Showing papers on "Reaction rate constant published in 1974"

Fluorescence correlation spectroscopy. I. Conceptual basis and theory

Abstract: We describe a method for determining chemical kinetic constants and diffusion coefficients by measuring the rates of decay of spontaneous concentration fluctuations. The equilibrium of the system is not disturbed during the measurement. We measure the number of molecules of a specified type in a defined open volume as a function of time and compute the time course of the deviations from the thermodynamic mean concentration. The method is based on the principle that the rates of decay of spontaneous microscopic fluctuations are determined by the same phenomenological rate coefficients as those of macroscopic departures from equilibrium which result from external perturbations. Hence, an analysis of fluctuations yields the same chemical rate constants and diffusion coefficients as are measured by conventional procedures. In practice the number of the specified molecules is measured by a property such as absorbance or fluorescence which is specific and sensitive to chemical change. The sample volume is defined by a light beam which traverses the cell. As the molecules appear in or disappear from the light beam, either due to diffusion or chemical reaction, their concentration fluctuations give rise to corresponding fluctuations of the intensity of absorbed or emitted light. This paper presents the theory needed to derive chemical rate constants and diffusion coefficients from these fluctuations in light intensity. The theory is applied to three examples of general interest: pure diffusion in the absence of chemical reaction; the binding of a small rapidly diffusing ligand to a larger slowly diffusing macromolecule; and a unimolecular isomerization. The method should be especially useful in studying highly cooperative systems, relatively noncooperative systems with intermediate states closely spaced in free energy, small systems, and systems not readily subject to perturbations of state.

Fluorescence Correlation Spectroscopy. II. An Experimental Realization

Abstract: synopsis This paper describes the first experimental application of fluorescence correlation spectroscopy, a new method for determining chemical kinetic constants and diffusion coefficients. These quantities are measured by observing the time behavior of the tiny concentration fluctuations which occur spontaneously in the reaction system even when it is in equilibrium. The equilibrium of the system is not disturbed during the experiment. The diffusion coefficients and chemical rate constants which determine the average time behavior of these spontaneous fluctuations are the same as those sought by more conventional methods including temperature-jump or other perturbation tecliniques. The experiment consists essentially in measuring the variation with time of the number of molecules of specified reactants in a defined open volume of solution. The concentration of a reactant is measured by its fluorescence; the sample volume is defined by a focused laser beam which excites the fluorescence. The fluorescent emission fluctuates in proportion with the changes in the number of fluorescent molecules as they diffuse into and out of the sample volume and as they are created or eliminated by the chemical reactions. The number of these reactant molecules must be small to permit detection of the concentration fluctuations. Hence the sample volume is small (10-8 rnl) and the concentration of the solutes is low (-10-9 M). We have applied this technique to the study of two prototype systems: the simple example of pure diffusion of a single fluorescent species, rhodamine 6G, and the more interesting but more challenging example of the reaction of macromolecular DNA with the drug ethidium bromide to form a fluorescent complex. The increase of the fluorescence of the ethidium bromide upon formation of the complex permits the observation of the decay of concentration fluctuations via the chemical reaction and consequently the determination of chemical rate constants.

The mechanism of action of superoxide dismutase from pulse radiolysis and electron paramagnetic resonance. Evidence that only half the active sites function in catalysis

Abstract: 1. Detailed studies on the mechanism of the enzymic reaction of bovine superoxide dismutase were carried out by using pulse radiolysis and electron paramagnetic resonance (e.p.r.). 2. The second-order rate constant for reaction between superoxide dismutase and the superoxide ion was redetermined as (2.37±0.18)×109m−1·s−1 at 25°C. This reaction governs the turnover, and any first-order steps must have rate constants higher than about 106s−1. Turnover has a low activation energy and is slowed substantially when the viscosity is increased with glycerol, confirming that the reaction rate is near the limit for diffusion control. In water a reversible conformation change to a less active form appears to take place above about 40°C. 3. Pre-steady-state rates of reduction and reoxidation of copper in the enzyme are consistent with these processes being rate-limiting in enzyme turnover. 4. Examination, with the help of computer simulation, of the e.p.r. spectra at 9 and 35GHz of native superoxide dismutase indicated that, apart from 10–20% of impurities, only one species of Cu2+ is distinguishable. Further, the specific activity of our enzyme preparations, measured by pulse radiolysis, is at least as high as that obtained by other workers. 5. Nevertheless, measurement of the proportion of copper present as Cu2+ (determined both optically and by e.p.r. spectroscopy) in the steady states approached from both the oxidized and the reduced forms of the enzyme, indicates (after allowing for the impurities) that only half of the copper atoms participate in turnover. E.p.r. spectroscopy provided no evidence for differences between functioning and non-functioning Cu2+ atoms. 6. It is suggested that the results may be best interpreted in terms of an allosteric type of mechanism, with two initially indistinguishable copper atoms in the enzyme. Reaction of one of these with a superoxide ion then renders the other, at least transiently, unreactive.

