Electrically bimolecular reaction rate constant

NAD (P)+-dependent enzymes, electrically contacted with electrode surfaces, can provide efhcient bioelectrocatalysis for the NAD(P)H oxidation. For example, diaphorase (DI) was applied to oxidize NADFl, using a variety of quinone compounds, several kinds of flavins, or viologens as mediators between the enzyme and electrode [217, 218]. The bimolecular reaction rate constants between the enzyme and mediators whose redox potentials are more positive than -0.28 V at pH 8.5 can be as high as 10 s , suggesting that the reac-... 

In the first group of studies, involving kinetic inhibition studies, comparisons of the uilibrium (K ), phosphorylation (IC), and inhibition constant (K.) for the inhibition of electric eel and human erythrocyte AChE by ANTX-A(S) and DFP were done (Table II). From Table II it is seen that ANTX- A(S) has a higher affinity for human erythrocyte AChE (K =0.253 fiM) than electric eel AChE (K j=3.67 aM). AN DC-A(S) also shows greater affinity for AChE than DFP (K =300 fiM). And finally the bimolecular rate constant, Kj, which indicates the overall rate of reaction, shows AChE is more sensitive toward inhibition by ANTX-A(S) (Kj=1.36 pM- min- ) than DFP (K, = 0.033 /iM- min ). These studies add information to the comparative activity of ANTX-A(S) and other irreversible AChE inhibitors but do not show the site of inhibition. 

The simple collision theory for bimolecular gas phase reactions is usually introduced to students in the early stages of their courses in chemical kinetics. They learn that the discrepancy between the rate constants calculated by use of this model and the experimentally determined values may be interpreted in terms of a steric factor, which is defined to be the ratio of the experimental to the calculated rate constants Despite its inherent limitations, the collision theory introduces the idea that molecular orientation (molecular shape) may play a role in chemical reactivity. We now have experimental evidence that molecular orientation plays a crucial role in many collision processes ranging from photoionization to thermal energy chemical reactions. Usually, processes involve a statistical distribution of orientations, and information about orientation requirements must be inferred from indirect experiments. Over the last 25 years, two methods have been developed for orienting molecules prior to collision (1) orientation by state selection in inhomogeneous electric fields, which will be discussed in this chapter, and (2) bmte force orientation of polar molecules in extremely strong electric fields. Several chemical reactions have been studied with one of the reagents oriented prior to collision. ... 

Proton transfer reactions occur, as a rule, vay promptly. Special methods were developed to measure rates of these reactions meAods of temperature jump, pressure drop, electric field pulse, and dielectric absoption ultrasonic method several electrochemical methods, and method of absorption line broadening of protons and in NMR spectra (see Chapter 8). These m ods allow the measurement of rate constants of bimolecular reacticms in the 10 - lO" l/(mol-s) interval. Results of measurements by different methods s netimes diverge dramatically. For example, ftx the reaction of H30 and CH3COO" the rate constant values obtained by different electrochemical methods lie in an interval of (l-9)-10 l/(mol-s) (H20,298 K). 

Among the extreme cases, the analysis of chemical electric field effects is simplest when the rotational equilibria are established faster than the diffusion-limited chemical processes. The other extreme is the complete control of the chemical processes by the rate of the orientational relaxations. As seen in Table 3, bimolecular chemical reactions exhibit a characteristic dependence of time constant and amplitude on concentration. 

---