Primary deuterium isotope

Because solvent viscosity experiments indicated that the rate-determining step in the PLCBc reaction was likely to be a chemical one, deuterium isotope effects were measured to probe whether proton transfer might be occurring in this step. Toward this end, the kinetic parameters for the PLCBc catalyzed hydrolysis of the soluble substrate C6PC were determined in D20, and a normal primary deuterium isotope effect of 1.9 on kcat/Km was observed for the reaction [34]. A primary isotope effect of magnitude of 1.9 is commonly seen in enzymatic reactions in which proton transfer is rate-limiting, although effects of up to 4.0 have been recorded [107-110]. 

A primary isotope effect results when the breaking of a carbon-hydrogen versus a carbon-deuterium bond is the rate-limiting step in the reaction. It is expressed simply as the ratio of rate constants, i wlky,. The full expression of k /kn measures the intrinsic primary deuterium isotope for the reaction under consideration, and its magnitude is a measure of the symmetry of the transition state, e.g., -C- H- 0-Fe+3 the more symmetrical the transition state, the larger the primary isotope effect. The theoretical maximum for a primary deuterium isotope effect at 37°C is 9. The less symmetrical the transition state, the more product-like or the more substrate-like the smaller the intrinsic isotope effect will be. 

A label (L = D or T) can be introduced in the transferable position (left subscript of the rate constant) or in the non-transferable position (right subscript of the rate-constant) Then A hh/ dh hd/ dd primary deuterium isotope effects, while hh/ hd dh/ dd secondary deuterium isotope effects. In the general case, we can relate each of the two effects by ... 

In 1895, Emil Ficher proposed an enediol intermediate for this isomerization. As would be expected, the enzyme-catalyzed isomerization of glucose-6-phosphate in 2H20 is accompanied by incorporation of deuterium into the product fructose 6-phosphate at C-l. In the reverse reaction 2H-containing fructose 6-phosphate was found to react at only 45% of the rate of the 1H-containing compound. Thus, the primary deuterium isotope effect expected for a rate-limiting cleavage of the C-H bond was observed (see Chapter 12, Section B,3). 

Acyl complex XVI was prepared from 3-perdeuteriophenyl-3,3-dideu-teriopropionyl chloride (XV), and its rate of decarbonylation (Reaction 9) along with that of the undeuteriated complex was measured at 80 °C. The rate constants (KH = 3.34 X 10 5, Kv = 4.75 X 10"6) show a primary deuterium isotope effect of 7.04. These results are also consistent with... 

Ab initio calculations at the MP2/6-31+ G level have been performed for gas-phase El elimination reactions of CH3CH2X (X = NH3"1", Br, Cl, F, SH) promoted by NH . OH-, F-, PH2. SH-, and Cl- in order to determine how changes in transition-state geometry, from reactant-like to product-like, influence kinetic isotope effects.9 Secondary isotope effects (a-H) on leaving group departure are correlated with the hybridization at C7 in the transition state, whereas there is no such correlation between secondary (/5-H) isotope effects and the transition state hybridization at C/ . The primary deuterium isotope effect is influenced markedly by the nucleophilic atom concerned but approach to a broad maximum for a symmetric transition structure can be discerned when due allowance is made for the element effect. 

Elimination reactions of ( )- and (Z)-benzaldehyde Opivaloyloximes (19a) and (19b) with DBU in MeCN have been found to occur by a nitrile-forming E2 mechanism which is ca 2000-fold faster for the latter isomer in each case.15 The corresponding Hammett substituent constants, activation parameters, and primary deuterium isotope effects, suggest that the anti elimination from (19b) (for which p = 2.4 0.1, H/ D = 2.7 0.3, A/H = 12.5 0.2 kcal mol-1, and A= —31.0 0.6eu) proceeds to (20) via a more symmetrical transition state with a smaller degree of proton transfer, less charge development at the jS-carbon and greater extent of triple bond formation than for syn elimination from (19a) (for which p = 1.4 0.1, kn/kn = 7.8 0.3, AH = 8.8 0.1 kcal mol 1 and A= -23.6 0.4 eu). 

The experiments above indicate that the solvent dependence of < >d arises primarily from the solvent dependence of the triplet state reactivity. The absence of a primary deuterium isotope effect makes it unlikely that protic co-solvents affect < >d through a bond-breaking interaction (e.g., photochemical proton transfer). Also, since... 