Elemental mercury evolution mediated by humic Acid.

Abstract: Elemental mercury is formed in aqueous solution by the chemical reduction of mercuric ion in the presence of humic acid. The reduction proceeds via first order kinetics (rate constant, 0.009 hour-1) and is depndent on pH. The reaction mechanism involves interaction of the ionic metal species with the free radical electrons of the humic acid.

Temperature dependence of some ionospheric ion-neutral reactions from 300°–900°K

Abstract: A flowing afterglow system that operates from 300° to 900°K has been used to measure the temperature dependence of a number of aeronomically important reaction rate constants over this temperature range. This extends previous measurements in an earlier system that operated up to 600°K. The reactions measured include O+ with N2, O2, and CO2; N+, N2+, and CO2+ with O2; O2+ with NO; and He+ and He2+ with N2. The measurements are compared with the previous measurements that exist and with measurements of rate constants as a function of ion kinetic energy alone.

Atomic mechanism of fracture of solid polymers

Abstract: Macromolecular chain scission under mechanical stress has been studied by infrared spectroscopy. The dependence of accumulation of chemical bond scissions on temperature T and uniaxial tensile stress σ has been investigated. The rate constant K for bond dissociation under mechanical stress has been found to obey the modified Arrhenius equation: K = K0 exp{ − (EA − ασ)/RA}. The quantitative connection between the rate constant for bond dissociation and mechanical lifetime τ has been established. Analysis of the experimental data indicates that the strength and mechanical lifetime of polymers is determined by the kinetics of mechanochemical scission of the main chains of polymer molecules.

Pyruvate Formate‐Lyase of Escherichia coli: the Acetyl‐Enzyme Intermediate

Abstract: The reaction pyruvate + CoA ⇌ acetyl-CoA + formate, catalyzed by pyruvate formate-lyase of Escherichia coli, occurs by the succeeding half-reactions (a) E + pyruvate ⇌ E-acetyl + formate; (b) E-acetyl + CoA ⇌ E + acetyl-CoA. Making use of coupled optical assays, a ‘ping-pong mechanism’ was derived from the complete kinetic investigation of the forward and reverse reactions. The thermodynamic equilibrium constant of the overall reaction was calculated from the kinetic constants to be K= 750 (30°C, pH 8.1), which agrees with chemically determined values. The intermediate acetyl-enzyme, which had been previously proposed from the [14C]formate-pyruvate exchange, was detected by product-pulse experiments with [2-14C]pyruvate and trapped by acid precipitation. The acetyl group is linked to a sulfhydryl group of the protein. The value of the equilibrium constant of the first half-reaction is about 50, as directly measured and calculated from the kinetic data. It was concluded that the standard free energy of hydrolysis of acetyl-enzyme is about 1.7 kcal (7.1 kJ) more negative than that of acetyl-CoA. The intermediate was found to react with dithiothreitol with a second-order rate constant at 30°C and pH 7.6 of 1160 M−1× min−1. It resulted in a half-life of 4 s (or 20 s at 0°C) in the particular buffer which was required for enzyme stabilization. The enzyme (about 60 U/mg) was prepared by carrying its purified inactive form through the enzyme-II-dependent activation reaction, employing photoreduced flavodoxin along with the effector compounds S-adenosylmethionine and oxamate (as a pyruvate analogue).

Temperature dependence of de‐excitation rate constants of He(23S) by Ne, Ar, Xe, H2, N2, O2, NH3, and CO2

Abstract: Reaction rate constants for the quenching of the 23S state of helium by Ne, Ar, Xe, H2, N2, O2, CO2, and NH3 have been measured as a function of temperature between 300 and 900°K in a flowing afterglow. All of these rate constants increased with temperature. This finding offers an explanation for the discrepancies among recently published values for the reaction rate constant of He(23S) with these gases obtained in different experiments.

