Apparent Activation Volume Research Articles

Activation volumes of hydrolyses of maltose and maltotriose over an immobilized glucoamylase catalyst

Abstract Initial rates of hydrolysis of maltose and maltotriose over an immobilized glucoamylase have been measured up to 127 MPa at 25±0.1°C. The observed rates have been analyzed showing the reaction pathways of both hydrolyses to be E+S⇌ES∗⇌ES7ast;E+P, where E, S, P, ES∗, and ES denote the enzyme, the substrate, the product, a substrate-subsite complex, and a substrate-active site complex, respectively. The apparent maximum rate rmand the apparent Michaelis constant Km as well as their respective pressure dependences in terms of the apparent activation volume Δ Vapp # and the apparent volume of reaction Δ Vapp have been evaluated. Small absolute values of Δ Vapp num; and Δ Vapp for both reactions have been discussed on the basis of the reaction mechanism.

The Strain Rate Sensitivity of Ni3(Al,Ta) Single Crystals.

ABSTRACTIn order to obtain more complete information about the strain rate sensitivity of the flow stress of L12 alloys, Ni3Al,Ta) single crystals have been deformed in compression, over a range of temperatures (293–1273K), at two different strain rates and in stress relaxation experiments.During the stress relaxation tests, at almost all the temperatures (except T~470K), a logarithmic decrease of the stress as a function of time has been observed. This is in fair agreement with the classical frame work of the thermal activation theory of dislocation glide. Thus, the corresponding apparent activation volumes have been determined and the nonmonotonic variation of this activation parameter with the temperature indicates that it is necessary to consider more than one plastic deformation process. These new results are discussed in terms of the previously published models which account for the plastic behaviours of the L12 compounds.

Friction stress for carbon composite and carbon yarn during high temperature creep

Friction stress σ 0 was measured on specially prepared, uniaxial. carbon yarn and carbon-carbon composite specimens up to a temperature of 2310°C. Relations between the friction stress, applied stress and other parameters are derived. The amount of work needed to activate the apparent activation volume V 0 is V 0( σ − σ 0) = 4 k Tn'. The friction stress and the applied stress relation is σ 0 = [ (n − n ') n ]σ , and the friction stress and the test temperature relations can be expressed as σ 0 ∝ exp ( l T ). These relations are consistent with test results.

Apparent activation volume for creep of copper and alpha brass at intermediate temperatures

Experimental measurements of the apparent activation volume for creep,V*, of Cu and Cu-30% Zn conducted at intermediate temperatures showed two types of strain dependencies. At the lower temperatures and higher stresses,V* decreased with increasing creep strain, e, while at higher temperatures and lower stresses,V* was essentially independent of strain. The low temperature-high stress behaviour for Cu and Cu-30% Zn was found to be consistent with the dominance of a dislocation intersection mechanism. The high temperature-low stress data for the pure metals suggest that the rate-controlling process involves the nonconservative motion of jogs on screw dislocations. For the latter conditions, an additional contribution from solute drag-limited dislocation glide also appears to be important in governing the creep behaviour of the alloy.

Creep behavior of copper at intermediate temperatures—I. Mechanical characteristics

Creep experiments were conducted on polycrystalline copper in the intermediate temperature range from 623 to 973 K, corresponding to 0.46−0.72 T m where T mm is the absolute melting temperature. The stress exponent for creep, n, is ∼ 6.6 at the lowest testing temperature but decreases to ∼ 4.2 at temperatures above ∼ 723 K. The activation energy for creep, Q c , decreases linearly with increasing normalized stress from ∼ 180 kJ mol −1 at σ/ G = 3 × 10 −4 to ∼ 135 kJ mol −1 at σ/ G = 2 × 10 −3, where σ is the applied stress and G is the shear modulus. The apparent activation volume for creep, V∗, decreases with increasing strain at the lower temperatures and higher stresses but V∗ is essentially independent of strain and of the order of 1600 b 3 at the highest testing temperature of 973 K, where b is the Burgers vector. An analysis of the experimental data shows that the creep behavior with n > 4.2 lies in the region of exponential creep above the power-law breakdown. Careful inspection of the data reveals no support for the concept of a region of low temperature climb controlled by diffusion along the dislocation cores.

