High Activation Enthalpy Research Articles

Energetics of base pair opening in a DNA dodecamer containing an A3T3 tract.

Nuclear magnetic resonance spectroscopy has been used to characterize the kinetics and energetics of opening of base pairs in the DNA dodecamer [d(CGCAAATTTGCG)]2. The dodecamer contains an A3T3 tract that induces intrinsic curvature of the helix axis. Previous studies from this and other laboratories have shown that the kinetics of base pair opening in AnTn tracts is unique: the opening rates of the A.T base pairs in the interior of the tract are much lower than that of the A.T base pair at the 5'-end of the tract. In the present work, we have investigated the energetics of the pathways for opening of the A.T base pairs in the A3T3 tract. The energetic parameters of the activated state(s) are obtained from the temperature dependence of the opening rate constants. The lower opening rates for the A.T base pairs situated in the interior of the tract are shown to originate from higher activation enthalpies which are compensated, in part, by increases in the activation entropies. We have also obtained an energetic characterization of the open state(s) of the A.T base pairs in the dodecamer by measuring the equilibrium constants for base pair opening and their temperature dependence. The results suggest that the transitions from closed to open state(s) in the A.T base pairs of the A3T3 tract are energetically similar.

High temperature damping in dispersion-strengthened aluminium alloys

Dispersion-strengthened alloys are promising candidates for application at high homologous temperatures. Mechanical loss measurements were carried out on a dispersion-strengthened Al alloy with about 10 vol.% of incoherent Al 2O 3 and Al 4C 3 particles. We observe a loss maximum at about 590 K for a frequency f = 1 Hz which exhibits an exceptionally high activation enthalpy ( H = 2.2 eV) compared with that for self-diffusion ( H D = 1.48 eV). The peak height increases with oscillation frequency. Additionally, transmission electron microscopy studies were carried out for a correlation of the loss spectrum with the microstructure. To explain the experimental results, a relaxation model is proposed which considers jog nucleation on climbing edge dislocations as the time-limiting mechanism.

Thermal activation of a group II intron ribozyme reveals multiple conformational states.

Conformational changes often accompany biological catalysis. Group II introns promote a variety of reactions in vitro that show an unusually sharp temperature dependence. This suggests that the chemical steps are accompanied by the conversion of a folded-but-inactive form to a differently folded active state. We report here the kinetic analysis of 5'-splice-junction hydrolysis (SJH) by E1:12345, a transcript containing the 5'-exon plus the first five of six intron secondary structure domains. The pseudo-first-order SJH reaction shows (1) activation by added KCl to 1.5 M; (2) cooperative activation by added MgCl2, nHill = 4.1-4.3, and [MgCl2]vmax/2 approximately 0.040 M; and (3) a rather high apparent activation energy, Ea approximately 50 kcal mol-l. In contrast, the 5'-terminal phosphodiester bond of a domain 5 transcript (GGD5) was hydrolyzed with Ea approximately 30 kcal mol-1 under SJH conditions; the 5'-GG leader dinucleotide presumably lacks secondary structure constraints. The effect of adding the chaotropic salt tetraethylammonium chloride (TEA) was also investigated. TEA reduced the melting temperatures of GGD5 and E1:12345. TEA also shifted the profile of rate versus temperature for SJH by E1:12345 toward lower temperatures without affecting the maximum rate. TEA had little effect on the rate of hydrolysis of the 5'-phosphodiester bond of GGD5. The high apparent activation enthalpy and entropy for SJH along with the effect of TEA on these parameters imply that conversion of an inactive form of E1:12345 to an active conformation accompanies enhanced occupation of the transition state as the temperature is raised to that for maximum SJH. Analytical modeling indicates that either a two-state model (open and disordered, with open being active) or a three-state model (compact, open, and disordered) could account for the temperature dependence of kSJH. However, the three-state model is clearly preferable, since it does not require that the activation parameters for phosphodiester bond hydrolysis exhibit exceptional values or that the rates for the chemical steps of SJH respond directly to TEA addition.

