Keith J. Laidler

Molecular kinetics of muscle adenosine triphosphatase

Abstract 1. 1. The kinetics of the dephosphorylation of ATP by myosin follow the law, −d [ ATP ] dt = k 2 K[ myosin ][ ATP ] (1 + K [ ATP ]) . The apparent heats and entropies associated with K2 and K are, respectively, Δ H 2 ‡ = 12.4 kcal, mole −1 ; Δ S 2 ‡ = −8 cal. mole −1 deg. −1 ΔH = 8 kcal. mole−1; ΔS = 49 cal. mole−1 deg.−1 assuming a univalent enzyme particle with a molecular weight of 2.0 × 107. 2. 2. There is evidence to suggest that K is a true equilibrium constant for (enzyme-substrate) complex formation, and that this formation involves a charge neutralization. 3. 3. Preliminary measurements of rate of thermal deactivation of myosin yield a first-order rate constant of 5.6 × 10−4 sec.−1 at 35.2 °C., an apparent Δ H ‡ of 70 kcal, mole−1, and an apparent ΔS ‡ of 150 cal. mole−1 deg.−1. In the presence of 0.001 M ATP, the rate constant of deactivation is reduced below 10−4 sec.−1. 4. 4. The incorporation of pH and ion effects into the expression for the steady-state reaction rate is discussed.

The molecular kinetics of the urea-urease system. IV. The reaction in an inert buffer.

Abstract A kinetic study has been made of the urease-catalyzed hydrolysis of urea, the work being done in a trishydroxymethylaminomethane-H2SO4 buffer, which was shown to have no activating or inhibiting effect on the reaction. The rate was found to pass through a sharp maximum at pH 8.00, and detailed studies were made at pH values of 8.00 and 7.13, over a wide range of substrate concentrations and temperatures. The Michaelis law was found to be obeyed accurately up to a substrate concentration of 0.25 M at pH 8.00 and of 0.33 M at pH 7.13; at higher concentrations there was some falling off of the rate. The absolute enzyme concentration was determined by comparison with Sumners data in phosphate buffer, and values of entropies and heats of activation were calculated.

The Kinetics of Membrane Processes. I. The Mechanism and the Kinetic Laws for Diffusion through Membranes

The equations for diffusion in binary systems are extended to the case of diffusion through a membrane. Three elementary rate processes are considered at a solution‐membrane interface: (1) adsorption of the diffusing species, (2) desorption back into the solution, and (3) diffusion into the membrane, and an expression for the rate constant of the over‐all process of surface penetration is developed in terms of three specific rate constants. Various special cases are considered and discussed with reference to the experimental data. A general expression for the rate of diffusion of a species through a membrane under steady‐state conditions is derived, the rate being expressed as a function of activities. It is shown that the application of this expression to the case of a solvent passing through a membrane which is impermeable to the solute leads to the thermodynamically exact equation for the osmotic pressure. Expressions are given for the rate of flow of solvent and solute through a membrane as a function...

Molecular kinetics of muscle adenosine triphosphatase. II. Solvent and structural effects.

Abstract 1. 1. The kinetics of the myosin-adenosine triphosphate system have been studied in various methanol-water and dioxane-water mixtures, at both low and high substrate concentrations. The rates are found to increase as the concentrations of methanol and of dioxane are increased. 2. 2. From the results are calculated the electrostatic contributions to the entropy of formation of the enzyme-substrate complex and to that of the breakdown of the complex. Using the data of a previous paper the corresponding nonelectrostatic entropies are obtained. 3. 3. The results are shown to be consistent with charge formation, while during complex breakdown there is apparently a separation of like (negative) charges.

The Kinetics of Membrane Processes. III. The Diffusion of Various Non‐Electrolytes through Collodion Membranes

Measurements have been made of the rates of diffusion of various non‐electrolytes, and of water, through a collodion membrane separating a dilute solution from pure water. The determinations were made by measuring the rate of rise and fall in a capillary tube in the solution, and analyzing the results using the procedure described in a previous paper. The work was done with 0.25N solutions of sucrose, lactose, raffinose and mannitol, and was carried out at different temperatures so that energies and entropies of activation could be determined. The energy of activation for diffusion through the membranes was found in all cases to be slightly less than for free diffusion, a result that is shown to be consistent with a positive heat of adsorption of the solute and solvent at the membrane. Analogously the entropies of activation for membrane diffusion are somewhat less than for free diffusion, since the process of adsorption brings about a decrease in entropy. The rate of diffusion of water through a membrane...