Creep of carbon yarn and a carbon-carbon composite at high temperatures and high stresses

Creep experiments were performed on specially prepared, uniaxial, carbon yarn and carbon-carbon composite specimens. Temperatures ranged up to 2500°C and stress levels approached 850 MPa. For steady-state creep the apparent activation energy was determined to be 1082 kJ/mol while the stress exponent had a value of 7.5 that was independent of the loading history. Values of the preexponential constant and the apparent activation volume are provided as well as a discussion on how the well-bonded matrix affects carbon yarn creep.

Volume Profile of the Reversible Dimerization Reaction of 2‐Methyl‐2‐nitrosopropane: Validity of the Hard Sphere Fluid Model and the Transition State Theory

AbstractThe volume profile of the reversible dimerization reaction of 2‐methyl‐2‐nitrosopropane in carbon tetrachloride was determined at 25.0°C on the basis of the study of the pressure effect and the measurement of the solution density at 0.1 MPa. For the dissociation process, the configurational reaction volume was +30.3 ± 0.8 cm3 mol−1 and the apparent activation volume was +7.7 ± 0.3 cm3 mol−1 at 25.0°C and 61.3 MPa. The configurational reaction volume at 0.1 MPa was estimated as +36 cm3 mol−1 from the high pressure study in agreement with the result of the density measurement. The experimental values of the reaction and activation volumes were successfully reproduced by using the hard sphere fluid model. The agreement of experiment and theory indicates that the hard sphere fluid model is valid for describing the volume change for the molecular reaction, and implies that the transition state theory (TST) is valid for interpreting the pressure dependence of the rate constant. A simple illustration of the validity of TST is presented; the energy relaxation process plays a key role in determining the validity.

Temperature, pressure and cholesterol effects on bilayer fluidity; a comparison of pyrene excimer/monomer ratios with the steady-state fluorescence polarization of diphenylhexatriene in liposomes and microsomes

Pyrene excimer/monomer ( E/M) ratios have been compared with the steady-state fluorescence polarization ( P) of diphenylhexatriene (DPH) in multilamellar liposomes of dilaurylphosphatidylcholine and rat liver microsomes. The purpose was to use the well-understood properties of DPH to reveal the nature of bilayer fluidity which pyrene manifests as an E/M ratio. Reducing the temperature (from 37°C to 8°C), increasing the hydrostatic pressure (from 0.1 to 70 MPa), and, in liposomes, cholesterol enrichment (up to 0.30 mole fraction) separately decreased the E/M ratios and increased P. The pyrene membrane/buffer partition coefficient was affected by temperature but not by pressure, and in the case of cholesterol enrichment, it was assumed to be unaffected. Plots of P as a function of the E/M ratio showed the two to be closely correlated ( r = 0.99) in liposomes and 0.96 in microsomes), independent of the treatment used to reduce fluidity. The apparent activation volume and enthalpy for excimer formation was calculated and compared with published data. Pyrene E/M ratios probably reflect the intermolecular volume (fluidity) of the outer region of the bilayer, which is reduced by a decrease in temperature and an increase in pressure and cholesterol. DPH reports the bilayer interior, which is similarly ordered by the experimental treatments. The regional distinction between the two probes, however, accounts for the divergence of E/M ratios and P, which has been reported in membranes enriched with fluidizing fatty acids.

Single acetylcholine receptor channel currents recorded at high hydrostatic pressures.

A technique for performing patch-clamp experiments under high hydrostatic (oil) pressure is described. The method allows the transfer of whole cell or membrane patches in a recording configuration into a pressure vessel, where pressure can be increased up to 60 MPa (approximately equal to 600 bar). We have studied in this way the pressure dependence of single acetylcholine receptor channels in excised "outside-out" membrane patches from cultured rat muscle cells. In the range of 0.1 to 60 MPa the open channel conductance in 140 mM NaCl solutions did not vary by more than 2%, which implies that the translocation of sodium ions through the channel pore does not involve steps with significant activation volumes. At high acetylcholine concentrations (20 microM) bursts of single-channel activity allowed measurements of the mean open and mean closed times of the channel. Pressurization to 40 MPa increased both mean open and mean closed times giving apparent activation volumes of about 59 and 139 A3, respectively. This implies a net volume increase of 80 A3, associated with the transition from the agonist-free state to the open state of the channel, which may be partially associated with the agonist-binding step. All the observed pressure effects were reversible. The activation volumes for the gating of acetylcholine receptor channels are comparable to those of sodium and potassium channels in the squid giant axon, suggesting that there is some basic common mechanism in the operation of ion-channel proteins.

