Kinetic isotope effects remote

Secondary kinetic isotope effects are observed if an isotopic label is located adjacent to or remote from the bond that is being broken or formed during the reaction. Again, these depend on the internal energy of the decomposing ions. Secondary kinetic isotope effects, 4ec, are generally much smaller than their primary analogues. 

Example Secondary kinetic isotope effects on the a-cleavage of tertiary amine molecular ions occurred after deuterium labeling both adjacent to and remote from the bond cleaved (Chap. 6.2.5). They reduced the fragmentation rate relative to the nonlabeled chain by factors of 1.08-1.30 per D in case of metastable ion decompositions (Fig. 2.18), but the isotope effect vanished for ion source processes. [78] With the aid of field ionization kinetic measurements the reversal of these kinetic isotope effects for short-lived ions (lO -lO" s) could be demonstrated, i.e., then the deuterated species decomposed slightly faster than their nonlabeled isoto-pomers (Fig. 2.17). [66,76]... 

Birck MR, Schramm VL (2004) Binding causes the remote [5 -3H]thymidine kinetic isotope effect in human thymidine phosphorylase. J. Am. Chem. Soc. 126 6882-6883... 

Secondary kinetic isotope effects provide one of the most subtle probes of reaction mechanism currently available as the perturbation of the system under study is small. Unfortunately, the interpretation of isotope effects is far from straightforward. There is no general agreement about the mechanistic significance of a-deuterium isotope effects in solvolytic reactions. The interpretation of more remote deuterium isotope effects appears to be even more complex. 

HEAVY ATOM AND REMOTE HYDROGEN KINETIC ISOTOPE EFFECTS IN GLYCOSYL TRANSFER... 

Table 3.3 Kinetic isotope effects for hydrolysis of various glycosyl derivatives. The isotopic quasi-racemate method was used for all measurements except those for AMP, where the remote label method was used.

NMR spectroscopy is one among many probes that have been reported for the evaluation of kinetic isotope effects. Several NMR methods, as analogs of previous proton inventory techniques involving classical kinetic methods were reported, involving line-shap>e analyses and polarization transfer experiments on the exchanging protons or deuterons and/ or on the remote spins as functions of the deuterium atom fraction n in the mobile proton sites. Moreover, the kinetic isotope effects and the number of transferred protons originating from... 

Subsequent work confirmed this apparently abnormal behaviour. Deuteriation at remote sites (the S- or e-position) induces small inverse secondary isotope effects in a-cleavages occurring in the ion source, but normal isotope effects in the decomposition of metastable ions in the field-free regions94,95. The time dependence of the isotope effect was also studied by field ionization kinetics, which permit the analysis of fragmentations occurring after lifetimes as short as 10 12 s-1. It was found that the inverse isotope effect favouring loss of the deuteriated radical operates at times shorter than 10 9 s95. 

Table 1. Kinetic isotope and steric effects observed in the remote bond activation of metal-ion complexes of several functionalized alkanes3 ...

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