Collisions of F(2P1/2) with H2

Abstract: The role of spin‐orbit and nonadiabatic effects in the reaction of F atoms with H2 is discussed, Ground and excited FH2 potential energy surfaces and the required interactions between them are computed by the diatomics‐in‐molecules method. Using an approximate semiclassical procedure, thermal rate constants for reaction and quenching of F(2P1/2) are estimated to be 1.2 and 8.4×10−12 cm3 mol−1 · sec−1, respectively. Since the former is about an order of magnitude smaller than previously reported calculations of the rate constant for reaction of F(2P3/2) with H2, these results predict a substantial difference in reactivity between the 2P3/2 and 2P1/2 fine structure states of atomic fluorine.

Chemical reaction rates of quasi free electrons in nonpolar liquids

Abstract: A method is described for determining reaction rates of molecules dissolved in nonpolar liquids with electrons produced by ionization of the solvent. It is based on modification by the added substance of the current- growth curve which is seen when the liquid, in an electric field, is suddenly exposed to X-rays, and uhich is used for mobility determination by the Hudson method. The rate constants for recombination of electrons with positive ions, determined by the Langevin method in n-pentane, n-hexane, and tetramethylsilane, are diffusion controlled and hence proportional to the electron mobilities; this rate constant in tetramethylsilane is 5 x 10/sup 16/ M/sup -1/ sec/sup -1/. The electron reactions wit h CCl/sub 4/, CH/sub 3/I, and O/sub 2/ are not diffusion controlled; the rates are generally higher in solvents which show higher electron mobilities, but the increase is less than proportional. Different behavior is shown with C/sub 2/H/sub 5/Br; the electron reaction rate is lower in the highelectron-mobility solvents neopentane and tetramethylsilane than in hexane or 2,2,4-trimethylpentane, and its temperature coefficient, which is positive in hexane, becomes unexpectedly negative in the former solvents, and amounts to -- 1.7% per degree in neopentane. New electron mobility determinations are reported for methylcyclohexane,more » n-hexane, n-pentane, cyclohexane, neopentane, and tetramethylsilane.« less

Ion‐molecule reactions and vibrational deactivation of H2+ ions in mixtures of hydrogen and helium

Abstract: Use of ion cyclotron resonance methods to measure the thermal energy rate constants for a number of ion-molecule reactions involving hydrogen and hydrogen-helium mixtures Assuming that the distribution of initial vibrational states in the H2(+) ion is a near-Franck-Condon distribution, the occurrence of collisional deactivation of vibrationally excited H2(+) ions by He atoms is identified, and an approximate rate constant for the deactivation process and its dependence on vibrational energy are given

Negative ion‐molecule reactions with atomic hydrogen in the gas phase at 296 °K

Abstract: Gas phase reaction rates have been measured for the reactions of Cl−, I−, OH−, O2−, SF6, and some hydrates of Cl−, OH−, and O2− with atomic hydrogen in a flowing afterglow system at 296°K. In general, the reaction mechanism is associative detachment, X−+H→HX+e, and the rate constants are very large (∼ 10−9 cm3/sec). The reactivities of Cl−, OH−, and O2− are reduced by clustering with H2O. The reaction of I− with H is immeasurably slow, and SF6− reacts with H to form SF5− which does not react further.

Laser magnetic resonance study of the gas phase reactions of OH with CO, NO, and NO2

Abstract: A laser magnetic resonance spectrometer has been used in combination with a discharge‐flow system to measure the gas phase reaction rates of the OH radical with CO, NO, and NO2 at 296°K and over a pressure range 0.4–5 torr. For the bimolecular reaction OH + CO → CO2 + H we measure a rate constant, k = 1.56×10−13 cm3/molecule·sec. For the termolecular reactions OH + NO + M → HNO2 + M, M = He, k = 4.0×10−31 cm6/molecule2·sec; M = Ar, k = 4.4×10−31 cm6/molecule2·sec; M = N2, k = 7.8×10−31 cm6/molecule2·sec. For the reaction OH + NO2 + N2 → HNO3 + N2, k = 2.9×10−30 cm6/molecule2·sec. Laser magnetic resonance detection of radicals is shown to be extremely sensitive, linear, and versatile. A complete description of this technique is presented with a discussion of its potential in the study of the reactions of free radicals.