Crucial role for DNA supercoiling in Mu transposition: a kinetic study.

DNA supercoiling plays an indispensable role in an early step of bacteriophage Mu transposition. This step involves formation of a nucleoprotein complex in which the Mu ends synapse and undergo two concerted single-strand cleavages. We describe a kinetic analysis of the role of supercoiling in the Mu-end synapsis reaction as measured by the cleavage assay. We observe a dependence of the reaction rate on superhelical density as well as on the length of Mu donor plasmid DNA. The reaction has a high activation enthalpy (approximately 67 kcal/mol). These results imply that the free energy of supercoiling is used directly to lower the activation barrier of the rate-limiting step of the reaction. Only the free energy of supercoiling associated with DNA outside the Mu ends appears to be utilized, implying that the Mu ends come together before the supercoiling energy is used. Our results suggest an essential function for the bacterial sequences attached to the ends of Mu virion DNA.

Radiation Enhanced Diffusion in Mgo

AbstractMeasurements of marker spreading following 2.0 MeV Kr+irradiation at 25 to 1300°C have been performed on MgO samples containing O18, Ca and Zn buried tracer atoms. Ion beam mixing at room temperature on both sublattices was approximately 2.0 to 3.0 A5/eV. From 600 to 1000°C, the apparent activation enthalpy for diffusion on the anion sublattice (O18) was 0.35 eV and the diffusion coefficient was linear in the irradiation flux. From 1150 to 1300°C the measured activation enthalpy was 4.1 eV and the diffusion coefficient was proportional to the square root of flux. The measured activation enthalpy on the cation sublattice was roughly 0.30 eV up to 700°C for both Ca or Zn doped samples. Measurable extrinsic thermal diffusion from vacancies present for trivalent impurity charge compensation occurred above this temperature, complicating the analysis at higher temperatures. The observed kinetics in the lower temperature range are most likely controlled by interstitial loop formation. In the higher temperature range, vacancy traps with a binding energy of approximately 2 eV could account for the high activation enthalpy.

Electrochemistry of end-capped oligothienyls. New insights into the polymerization mechanism and the charge storage, conduction and capacitive properties of polythiophene

The kinetics of anodic coupling to dimers of thiophene oligomers ( n = 3–5), methyl protected at one α-terminal position, is second order in oligomer concentration and evidences high activation enthalpies and negative activation entropies. Activation free energies are linearly related to the inverse of the oligomer length n (the dimerization rate decreases as n is increased). Thin films of methyl end-capped thiophene oligomers ( n = 4, 6, 8 and 10) display reversible oxidations from a single one-electron step (tetramer) to a single two-electron step (octamer and decamer) through two separate one-electron steps (hexamer). ESR indicates strong magnetic dimerization for the one-electron-oxidized hexamer. The close resemblance of the electrochemical and ESR behaviour of the hexamer with that of polythiophene suggests that oxidation of the latter occurs via hexameric spin-dimerized polarons. The conductive and capacitive properties of the end-capped oligomers ( n = 6, 8 and 10) were investigated by in situ conductivity and chronopotentiometry. While conductivity of octamer and decamer is displayed at the two-electron (bipolaron) state, the hexamer, insulating at this state, is conducting at the mixed-valence polaron-bipolaron state; capacitive responses are evidenced at the bipolaron state for the octamer and decamer only. The difference of conductive and capacitive behaviour between the hexamer and the higher oligomers is explained by charge localization in hexameric segments.

