Tritium kinetic isotope effects

TABLE 11. The primary hydrogen-tritium kinetic isotope effects found in the reactions between various alkyl radicals and tributyltin hydride and tributyltin hydride-t... 

TABLE 9. The carbon-12/carbon-14 and secondary alpha hydrogen-tritium kinetic isotope effects for the Sat2 reactions between Y-substituted /VA-dimethylanilines and Z-substituted benzyl X-substituted benzensulfonates in acetone at 35°C ... 

Karsten, W.E., Hwang, C.C. and Cook, P.F. (1999). Alpha-secondary tritium kinetic isotope effects indicate hydrogen tunneling and coupled motion occur in the oxidation of L-malate by NAD-malic enzyme. Biochemistry 38, 4398-4402... 

Isotope effects have also been applied extensively to studies of NAD+/NADP+-linked dehydrogenases. We typically treat these enzymes as systems whose catalytic rates are limited by product release. Nonetheless, Palm clearly demonstrated a primary tritium kinetic isotope effect on lactate dehydrogenase catalysis, a finding that indicated that the hydride transfer step is rate-contributing. Plapp s laboratory later demonstrated that liver alcohol dehydrogenase has an intrinsic /ch//cd isotope effect of 5.2 with ethanol and an intrinsic /ch//cd isotope effect of 3-6-4.3 with benzyl alcohol. Moreover, Klin-man reported the following intrinsic isotope effects in the reduction of p-substituted benzaldehydes by yeast alcohol dehydrogenase kn/ko for p-Br-benzaldehyde = 3.5 kulki) for p-Cl-benzaldehyde = 3.3 kulk for p-H-benzaldehyde = 3.0 kulk for p-CHs-benzaldehyde = 5.4 and kn/ko for p-CHsO-benzaldehyde = 3.4. 

The mechanism of hydrogen evolution has been investigated by impedance measurements [371] and hydrogen-tritium kinetic isotope effects [375]. The effect of halides dissolved in solution has been studied [372, 376] these ions increase the overpotential in the sequence Cl- Adsorption isotherms for halides have been derived. They conform to the Temkin adsorption model with partial charge transfer. The lateral interaction between adsorbed particles has been calculated. It is higher for Br- than for I- and increases with overpotential on account of the weakening in the metal-halide bond. Thus, halides are substantial poisons for hydrogen evolution on iron. Poisons also include metal ions such as Cd2+, Zn2+, and Mn2+ [26]. 

The data given in Table 7 indicate that the deuterium-tritium kinetic isotope effect in hydrogen exchange with bases is in fact positive and lies within the limits of approximate theoretical estimates (Bigeleisen, 1949 Melander, 1960 Zollinger, 1958). The differences in the values... 

Protium/deuterium/tritium kinetic isotope effects are often used to support hydride transfer mechanisms over single electron transfer mechanisms. However, sequential electron/proton/electron transfer mechanisms can easily show isotope effects as well. Even though the rate limiting step in the overall two electron reduction of flavin or NADH may be the isotope independent endergonic electron tunneling to form a radical intermediate state, once formed, this radical state can return the electron to recreate the... 

Evidence for the dual proton and electron tunneling process required for simultaneous electron and proton transfer has been obtained from both enzymatic and model systems. In biomolecular systems, the strongest evidence for proton tunneling processes is obtained from kinetic isotope effects, where both anomolously high protium-tritium kinetic isotope effects and distinct temperature profiles for these isotope effects can be used as diagnostic tools for proton tunneling events [57]. 

Large primary kinetic isotope effects have been measured for the H-atom transfer steps from substrate to dAdo and from dAdo to the product radical in a number of AdoCbl-dependent enzymes as indicated in Table 19.1. In methylmalonyl-CoA mutase, the steady-state deuterium isotope effect is 5-6 in the forward direction, and the intrinsic isotope effect of step (i) in Scheme 19.3 is masked by the kineti-cally coupled but slower later steps [37-39]. The steady-state tritium kinetic isotope effect kii/kj) in the forward direction has been reported to be 3.2 [38]. Note that the experiments with deuterium were performed with a fully deuterated methyl group, while those with tritium were carried out at the trace level and correspond to a single isotopic atom therefore these two isotope effects should not be directly compared. For the reverse reaction, the deuterium kinetic isotope effect is also par-... 

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