A flash photolysis‐resonance fluorescence study of the reaction of atomic hydrogen with molecular oxygen H + O2 + M → HO2 + M

Abstract: Using the technique of flash photolysis-resonance fluorescence, absolute rate constants have been measured for the reaction H + O2 + M HO2+M over a temperature range of 220–360°K. Over this temperature range, the data could be fit to an Arrhenius expression of the following form: The units for kAr are cm6/mole-s. At 300°K the relative efficiencies for the third-body gases Ar:He:H2:N2:CH4 were found to be 1.0:0.93:3.0:2.8:22. Wide variations in the photoflash intensity at several temperatures demonstrated that the reported rate constants were measured in the absence of other complex chemical processes.

A kinetic approach to the study of absorption of solutes by isolated perfused small intestine

Abstract: 1. A new technique has been developed for making serial measurements of water and solute absorption from the lumen of isolated small intestine.2. The isolated intestine is perfused in a single pass with a segmented flow of slugs of liquid separated by bubbles of oxygen-carbon dioxide mixture. Simultaneous collections are made of effluent from the lumen and of the fluid which is transported across the mucosa. This latter fluid appears to be a fair sample of the tissue fluid.3. Conditions in the lumen can be changed within less than 5 min. The effects of two or more treatments applied to the same segment of intestine can be determined and the time course of a change in luminal conditions.4. The rate of appearance of solutes on the serosal side depends on the rate of water absorption, and changes exponentially towards a steady state. The rate constant is a function of tissue fluid volume.5. In the steady state the concentration of glucose in the tissue fluid is 71 mM when the luminal concentration is 28 mM, and is 45 mM when the luminal concentration is 8.3 mM.6. For solutes such as glucose for which reflux from tissue fluid to lumen is small relative to flux from lumen to tissue fluid, the time of attainment of a steady state in secretion is usually 50-60 min.7. For solutes such as sodium for which the reflux is relatively high, the steady state may be reached in 15-20 min.8. The K(m) for glucose absorption (14-19 mM) is much lower than is found with unsegmented flow perfusion.9. These findings emphasize problems in interpreting results from other types of intestinal preparation.10. The rate of glucose absorption from the lumen falls only gradually when the luminal sodium concentration is reduced abruptly. In contrast the rate of glucose absorption falls suddenly when the luminal glucose concentration is reduced abruptly. This suggests that glucose absorption is not directly dependent on luminal sodium ions.

Flash photolysis‐resonance fluorescence kinetics study: Temperature dependence of the reactions OH + CO → CO2 + H and OH + CH4 → H2O + CH3

Abstract: Two reactions involving the transient chemical species OH(ν″ = 0) and the reactants CO and CH4 have been investigated over a temperature range of nearly 140°C. Of particular importance were the measurements made below 300°C where data has heretofore been lacking. The rate constant expressions in Arrhenius form are kCO = (2.15±0.19)×10−13 exp[−(160±80 cal mol−1)/R T] and kCH4 = (2.36±0.21)×10−12 exp[−(3400±175 cal mol−1)/R T]. Units are cm3 mol−1 · sec−1. Wide variations in the total pressure, H2O concentration, initial OH concentration, and the nature of the diluent gas showed no indication of complex secondary reactions. The activation energies reported in this work for both processes, virtually zero for Reaction (1) and 3400 cal mol−1 for Reaction (2), are incompatible with those same quantities previously reported at elevated temperatures and indicate either nonlinear Arrhenius behavior for these OH reactions or possibly errors in experimental measurements.

A Critical Review of H‐Atom Transfer in the Liquid Phase: Chlorine Atom, Alkyl, Trichloromethyl, Alkoxy, and Alkylperoxy Radicals

Abstract: This review covers hydrogen‐atom transfer from carbon‐hydrogen bonds in organic compounds to chlorine atom, methyl, ethyl, trichloromethyl, t‐butoxy and alkylperoxy radicals in the liquid phase. Rate constant data are presented in 38 tables. Literature is covered through most of 1972. The review is divided into six sections; an introduction plus five sections each dealing with specific radicals. Hydrogen‐atom transfer to chlorine atom are presented as relative rate constants. For hydrogen‐atom transfer to methyl, ethyl, trichloromethyl, and t‐butoxy radicals, both relative and absolute rate constants are tabulated. For alkylperoxy radicals only absolute rate constants are listed. Each absolute rate constant has a tabulated set of rate parameters where A has been assigned and E derived from the Arrhenius equation.