Effects of hydrostatic pressure on lipid bilayer membranes. I. Influence on membrane thickness and activation volumes of lipophilic ion transport

Measurements of membrane capacitance, Cm, were performed on lipid bilayers of different lipidic composition (diphytanoyl phosphatidylcholine PPhPC, dioleoyl phosphatidylcholine DOPE, glycerylmonooleate GMO) and containing n-decane as solvent. In the same membranes, the absorption of the lipophilic ions dipicrylamine (DPA-) and tetraphenylborate (TPhB-), and the kinetics of their translocation between the two membrane faces have been studied. The data were obtained from charge pulse relaxation measurements. Upon increasing pressure the specific capacity Cm increased in a fully reversible and reproducible way reflecting a thinning of the membrane that is attributed to extrusion of n-decane from the black membrane area. High pressure decreased the rate constant, ki, for lipophilic ion translocation. After correcting for changes in the height of the energy barrier for translocation due to membrane thinning the pressure dependence of ki yields an apparent activation volume for translocation of approximately 14 cm3/mol both for DPA- and TPhB-. Changes in lipophilic ion absorption following a step of pressure developed with a rather slow time course due to diffusion limitations in solution. The stationary concentration of membrane absorbed lipophilic ions increased with pressure according to an apparent volume of absorption of about -10 cm3/mol. The relevance of the results for the interpretation of the effects of pressure on nerve membrane physiology is discussed.

Kinetics of pressure-induced inactivation of bovine liver glutamate dehydrogenase

Kinetics of pressure-induced denaturation of bovine liver glutamate dehydrogenase (EC 1.4.1.3) were investigated in the pressure range 1.8–2.8 kbar by observing the residual activity after the pressure-release and the scattered light intensity during the incubation at high pressure. The residual activity decreased exponentially with the incubation time, whereas the scattered light intensity showed a bimodal profile indicating parallel aggregation and dissociation reactions. The latter suggested that two kinds of aggregates were formed during the incubation under pressure. The observed first-order rate constant for the inactivation, k obs ; showed a minimum around 30°C. These experimental results were interpreted in terms of the following reaction scheme; ▪ where N represents the enzyme entity with native structure, D 1 the partially denatured intermediate, D 2 the irreversibly denatured state, and A 1 and A 2 the two kinds of aggregates, one of which A 1 is reversibly formed at an early stage of the incubation under high pressure. The apparent activation volume for the inactivation reaction was estimated to be ΔV app ∗ = − 113±5 cm 3· mol −1 from the pressure dependence of k obs . The effect of coenzyme, NAD +, on the pressure-induced inactivation was also studied. The inactivation was retarded by the presence of the coenzyme, whereas the apparent activation volume for the holoenzyme ΔV app ∗ = − 104±2 cm 3· -1 did not differ significantly from that for the apoenzyme.

Dielectric relaxation of 4:1 v/v methanol–ethanol to 90 kbar

The effect of pressure on dielectric relaxation in a 4:1 v/v methanol–ethanol mixture has been measured in the range −44 –+22 °C to a maximum pressure of 90 kbar using a tungsten–carbide anvil apparatus. Graphs of the complex capacitance in the complex plane are not semicircles, but are approximately skewed arcs. The frequencies of maximum loss were fitted to a general quadratic in the deviations of the pressure and the reciprocal temperature from a reference pressure and temperature near the middle of the field, and apparent activation enthalpies and volumes were calculated. The model in which molecules are represented by spheres of radius a immersed in a medium of viscosity η predicts that the quantity fmη/T, where fm is the frequency of maximum loss and T the temperature, is independent of temperature and pressure. Its value changes relatively little with temperature but changes by three orders of magnitude in the range 0–70 kbar at 21 °C. Our theories of the transport properties of fluids at very high pressures are therefore quite inadequate. As much of the interior of the earth is made of such fluids, our small understanding of them limits our understanding of the earth’s interior.

