Isotope effects, kinetic phenomenon

The general phenomenon of a primary kinetic isotope effect is caused by the higher zero-point vibration energy of a C-H bond compared to a C-D bond. If in the transition state where this bond is being broken the intermediate is linear (C-H-X), the energies of the deuterio and protio species are equal and... 

In the following year, Cleland and his coworkers reported further and more emphatic examples of the phenomenon of exaltation of the a-secondary isotope effects in enzymic hydride-transfer reactions. The cases shown in Table 1 for their studies of yeast alcohol dehydrogenase and horse-liver alcohol dehydrogenase would have been expected on traditional grounds to show kinetic isotope effects between 1.00 and 1.13 but in fact values of 1.38 and 1.50 were found. Even more impressively, the oxidation of formate by NAD was expected to exhibit an isotope effect between 1.00 and 1/1.13 = 0.89 - an inverse isotope effect because NAD" was being converted to NADH. The observed value was 1.22, normal rather than inverse. Again the model of coupled motion, with a citation to Kurz and Frieden, was invoked to interpret the findings. 

These studies had therefore found the tunneling phenomenon, with coupled motion, as the explanation for failures of these systems to conform to the expectations that the kinetic secondary isotope effects would be bounded by unity and the equilibrium effect and that the primary and secondary effects would obey the Rule of the Geometric Mean (Chart 3), as well as being consistent with the unusual temperature dependences for isotope effects that were predicted by Bell for cases involving tunneling. 

Increasing numbers of nitrogen atoms increase not only the kinetic susceptibility toward attack but also the thermodynamic stability of the adducts. Reversible covalent hydration of C = N bonds has been observed in a number of heterocyclic compounds (76AHC(20)117). Pyrimidines with electron-withdrawing groups and most quinazolines show this phenomenon of covalent hydration . Thus, in aqueous solution the cation of 5-nitropyrimidine exists as (164) and quinazoline cation largely as (165). These cations possess amidinium cation resonance. The neutral pteridine molecule is covalently hydrated in aqueous solution. Solvent isotope effects on the equilibria of mono- (166) and dihydration (167) of neutral pteridine as followed by NMR are near unity (83JOC2280). The cation of 1,4,5,8-tetraazanaphthalene exists as a bis-covalent hydrate (168). 

Proof for single-electron transfer in reactions of allyl- and crotylmagnesium bromide with benzophenone had been reported a few years before, in 1988, based on measurements of carbonyl carbon kinetic isotope effects (which were nonexistent) and substituent effect techniques [65]. Doubt about the correct interpretation of the observed phenomenon was discussed earlier in this chapter (see p. 237 [13]). 

Is any reasonable mechanism consistent with the data The answer lies in an observation of a probable isotope effect in a coupled nonenzymic phenomenon. The double-isotope fractionation method does not enter into the analysis. The keto group of glyoxalate is actually present as a covalent hydrate to the extent of about 99% of the total glyoxalate concentration (27). However, the ketone is the form that will react in the enzymic process and the concentration of ketone determines the rate of reaction and binding to the enzyme. The equilibrium between ketone and hydrate is not catalyzed by the enzyme and as a result the isotope effect on this equilibrium will appear in the measured kinetic isotope effects. Of course, the extent of this equilibrium will not be affected by deutera-tion of the methyl group of acetyl-CoA. Therefore, the observed HVIK) is not an indication of kinetically significant carbon-carbon bond formation but of a preequilibrium hydration, a process that is independent of the enzyme. The value for HV/K) of 1.0037 is consistent with measured equilibrium isotope effects in related molecules (23). Therefore, the deuteration of acetyl-CoA has no effect on the observed kinetic because that value in fact is due to a preequilib-... 

Molecules that are chemically identical except for containing different isotopes react at different rates. For example, it is the difference in rates of electrolysis that allows D2O to be obtained by the electrolysis of water, even though the relative abundance of D compared to H is 1 6000. This phenomenon is known as the kinetic isotope effect. A primary kinetic isotope effect occurs when isotopic substitution has been accomplished so that the... 

After the above discussion of several electrophilic aromatic substitutions which show primary kinetic isotope effects, it might be appropriate to summarize the structural characteristics which cause the occurrence of a rate-limiting proton release and, therefore, an isotope effect. There is one well established structural phenomenon known today, namely steric requirements of the intermediate, and another which is tentatively proposed here, namely formation of a stable quinoid intermediate, the... 

The equality A17 = -[(1 -a)/a]Ai i can be checked as follows. As is known, the kinetic isotopic effect in the case of hydrogen ion discharge is, to an appreciable extent, dependent on the potential. Irrespective of how this phenomenon is explained, it is obviously determined by the electric field directly acting upon the ion, i.e.,... 

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