Gas phase recombination of OH with NO and NO2

Abstract: The termolecular recombination of OH with NO and with NO2 is studied for M=He, Ar, and N2 at pressures from 1–10 torr and temperatures from 230–450°K in a flow tube using resonance fluorescence detection of OH radicals. The reactions are in their low pressure, third-order limit. The rate constants for the NO reaction at 295°K are 3.3, 3.4, and 5.8×10−31 cm6 sec−1 in the above order of M gases. For the NO2 reaction the corresponding values are 1.0, 1.0, and 2.3×10−30, all with σ=±20% including estimates of systematic errors. Both recombinations show the expected negative temperature dependence, the Arrhenius activation energies being −1.7 kcal for NO and −1.8 kcal for NO2. These results are compared with all other published data and with RRKM unimolecular calculations.

The Reaction of the Iron‐Sulfur Protein Hydrogenase with Carbon Monoxide

Abstract: The iron-sulfur protein hydroenase from Clostridium pasteurianum was shown to be specifically inhibited by carbon monoxide. Cyanide (10 mM), azide (10 mM), and fluoride (10 mM) were without effect. The inhibition of hydrogenase by carbon monoxide was studied under pre-steady-state and under steady-state conditions. The data obtained from both studies indicate a 1:1 stoichiometry of the reversible reaction between an independent active site of hydrogenase and carbon monoxide. The second-order rate constant of the formation (k+1) and the first-order rate constant of the dissociation (k−1) of the hydrogenase · carbon-monoxide complex at pH values between 6.5 and 8.5 and at 25°C were found to be 5.7 s−1mM−1 and 0.013 s−1, respectively. The equilibrium constant K was calculated from pre-steady-state data to be 440 mM−1 and from steady-state data to be 420 mM−1. The carbon monoxide inhibition of hydrogenase could be reversed bylight. The light sensitivity indicates that in the active site iron is being attacked by carbon monoxide.

Chemical reactions in solid polymeric systems. Photomechanical phenomena

Abstract: Poly(ethyl acrylates) crosslinked with 0.5-1 mole-% of photochromic bis-(spirobenzopyran dimethylacrylate) show in the solid state a photomechanical behaviour. On irradiation, contraction (more than two per cent) occurs in isothermal conditions, while in the dark, length-recovery takes place, the process being reversible. The dependence of this photocontractile behaviour under constant stress on temperature, and inversely at constant temperature, on increasing stress has been examined by plotting the relative shrinking δL/L as a function of time. This photomechanical effect is much more pronounced for samples swollen in benzene. The photostationary relative contraction of copolymer C increases with the relative elongation λ = L/L 0 at low stresses, passes through a maximum for λ = 2.3, and then decreases. This photomechanical shrinking is interpreted in terms of an increase of entropy of the polymer chains due to the isomerization of the rigid bis-spiro-structure into the planar merocyanine one. The photochromic behaviour of copolymer C, unstretched and stretched (50 per cent) has also been followed at different temperatures. Stretching causes a strong decrease of decoloration rate constants, being one-third of that in unoriented state. When, in contrast, poly(ethyl acrylate) is crosslinked with ethylene glycol dimethacrylate and carries pendant similar photochromic groups (0.7 per cent), no effect on the photochrome and photomechanical behaviour can be shown. The involvement of the strained biphotochromes in the crosslinks is thus required.

A study of the collisional quenching of O(21D2) by the noble gases employing time-resolved attenuation of atomic resonance radiation in the vacuum ultraviolet

Abstract: Electronically excited oxygen atoms O(21D2) have been generated by the pulsed irradiation of ozone in the Hartley-band continuum and monitored photoelectrically in absorption by time-resolved attenuation of atomic resonance radiation at λ = 115.2 nm [O(31D2°) O(21D2)]. Collisional quenching of the excited atom has been investigated for all the noble gases, and the first absolute values for the second-order deactivation rate constants are reported. The resulting rate data are discussed in terms of a curve-crossing mechanism based on existing spectroscopic data for the noble gas oxides. The absolute rate constants are compared with previous relative rate data for the deactivation of O(21D2) by the noble gases.