Kinetics and mechanism of the reaction of bis-μ-diethylphosphidobis(tetracarbonylmanganese) with tri- n-butylphosphine

The dinuclear phosphido-bridged manganese complex [(OC) 4Mn(μ-PEt 2) 2Mn(CO) 4] undergoes thermal CO substitution by P nBu 3 in decalin to give [(OC) 4Mn(μ-PEt 2) 2Mn(CO) 3(P nBu 3)] and [( nBu 3P)(OC) 3Mn(μ-PEt 2) 2Mn(CO) 3(P nBu 3)]. A kinetic study of the first CO/P nBu 3 exchange indicates that all substitutions for the forward and reverse reactions occur via a dissociative mechanism. The rate of CO dissociation is rather low ( k = 6.27 × 10 −5 s −1 at 130°C), mainly as a consequence of a high activation enthalpy (Δ H* = 165 kJ mol −1). The discriminating ability of the resulting coordinatively unsaturated intermediate [(OC) 4 Mn(μ-PEt 2) 2Mn(CO) 3] is only slightly in favour of attack of the more basic P nBu 3 compared to CO, as indicated by the values of the competition rate ratio [ k(CO)/ k(P nBu 3)=0.36-0.47]. This behaviour is compared with that of the isostructural chromium complex and discussed in the light of the electronic and structural features related to the metal-metal distance in the two complexes (MnMn = 3.675 Å, CrCr = 2.905 Å, for the μ-PMe 2 derivatives).

Effect of copper and zirconium upon the high temperature mechanical properties of AlMgZn alloys

High temperature mechanical properties of AlMgZn alloys are investigated by impression creep tests and the strain rate sensitivities and activation energies obtained are presented. It is found that the steady-state impression creep rate is strongly influenced by Cu and Zr addition. The simultaneous addition of Cu and Zr results in a fine-grained microstructure due to which the material exhibits superplastic properties at high temperatures. Together with the superplastic behaviour the occurrence of a threshold stress is also proved. On the contrary, if the deformation is not superplastic, threshold stresses are not observable. It is shown that the relatively high apparent activation enthalpy obtained for the superplastic region of deformation is due to the strong temperature dependence of the threshold stress, and taking into account this fact a true activation enthalpy corresponding to that of the bulk diffusion can be assessed.

Mechanisms underlying the low and high enthalphy death induced by short-chain monocarboxylic acids and ethanol in Saccharomyces cerevisiae

In glucose-grown cells of Saccharomyces cerevisiae IGC 4072, propionic, butyric and pentanoic acids induced death at high temperatures with the same high enthalpy of activation as thermal in the absence of acids (HED). In addition, a second type of death with a much lower enthalpy of activation (LED) was observed at intermediate and lower temperatures. While the underlying molecular mechanism of HED appeared to be similar to that of thermal death, the effects of the acids as well as of ethanol on LED could be considered a consequence of acidification of the cytoplasm. Indeed, the addition of pentanoic, propionic, acetic and butyric acids as well as of ethanol at increasing concentrations, resulted in an enhancement of the specific death rate correlated with a drop in internal pH.

Mechanical properties and dislocation structure in L12long-range ordered Ni3Fe

Abstract Although Ni3Fe belongs to the group of L12 long-range ordered intermetallics the critical resolved shear stress (CRSS) does not depend strongly on the deformation temperature. Dipoles and bundles of dipoles near edge orientations are characteristic features in the dislocation structure. Few screw superlattice dislocations are observed. There are striking similarities to the dislocation structure of pure f.c.c. metals or dilute solid solutions after low and medium plastic deformation. Cross-slip of screw superlattice dislocations onto cube (010) planes plays a minor role and is not regarded to be the rate-controlling factor for dislocation motion. The reason for the absence of cube cross-slip is the high activation enthalpy necessary to constrict widely dissociated Shockley partials to form unit screw superlattice partials.

Zinc-activated alcohols in ternary complexes of liver alcohol dehydrogenase.

