reverse micelles composed of sodium dodecyl sulfate, SDS, BuOH, and water, are miscible between 45,80%. Two major and two minor dediazoniation products are observed, 2-cresol, ArOH, 2-butyl-tolyl-ether, ArOBu, and small amounts of 2-chlorobenzene, ArCl (from HCl added to control solution acidity) and toluene, ArH (a reduction product). Product yields depend on experimental conditions, but quantitative conversion to products is achieved over the entire composition ranges investigated. The observed rate constants, kobs, obtained by monitoring 2MBD loss or by monitoring ArOH or ArOBu formation, are the same and they are only modestly affected by changes in the solution composition. The activation parameters obtained from the effect of temperature on kobs show that the enthalpy of activation is relatively high compared to those found in bimolecular reactions and the entropy of activation is small but positive. The results suggest that 2MBD is mainly sampling in the BuOH-H2O rich interfacial region of the reverse micelle and are consistent with 2MBD decomposing through a DN,+,AN mechanism, i.e., a rate limiting formation of an aryl cation that reacts immediately with nucleophiles. Copyright © 2008 John Wiley & Sons, Ltd. [source] Ionic and radical fragmentations of alkoxyhalocarbenes , a perspectiveJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 4 2009Robert A. Moss Abstract Fragmentations of secondary or tertiary alkoxyhalocarbenes in polar solvents generate carbocations as components of ion pairs. A variety of carbocations can be produced including acyclic, alicyclic, benzyl, bridgehead, cyclopropyl, cyclopropylcarbinyl, and norbornyl examples. Laser flash photolysis (LFP) studies provide kinetics and activation parameters for the carbene fragmentations, which are orders of magnitude faster, and require considerably reduced activation energies, compared to analogous solvolytic carbocation-generative processes. In some cases, the time required for solvent and anion equilibration of the ion pairs can be estimated. In nonpolar solvents, the gas phase, or cryogenic matrices, homolytic carbene fragmentation may, in certain cases, supplant heterolytic fragmentation. Copyright © 2008 John Wiley & Sons, Ltd. [source] Hydrolysis of 2-(p -nitrophenoxy)tetrahydropyran: solvent and ,-deuterium secondary kinetic isotope effects and relationships with the solvolysis of simple secondary alkyl arenesulfonates and the enzyme-catalyzed hydrolysis of glycosides,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 6-7 2004Imran A. Ahmad Abstract The effect of solvent composition in aqueous ethanol, trifluoroethanol and hexafluoropropan-2-ol on the rate constant and activation parameters for the uncatalysed hydrolysis of 2-(p -nitrophenoxy)tetrahydropyran (1) was investigated, and the m(YOTs) value is 0.60. This appreciable but less than maximal value is in accordance with an SN1 mechanism with rate-limiting ionization. The ,-deuterium secondary kinetic isotope effect (,-kie) for the uncatalysed hydrolysis of 1 is 1.17 in water (46°C), 1.15 in aqueous trifluoroethanol (50% mole fraction, 70.6°C) and 1.13 in aqueous ethanol (50% mole fraction, 70.6°C). These values correspond to about 1.19 at 25°C, which is characteristic of rate-limiting ionization in an SN1 reaction and appreciably higher than values for enzyme-catalysed glycolysis. The ,-kie is smaller under aqueous acidic conditions (1.07, 0.1,mol,dm,3 hydrochloric acid, 20.2°C) when 1 hydrolyses with acid catalysis. The previously reported ,-kie for the hydrolysis of 1 in buffered aqueous dioxan (1.063, 25°C) is now seen to correspond to acid-catalysed hydrolysis. These new results for 1 indicate that transition structures in enzyme-catalysed glycolyses with ,-kie values of less than about 1.15 at 25°C involve a lower degree of carbenium ion character than has hitherto been assumed. Copyright © 2004 John Wiley & Sons, Ltd. [source] Evidence for extended ,,n-participation in solvolysis of some benzyl chloridesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 12 2003Sandra Juri Abstract Chlorides 3 (1-aryl-1-chloro-4-methyl-7-methoxy-4-heptene) and 4 (1-aryl-1-chloro-4-methyl-4-hexene) with various phenyl substituents were prepared (Y=p -OCH3, p -CH3, H and m -Br) and the solvolysis rates were measured in 80% (v/v) aqueous ethanol. The rate constants of 3 correlate well with ,+, and the ,+ value obtained is ,1.45±0.15, whereas with 4 breakdown of the Hammett plot occurs, and the ,+ value without the p -anisyl group is ,2.55±0.20, indicating extended ,,n-participation in 3 and simple ,-participation in 4. The drastically smaller activation parameters obtained with 3 than with 4 are consistent with the proposed mechanism in which the high degree of order required in the transition state (large negative ,S,) is overcompensated by a small ,H,. Copyright © 2003 John Wiley & Sons, Ltd. [source] pH-independent hydrolysis of 4-nitrophenyl 2,2-dichloropropionate in aqueous micellar solutions: relative contributions of hydrophobic and electrostatic interactionsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8 2001Omar A. EI Seoud Abstract The pH-independent hydrolysis of 4-nitrophenyl 2,2-dichloropropionate (NPDCP) in the presence of aqueous micelles of sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, alkyltrimethylammonium chlorides, alkyldimethylbenzylammonium chlorides (alkyl,=,cetyl and dodecyl) and polyoxyethylene(9) nonylphenyl ether was studied spectrophotometrically. The observed rate constants, kobs, decrease in the following order: bulk water >cationic micelles >anionic micelles >non-ionic micelles. This order is different from that observed for pH-independent hydrolysis of 4-nitrophenyl chloroformate (NPCF), whose reaction is faster in cationic micelles than in bulk water. A proton NMR study on solubilization of a model ester, 4-nitrophenyl 2-chloropropionate, showed that the methylene groups in the middle of the surfactant hydrophobic chain are most affected by the solubilizate. Lower polarity and high ionic strength of interfacial water decrease the rates of hydrolysis of both NPCF and NPDCP, but the fraction of the former ester that diffuses to the interface is probably higher than that of the latter. Therefore, whereas the (negatively charged) transition state of NPCF is stabilized by cationic interfaces and destabilized by anionic interfaces, that of NPDCP is negligibly affected by ionic interfaces, which explains the observed rate retardation by all ionic micelles. Calculated activation parameters corroborate our explanation. Copyright © 2001 John Wiley & Sons, Ltd. [source] | |||||||||||||||||||||||||||||||