Isotope effects hydrogen migration

The specificity of the enzyme for the diastereotopic hydrogen atoms at the C-l of glycerol was also determined using (1 / , 2 / )-[ 1 -2 H, ]glycerol as a substrate (see Chapter 3 for the method of preparation). Again, in spite of an isotope effect deuterium migration was observed exclusively to furnish, after chemical oxidation, ( + )-(S)-3-[2-2H,]hydroxypropionic acid (Fig. 13). 

The large isotope effect for migrating hydrogen relative to migrating deuter-... 

OTHER EXAMPLES OF KINETIC ISOTOPE EFFECTS. The power of kinetic isotope effects in enzymol-ogy is well illustrated in the work of Rose ° and Knowles deahng with hydrogen effects in proton transfer to and from carbon. Abstraction of a proton from a tetrahedral carbon is a fundamental step in many enzyme-catalyzed reactions. Intramolecular proton transfer as well as partial loss (wash-out) migrating protons have provided important clues in mechanistic investigations. Enol and enediolate formation constitute several... 

The major reaction product formed from the solvolysis of the trans y-silyl ester was cyclohexene formed from the carbocation 138 (Scheme 18) by 1,2-hydrogen migration to give the p-silyl cation 139 followed by loss of the silicon substituent. In contrast to the behavior shown by 136, the cis y-silyl ester 134 exhibited a small inverse p-d4 isotope effect (kH/kn = 0.97) attributed to the inductive effect of the P-deuteriums and implies that there is very little hyperconjugative stabilization of... 

The mass spectra of 7, obtained from the NADPH/02/P-450 oxidation of 5, and 8 are compared in Table IV. Three isotopic species are expected for the allylic hydroxylation of 5. Oxidation with removal of hydrogen will produce 7-d3and 7a-d3. Extensive controls have demonstrated that 7-d3 and 7a-d3 are indistinguishable by mass spectrometry because of rapid 1,3-migration of the siloxy group upon electron impact. Oxidation with deuterium removal leads to the production of 7-d2 (Scheme 4). By this analysis, the isotope effect is simply the ratio of 5-d3 (and 5a-d3) to 5-d2 in the oxidized sample. Deconvolution of the parent region (with appropriate correction for carbon and silicon... 

Fig. 18. The expected percentages of various labelled products of the dioldehydratase reaction using 25 as substrate. The calculation was based on the following facts and assumptions (1) The enzyme does not differentiate between the enantiotopic hydrogen positions (conclusion from experiments with species 17 and 18 shown in Fig. 14) (2) in the competition between vicinal hydrogen atoms there is an intramolecular kinetic deuterium isotope effect of 2.6 (Fig. 15) (3) this effect is 10 for geminal hydrogen atoms (4) the migrating hydroxyl group substitutes one of the hydrogen atoms in the vicinal position stereospecifi-cally (i.e., with inversion).

---