DSC and high pressure conductivity and electrical relaxation measurements in PPO and PPO complexed with lithium salts

Differential scanning calorimetry and high pressure electrical conductivity studies have been carried out on poly(propylene oxide) after the addition of various lithium salts. In addition, high pressure electrical relaxation measurements have been performed in the glass transition region on uncomplexed PPO. The vacuum conductivity measurements show that T 0 is 30–40°C lower than the DSC Tg. Next, it is found that both T 0 and Tg increase several K/kbar. In terms of the configurational entropy model, the isobaric data yield a reasonable value for the activation volume which is consistent with values previously reported for PEO complexed with alkali metal ions. The isothermal data yield a negative activation volume for PPO complexed with LiSCN. Finally, DSC and isothermal conductivity measurements show that neither T g nor the apparent activation volume scale directly with ion size.

Influence of dynamic strain aging on the apparent activation volume for deformation

There is sufficient evidence that dynamic strain aging is a very significant factor in the deformation of commercial titanium in the intermediate temperature range (0.3–0.4)T m where T m is the melting temperature. In this paper the influence of dynamic strain aging on the apparent activation volume for deformation is examined on the basis of the published data available. The rapid increase in activation volume above about 0.3T m can be explained very well by the dynamic strain aging associated with oxygen atoms. The gradual decrease in activation volume below 200 K could result from a decrease in the Taylor factor with the increasing contribution of twinning.

Apparent activation volumes of hydrophobic ions and carriers in planar lipid bilayers.

A gas-free high-pressure cell has been developed to measure planar bilayer conductances induced by hydrophobic ions and ionophores as a function of hydrostatic pressure. Plots of log conductance versus pressure for valinomycin and nonactin-mediated potassium transport in egg phosphatidyl cholinedecane membranes are essentially linear over a pressure range of 1 to 818 atm. Calculated activation volumes give similar results for both nonactin and valinomycin yielding values of + 48 and + 42 cc/mole, respectively. The valinomycin activation volume agrees reasonably well with the results obtained by Johnson and Miller (Biochim. Biophys. Acta 375:286-291, 1975) for K+-valinomycin transport in liposomes. In contrast to the activation volumes for nonactin and valinomycin, relaxation measurements of tetraphenyl boron (TPB) and dipicrylamine (DPA) give very small values of less than 5 cc/mole for the translocation rate constant, ki. Similarly, steady-state conductance measurements on tetraphenyl arsonium (TPA) and carbonylcyanide m-chlorophenylhydrazone (CCCP), give small values of 6 and 7 cc/mole, respectively. These low figures do not support transport theories based on the formation of bilayer holes or kinks (H. Träuble, J. Membrane Biol. 4:193-208, 1971). The low values for TPB and TPA are especially interesting because their cross-sectional areas are not much different than those of valinomycin and nonactin. Pressure-induced changes in membrane dielectric constant and thickness which lower the bilayer electrostatic barrier could explain the low values for the hydrophobic ions. Additionally, larger activation volumes might be expected for carriers such as nonactin and valinomycin that undergo significant rearrangement and change in hydration during surface complexation of cations.

Relaxation strain measurements in cellular dislocation structures

The conventional picture of what happens during a stress relaxation usually involves imagining the response of a single dislocation to a steadily decreasing stress. The velocity of this dislocation decreases with decreasing stress in such a way that we can measure the stress dependence of the dislocation velocity. Analysis of the data from a different viewpoint enables us to calculate the apparent activation volume for the motion of the dislocation under the assumption of thermally activated glie. Conventional thinking about stress relaxation, however, does not consider the eventual fate of this dislocation. If the stress relaxes to a low enough level, it is clear that the dislocation must stop. This is consistent with the idea that we can determine the stress dependence of the dislocation velocity from relaxation data only for those cases where the dislocation's velocity is allowed to approach zero asymptotically, in short, for those cases where the dislocation never stops. This conflict poses a dilemma for the experimentalist. In real crystals, however, obstacles impede the dislocation's progress so that those dislocations which are stopped at a given stress will probably never resume motion under the influence of the steadily declining stress present during relaxation. Thus one couldmore » envision stress relaxation as a process of exhaustion of mobile dislocations, rather than a process of decreasing dislocation velocity. Clearly both points of view have merit and in reality both mechanisms contribute to the phenomena.« less