Activation parameters for each reaction step in the kinetic mechanism of liver alcohol dehydrogenase have been measured for the oxidation of ethanol and the reduction of acetaldehyde. In the oxidation process, the highest enthalpy of activation, 9.7 kcal/mol, occurs for the turnover of the liver alcohol dehydrogenase-NAD(+)-ethanol ternary complex. To investigate if this enthalpy requirement represents a change in the ionization state of ethanol bound in the ternary complex, inhibition of ethanol oxidation was determined using the following series of small, electronegative alcohols with pKa values ranging from 12.37 to 15.5: 2,2,2-trifluoroethanol, 2,2,2-trichloroethanol, 2,2,2-tribromoethanol, 2,2-dichloroethanol, 2,2-difluoroethanol, propargyl alcohol, 3-hydroxypropionitrile, 2-chloroethanol, 2-iodoethanol, 2-methoxyethanol, ethylene glycol, and methanol. The observed inhibition patterns were analyzed according to several kinetic inhibition models; in each case, the best fit model was used to determine the substrate competitive inhibition constant. A plot of the logarithm of these inhibition constants is shown to be dependent on the pKa values of the inhibiting alcohols with a slope approaching -1, indicating that inhibition is controlled by a proton loss from the alcohol. The observed competitive inhibition behavior, coupled with crystallographic studies depicting a direct ligation of an alcohol oxygen to the catalytic zinc ion, indicates that inhibition is controlled by the formation of a zinc-bound alkoxide. Because the inhibiting alcohols are structurally homologous to ethanol, a relationship between the inhibition constant and the inhibiting alcohol's pKa can be derived to show that the pKa of an alcohol bound in a ternary complex is also dependent on its pKa as a free alcohol. Ternary complex pKa values have been determined for ethanol and the inhibiting alcohols.

Glass-liquid transition of water and ethylene glycol solution in poly(2-hydroxyethyl methacrylate) hydrogel

Water in PHEMA hydrogels unergoes a glass-to-liquid transition in two steps: the first at ∼132 K, which is caused by the nonfreezing water fraction, and the second at ∼162 K, which is caused by the freezable water fraction. The H-bonded structure of the latter water fraction in the hydrogel has a narrower distribution of relaxation times and/or higher activation enthalpy than in pure glassy water. The glass transition temperature of a 50 wt% ethylene glycol-water solution in the PHEMA hydrogel is the same as in its pure state, but the distribution of structural relaxation times is broader in the former than in the latter

Kinetics and mechanism of Ru(III)-edta catalyzed oxidation of cis-cyclootene by molecular oxygen

The homogeneous oxidation of cis-cyclooctene with molecular oxygen catalyzed by potassium chloro(ethylenediaminetetraacetato)ruthenium(III), K[Ru(H-EDTA)Cl]·2H 2O, was investigated in the temperature range 310–335 K,μ = 0.1 M KC1 in 1:1 water-dioxane mixture. The rate of oxidation of cis-cyclooctene was found to be first order with respect to catalyst and cyclooctene concentrations, and one-half order in molecular oxygen concentration. Based on the kinetic data, a μ-peroxoruthenium(IV)cyclo-octene complex is suggested as an active intermediate for the formation of cyclooctene oxide as a major product. A small quantity of 2-cycloocten-1-ol was also obtained in the reaction. The formation of 2-cycloocten-1-ol may be explained on the basis of the formation of a peroxometallocycle involving the catalyst, cyclooctene and dioxygen, which gives rise to an ene insertion product. A free radical route for the formation of 2-cycloocten-1-ol can be ruled out on the basis of the absence of any hydroperoxide intermediate. The activation parameters for the oxidation step corresponding to the rate constant were also calculated. The high enthalpy of activation Δ H ‡ of 26.8 kcal mol −1 supports the O-O bond cleavage in either mechanism. The products were analysed by GLC, IR and NMR spectroscopies and by derivatization of the products.