Intermolecular kinetic isotope effects depend strongly upon reactant properties. Consider the extreme case of a primary deuterium isotope effect in a molecule with only one hydrogen replaced by deuterium, e.g. the rate coefficient for (CDCl3)f - (CC13)+ + D compared with that for... 

The reaction of 3-unsubstituted isoxazoles with bases, leading to ring opening, has been known for almost 90 years, and has many applications. The mechanism of the reactions with hydroxide ion has been studied by Italian workers.156 157 The reactions showed second-order kinetics (first-order in base and in substrate) and the UV spectra of reaction mixtures showed sharp isosbestic points, indicating the transformation of reactants into products without formation of an intermediate. A primary deuterium isotope effect indicated that fission of the C3—H bond was rate-determining. 

In several investigations of the azo coupling of arenediazonium ions with aromatic amines in organic solvents general base catalysis and consequently a primary deuterium isotope effect were observed... 

The other method for determining intrinsic isotope effects involves measuring deuterium and isotope effects on the same step (21). Equation 26 is used for primary and secondary deuterium isotope effects, and for isotope effects with unlabeled and primary or secondary deuterated substrates. In the latter cases, the commitments are reduced by the size of the intrinsic deuterium isotope effects, which gives five equations in the five unknowns P k, °k, k, Cf, and Cr. Although the errors for the intrinsic isotope effects are reasonable, those on Cf and Cr are large, but their sum is well determined (22). For irreversible decarboxylations where Cr is small, one needs only the primary deuterium isotope effect and the one with unlabeled and deuterated substrates to solve for P k, k, and Cf. 

Figure 3 Evidence for a stepwise removal of hydrogen by membranous stearoyl A desaturase (SCD) initiated at C-9 Primary deuterium isotope effect observed at C-9 but not at C-10 (a) Sulfoxidation of S-9 preferred over S-10 thiasubstrates (b).

For PHBH to function as an efficient catalyst, the series of four conformational changes in a catalytic cycle have to be fast and coordinated compared with the chemical reactions of catalysis. For example, the observation that the reduction of flavin under optimal conditions for catalysis exhibits a full primary deuterium isotope effect (13) implies that the rate of reduction of flavin is limited by hydride transfer and not by conformational rearrangements. However, when the enzyme is stabilized in the in conformation (as with the mutant form, Ala45Gly), then a large fraction of flavin reduction becomes much slower under the same conditions and shows only a small deuterium isotope effect (21). 

The consistancy of the observed /4-factors, the magnitude of the activation entropies, the a correlations in the substituent effects at the a- and /8-carbon positions, the collective influence of all three substitutable centers on the reaction rates, the importance of charge stabilization in the transition state, and the primary deuterium isotope effects on the alkyl acetic acid ester decomposition, all favor the concerted polar 6-center transition state shown below (IV). However, an alternative possibility involving intimate ion-pair formation has been proposed by Scheer et al.. ... 

With proton transfer (k ) rate determining, the primary deuterium isotope effect is satisfied as well as the sensitivity of the reaction rates to (C-O) bond rupture. The latter is reflected in the (kjk2) ratio. It was noted that rearrangement in the carbonium ion of the ion pair (e.g., loss of optical activity at the a-carbon center in the reactant ester, Atj > fcj) would substantiate the ion pair mechanism. Results of this kind of experiment in the stereospecific m-elimination that is observed in ester pyrolyses is not compatible with the ion-pair mechanism. For example, the acetate of the erythro-isomsT of 2-deutero-l,2-dipenylethanol gave /runs-stilbene with predominant retention of deuterium (95.6 1.7%). In contrast, the threo-isomer gave Iranj-stilbene with predominant loss of deuterium (26.4+0.6% deuterium) . 

The normal primary deuterium isotope effect in the first part shows that an OH bond is being broken in tile rate-detertnining step. Imidazole is loo weak a base to remove the OH proton completely so its role must be as general base catalyst. Attack on the carbonyl is the slow step with (aster breakdown of the tetrahedral intermediate arid hydrolysis of the lactone. Lactones arc hydroly,sed faster than esters because they lack anomeric stabilization (p. 1134). [ he role ttf the OH group is intramolecular nucleophilic catalyst. 

Labels Stereochemistry and Chirality Kinetics Solvent Effects Substituent Effects Primary Deuterium Isotope Effects Barrier Data... 

---