Pressure effects on alamethicin conductance in bilayer membranes

We report here the first observations of the effects of elevated hydrostatic pressure on the kinetics of bilayer membrane conductance induced by the pore-forming antibiotic, alamethicin. Bacterial phosphatidylethanolamine-squalene bilayer membranes were formed by the apposition of lipid monolayers in a vessel capable of sustaining hydrostatic pressures in the range, 0.1-100 MPa (1-1,000 atm). Principal observations were (a) the lifetimes of discrete conductance states were lengthened with increasing pressure, (b) both the onset and decay of alamethicin conductance accompanying application and removal of supra-threshold voltage pulses were slowed with increasing pressure, (c) the onset of alamethicin conductance at elevated pressure became distinctly sigmoidal, suggesting an electrically silent intermediate state of channel assembly, (d) the magnitudes of the discrete conductance levels observed did not change with pressure, and, (e) the voltage threshold for the onset of alamethicin conductance was not altered by pressure. Apparent activation volumes for both the formation and decay of conducting states were positive and of comparable magnitude, namely, approximately 100 A3/event. Observation d indicates that channel geometry and the kinetics of ion transport through open channels were not affected by pressure in the range employed. The remaining observations indicate that, while the relative positions of free-energy minima characterizing individual conducting states at a given voltage were not modified by pressure, the heights of intervening potential maxima were increased by its application.

Sensitivity of various salt forms of Bacillus megaterium spores to the germinating action of hydrostatic pressure

Spores of Bacillus megaterium ATCC 19213 were subjected to a complete ion-exchange regimen, which included titration to a pH value of 2, heating at 60 °C for up to 18 h, back titration with various base solutions to a pH value of 8, and heating again at 60 °C. Spore populations maintained high levels of viability throughout this rigorous procedure, and the various salt forms prepared showed a wide range of sensitivities to the germinating effect of hydrostatic pressure. Native spores showed the expected germination response when subjected to pressures of 350–750 atm (1 atm = 101.325 kPa) at 24 °C. There was a threshold pressure for germination of some 300 atm, and the apparent activation volume for the process was calculated to be 188 mL/mol, indicating that these spores had approximately the same pressure sensitivity as those of Bacillus subtilis or Bacillus pumilus. The optimum pH for germination was approximately 8, and the optimum temperature was approximately 45 °C. The hierarchy of resistance of the various salt forms tested to the germinating action of 493 atm pressure was H > K > Ca, Mg, Na > native. The H form was particularly insensitive, even to pressures as high as 1020 atm, but germinated in response to chemical germinants. We concluded that the specific mineralization of bacterial spores has major influence on pressure-induced germination, which can occur even in the absence of added salts. Sensitivities to pressure could not be correlated with previously reported heat sensitivities, electrostasis, or states of dehydration.

High pressure mechanistic studies of the photochemical reactions of transition metal complexes. : II. Ligands field photolysis of Cr(NCS) 3−6, Cr(CN) 3−6 and Co(CN) 3−6 in aqueous solution

The photo-aquation reactions of Cr(NCS) 3− 6, Cr(CN) 3− 6 and Co(CN) 3− 6 have been studied as a function of pressure up to 1500 bar. The apparent volumes of activation for these reactions, determined from the pressure dependence of the quantum yields, are +2.1 ± 0.3, +2.7 ±0.2 and +1.3 ± 0.1 cm 3 mol −1, respectively. These dataare discussed in detail in reference to partial molar volume data and are interpreted evidence for a photo-auation mechanism of the d type.

Hydrostatic pressure and long-chain organic molecule diffusion through a polymer matrix

Earlier papers from our laboratory1–3 have described a sensitive but relatively simple technique using IR spectroscopy to study the diffusion of labelled organic molecules through a polymer matrix. The method showed that the behaviour agreed well with de Gennes4 model of reptation which assumes that long organic molecules diffuse by the motion of segments consisting of 20 to 30 units in the backbone. We describe here an extension of this work and deal with the effect of hydrostatic pressure on the diffusion of two hydrocarbons (suitably labelled) with backbone units N = 18 and 45 through molten low density branched polyethylene at pressures ranging from atmospheric to 1,300 atmospheres. The results show that the diffusion coefficient D decreases as the pressure P is increased. To a close approximation log D decreases linearly with P implying an apparent activation volume, V. Typical values for N = 18, are V = 55 A3 at 150 °C and 70 A3 at 120 °C implying a thermal expansion of the free volume of order 0.5 A3 K−1. For N = 45, V = 80 A3 at 120 °C.