The immune response towards beta-adrenergic ligands and their receptors—VII. Equilibrium and kinetic binding studies of l-alprenolol to a monoclonal anti-alprenolol antibody

Binding of the catecholamine β-adrenergic antagonist, l-alprenolol, by the IgG1 anti-alprenolol monoclonal antibody 37A4 was examined using the radioligand 3H-dihydroalprenolol as an extrinsic signal and the increase in antibody fluorescence upon l-alprenolol binding as intrinsic signal. Equilibrium binding studies based on both signals indicated that the binding process was exothermic with a positive entropy change. The difference in the affinity constants obtained by radioligand binding studies and by fluorescence analysis could be ascribed to the higher affinity of the hydrogenated tritiated l-dihydroalprenolol compared to the unsaturated l-alprenolol. The association rate constants determined by both signals were 10 4–10 5/ M/sec and showed a high activation enthalpy (8–10 kcal/mol), thus excluding a diffusion controlled reaction. At low temp (7°C), the fluorescence stopped-flow studies showed non-linear pseudo first order kinetics, indicating the existence of a fast pre-equilibrium of low affinity, followed by a conformational change leading to the tight binding of the ligand. The dissociation rate constants determined using both signals were very similar. Thus, the differences in affinity between the hydrogenated and non-saturated l-alprenolol could be ascribed to the association rate constants. Affinity constants and thermodynamic parameters calculated from the kinetic data were in close agreement with those determined by equilibrium binding. The mechanisms of ligand binding are discussed in terms of the interactions of idiotypes and anti-idiotypes in the anti-catecholamine immune response.

Crystal structure and electrical conductivity of the “one-dimensional solid electrolyte N, N-dimethylmorpholinium pentaiodotetraargentate

The new solid electrolyte N, N-dimethylmorpholinium pentaiodotetraargentate, [(CH 3)N(CH 2) 2-(CH 2) 2O]Ag 4I 5 (DMMAg 4I 5), has a crystal structure which allows the Ag + ions to move effectively in only one dimension. The channels for this motion result from the sharing of opposite faces of icosahedra, having iodides at their centers, forming infinite chains of icosahedra along thea axis. The joining of the icosahedra forms three more face-sharing tetrahedra at each junction, or a total of 23 tetrahedra per eight Ag + ions, which are distributed non-uniformly over the tetrahedral sites. The limited number of pathways for the Ag + ion diffusion leads to a low average conductivity, 1 × 10 −4 Ω −1 cm −1 and a high activation enthalpy, 0.35 eV. Crystals of DMMAg 4I 5 belong to space groupP2 12 12 1 (D 4 2) witha = 13.167(4), b = 23.437(3), c = 12.758(3)A˚; the unit cell contains eight formula units. The structure of dimethylmorpholinium iodide (DMMI) confirms the chair model for the DMM + ion. Crystals of DMMI belong to space groupP2 1m(C 2 2h), witha = 8.861(4), b = 8.020(2), c = 6.685(2)A˚, β = 101.07(3)°; each unit cell contains two formula units.

Kinetics of the reaction between testosterone and antitestosterone antiserum

In order to characterize from a kinetic viewpoint the antibody population mainly involved in the binding of testosterone by its homologous antiserum, the kinetics of the association reaction between [1,2,6,7- 3H]-testosterone and rabbit antiserum anti-testosterone-3-(O-carboxymethyl) oxime-bovine serum albumin (Ab R2603-1) was followed at pH 7.4 and at constant ionic strength, at temperatures ranging from 2°C to 37°C and at concentration near to work conditions for testosterone radioimmunoassay; dextran coated charcoal suspension was used for the bound/free separation. In the examined concentration range, the observed kinetics trends can be explained by assuming the existence of two classes of antibody binding sites, Ab 1 and Ab 2. The kinetics of the dissociation reaction of the testosterone-antibody complex was also followed after the addition of a large excess of unlabeled testosterone. At 22.0°C, association and dissociation rate constants are 2.1·10 7 s −1M −1 and 3.7·10 −3 s −1, respectively, for the Ab 1 class of antibody binding sites, and 3.6·10 6 s −1M −1 and 7.0·10 −4 s −1 for the Ab 2 class. Equilibrium constants obtained from kinetic data were very similar for both classes of antibody binding sites and in good agreement with the equilibrium values obtained from linear Scatchard plot. The order of magnitude of the second order rate constants and the high activation enthalpy for the forward and reverse reaction suggest a mechanism more complex than a simple second order.