Kinetics of the Decomposition of Diethyl Peroxide

The kinetics of the gas‐phase decomposition of diethyl peroxide have been studied in a flow system, using excess of toluene. The main reaction products were found to be ethane and formaldehyde, and there were smaller amounts of methane and dibenzyl. The frequency factor of the reaction is 2.1×1013, and the activation energy is 31.7 kcal per mole. The mechanism of the reaction is discussed, and it is shown that the measured rate must refer to the initial dissociation C2H5OOC2H5→2C2H5O. With a new value of 47.8 kcal for the heat of formation of gaseous diethyl peroxide, the heat of formation of the C2H5O radical is calculated to be 8.1 kcal, which agrees satisfactorily with estimates from other kinetic data. Thermochemical values for other reactions involving this radical are calculated.

Microwave Spectroscopic Investigation of the Kinetics of the Heterogeneous Ammonia‐Deuterium Exchange

A method is described for analyzing a mixture of gases using microwave spectroscopy. Relatively simple microwave measurements are employed on single resolved collision broadened spectral lines. For binary mixtures, only the peak absorption coefficients need to be measured if the microwave collision diameters are known. Mixtures of isotopically substituted molecules can also be analyzed because the microwave absorption lines of the isotopic species are easily resolved.The technique is applied to determining the mole fraction of NH3 in mixtures of deuterated ammonias, and to a study of the kinetics of the NH3—D2 isotopic exchange reaction on a singly promoted iron catalyst. It is found that rates of desorption of molecules from surfaces are markedly increased on adsorption of other molecules, a fact that can be explained only in terms of strong interactions between adsorbed molecules. The rate of the exchange reaction is proportional to the square root of the deuterium pressure. With increasing ammonia pres...

ELEMENTARY PROCESSES IN THE RADIATION CHEMISTRY OF WATER

Schematic potential‐energy diagrams are constructed for ground and excited states of H2O, H2O+ and H2O‐, making use of thermochemical, spectroscopic, and electron‐impact data. On electron impact a gaseous water molecule is raised to various states of H2O, H2O+ and H2O‐, which may subsequently decompose to give H, O, H2, and OH and the ions H+, H‐, O+, O‐, OH+, and H2+. The exact mechanisms by which these atoms, radicals, and ions are produced are discussed in detail with reference to the potential‐energy curves, due attention being paid to the appropriate correlation and selection rules. It is concluded that the production of O‐ at 173 kcal electron energy probably proceeds via a 2A1, 2B1, 2A2, or 2B2 state of H2O‐ which decomposes into 2H+O‐ without the prior formation of OH‐. The production of O+ at 437 kcal must necessarily proceed via a 4A2 or 4B2 state of H2O+, while H2+ at 530 kcal must involve the formation of a 2A2 or 2B2 state; both these latter states are stable with respect to the simple breaki...

The Mechanism of Processes Initiated by Excited Atoms III. Photo‐Sensitized Hydrocarbon Reactions

The photo‐sensitized reactions of the hydrocarbons are discussed in terms of the following reaction scheme, in which A* indicates the photo‐sensitizing atom and MH2 the hydrocarbon: A*+MH2→A+MH+H,A*+MH2→A+MH2*,MH2*+MH2→2MH2,MH2*→M+H2,MH2*→MH+H. The first and fifth reactions are followed by polymerizations. Expressions for the quenching rate, the rate of formation of M, and the rate of formation of polymer, are obtained in terms of the original rate constants, and a number of special cases of practical interest are treated. The relative rates of the reactions are discussed with reference to the experimental data, particular regard being paid to the energy relationships, the general kinetic behavior, and the influence of temperature. Evidence is adduced in favor of the following: (1) the strength of the C–H bond in ethylene is about 100 kcal., (2) there is a triplet‐excited state of acetylene with an excitational energy of 10–30 kcal., (3) conversion of a large amount of electronic energy into vibrational e...

Molecular kinetics of muscle adenosinetriphosphatase. III. Influence of hydrostatic pressure

Abstract 1. 1. An experimental study has been made of the kinetics of the myosinadenosine triphosphate system over a range of hydrostatic pressures (up to 10,000 lb./sq. in.). The work was done over a range of substrate concentrations and at three potassium chloride concentrations, namely 0.05, 0.3, and 0.6 M . 2. 2. The data have been analyzed in terms of the Michaelis-Menten mechanism of enzyme action, and volumes of activation calculated for the bimolecular interaction of enzyme and substrate and for the breakdown of the complex into products; these results are summarized in Table II. Some indication has also been obtained of the effect of KCl concentration on the kinetics. 3. 3. The results are shown to be consistent with previous hypotheses as to the nature of the enzyme-substrate interaction and of the breakdown of the complex. The volumes associated with the enzyme-substrate interaction are positive, which is consistent with there being a charge neutralization as the enzyme and substrate come together. The volumes associated with the breakdown are negative, a fact which is attributed to changes in the conformation of the enzyme. 4. 4. The data obtained for this reaction are compared with volume and entropy data for other enzyme reactions, and are found to show some features in common.