Efficiency and evolution of enzyme catalysis.

A new function derived from kinetic data, the efficiency function, can be used to quantify the efficiency of a catalyst. For freely diffusing species the maximum efficiency is unity. The enzyme triose phosphate isomerase has an efficiency of 0.6 and is thus almost a perfect catalyst. The efficiency of the acetate ion as catalyst for the same reaction is 2.5 × 10^?11. This increase in catalytic efficiency is discussed in terms of three types of evolutionary improvement: the uniform binding of the substrate to the enzyme, changes in the internal thermodynamics of the bound states, and more effective catalysis of elementary steps. These concepts are illustrated for triose phosphate isomerase.     

The kinetics of electron transfer in the thionine-coated electrode

Transient studies using potential jump and optical absorbance have been carried out on thionine-coated electrodes. Both types of transient are shown to fit a simple diffusion model. At pH 1.3 the characteristic diffusion time varies with the size of the anion present in the electrolyte, showing that diffusion of the counter ions is the rate-limiting process. At pH 5 faster, more uniform, rates are found which are attributed to rate-limiting electron hopping. The rate constant for this process is in rough agreement with the rate of the conproportionation reaction of thionine and leucothionine. The electron-hopping process also limits the maximum current that can be passed through the modified electrode in the steady state.     

The control of potential in continuous flow systems

The control of the potential of the working electrode in closed loop flowing systems is discussed. Various relative positions of the reference electrode are considered. No satisfactory arrangement of a three electrode configuration is possible for the general case; only when all but one of the current paths are blocked is a three electrode system satisfactory. Generally, however, a five electrode arrangement is adequate for all purposes. Electrical circuits for practical operation are given. With suitably designed tube electrodes reactions with very high exchange currents can be studied free from any uncertainty due to ohmic resistances. An application to the control of parasitic current corrosion is noted.     

Photogalvanic cells: Part VIII. The theory of the transparent rotating disc electrode

The theory of the transparent rotating disc electrode is first extended to cover the case where two photogenerated species react with second-order kinetics and where one of the species is electroactive and the other is not. Secondly the theory is developed to describe the case where the solution is bleached close to the electrode. Thirdly theory is presented for experiments where the light source is modulated and the phase shift and amplitude of the resulting modulated photocurrent are measured.     

Transport and kinetics at microheterogeneous electrodes: Part III. Application to the photoelectrochemical decomposition of water

The transport and kinetics in a system involving two sets of microheterogeneous electrodes, which are designed to use solar radiation to split water into H₂ and O₂, are described. A reaction scheme involving fourteen different processes is considered. The conditions for an efficient system are derived. It is shown that the main problems are electrode selectivity and the loss of H₂ and O₂ by back reactions.     

The tube electrode and e.s.r.: Second order kinetics

A radical species is generated on a tube electrode; it is then transported under conditions of laminar flow through an e.s.r. cavity. In the bulk of solution it decomposes by second order kinetics. The convective diffusion equation for this problem is treated by the simulation method. This method is greatly improved if the spatial co-ordinates are suitably distorted. The distortion used in this work is applicable to all problems involving tube and channel electrodes. The results of the calculation are tested using the reduction of nitrobenzene and are found to be in good agreement with experiment. An upper limit is estimated for the second order rate constant for the radical decomposition above which the radicals will not be detected.     

Photogalvanic cells: Part XI. The thionine-coated electrode

Studies using ring-disc electrodes, cyclic voltammetry, optical absorption and X-ray photoelectron spectroscopy show that up to twenty monolayers of thionine can be irreversibly coated on to platinum or tin oxide electrodes. On such electrodes the electrode kinetics of the thionine-leucothionine couple are still reversible or quasi reversible. In contrast the reduction of Fe(III) is much slower on a coated electrode compared to a clean electrode. Koutecky-Levich plots show that there is a second potential-independent rate-limiting step. This blocking of the Fe(III) reduction means that the thionine coated electrode is the ideal electrode for a photogalvanic cell using the iron thionine system.