Solid-state structure and reactivity in solution: V. Ring-closure in the (η 1-2,2,8,8-tetramethyl-3,7-dithianonane)tetracarbonyltungsten(0) intermediate

Pulsed laser flash photolysis of (η 2-DTN)W(CO) 4 (I, DTN = 2,2,8,8-tetramethyl-3,7-dithianonane) in 1,2-dichloroethane induces ring-opening to afford the solvated [(η 1-DTN)W(CO) 4] intermediate II. Rates of unimolecular ring-closure for this intermediate to afford I and of bimolecular combination of I with triisopropyl phosphite (L) have been determined, as have activation parameters for the ring-closure pathway. The relatively high enthalpy of activation (12.0(6) kcal/mol) suggests distortion in I induced upon chelate ring-closure. Rates of interaction of L at an “open” coordination site (one produced via CO loss from an octahedral (chelate)M(CO) 4 complex) are some two orders of magnitude faster than are those for a “shielded” site produced through ring-opening, suggestive of extensive steric interaction between the bidentate ligand and L for the combination process in the latter.

Anation of the cis-diaquabis(oxalato)chromate(III) complex ion by oxalate species

in the absence of catalysis. In the presence of the nitrate ion, direct chelation of the binoxalato-0 ligand of trans-Cr(C204)2(0COC02H)(H20)2- (k3l’) competes favorably with aquation (kZl’) to fully activate this anation pathway. At 50.0 OC and an ionic strength of 1.20 M, kl = (4.02 f 0.06) X lo-) s-I, kll/kz = 0.50 i 0.04 M-l, kll‘/k2 = 0.26 f 0.04 M-I, k12/k2 = 1.56 i 0.10 M-I, and k3l’/k21’ = 3.0 * 0.5 M-I. The activation parameters of kl were found to be AH* = 18.0 EL: 0.2 kcal mol-l and AS* = -14.0 f 0.5 cal mol-l K-I at an ionic strength of 1.20 M. The reaction mechanism has been extended to include data previously reported near pH 1. The combined data show that nitrate-catalyzed anation proceeding through trans-Cr(C204)2(0COC02H)(H20)2- is a process of high activation enthalpy and positive activation entropy. In contrast, substitution proceeding through cis intermediates has a low activation enthalpy and a negative activation entropy.

Desaktivierungsverhalten von Arenen und Heteroarenen. XXXIII. Zur statischen Löschung der Fluoreszenz von Azinium‐Anthracenen in Lösungsmitteln mit n‐Donatoreigenschaften

Deactivation Behaviour of Arenes and Heteroarenes. XXXIII. Static Fluorescence Quenching of Azinium‐Anthracenes by Solvents of n‐Donor PropertiesThe fluorescence of azinium‐anthracenes is quenched by solvents of n‐donor properties. A model of static quenching by electron transfer is developed analogous to the dynamic process. The rates of quenching correlate with the donor properties of the solvent (ionization potential) and the acceptor properties of the fluorophor (reduction potential). In the same way as in the case of dynamic quenching this static process is characterized by a high free enthalpy of activation. The existence of a thermally induced step in the process of the electron transfer affects the temperature dependence of the fluorescence quantum yields. The analysis of this temperature dependence on the basis of an Arrhenius equation shows a linear relationship.

Hyperfine Interaction Investigations of Helium Trapping in Metals

ABSTRACTHelium decorated vacancies may be trapped to a high degree and with a high activation enthalpy at impurities in metals. This effect is studied by means of the hyperfine interaction of the radio-active impurities 111In → 111Cd, 119Sb → 119Sn and 129mTe → 129I, implanted into a number of f.c.c. metals and into iron and cobalt. Large changes of the magnetic hyperfine field are caused at 119Sn impurities in Ni, Co and Fe and hiqh values of the electric quadrupole interaction strength are found at 111Cd impurities in Al, Cu, Ag, Ni and Pt when helium decorated vacancies are trapped at the impurities. These effects provide a new method for studying the trapping and desorption mechanism of helium in metals.