A-Secondary isotope effects

A kinetic isotope effect that results when the bond to the isotopic atom is not broken is called a secondary isotope effect. Here are two examples ... 

In these examples B is a base. The first example is called a secondary isotope effect of the first kind, the next one is a secondary isotope effect of the second kind. The distinction between these is that in the first kind bonds to the isotopic atom have undergone spatial (i.e., structural) change. Halevi has reviewed secondary isotope effects on equilibria and rates. 

Deuterium Substitution. The a and P secondary isotope effects affect the rate in various ways (p. 298). The measurement of a secondary isotope effects provides a means of distinguishing between SnI and Sn2 mechanisms, since for Sn2 reactions the values range from 0.95 to 1.06 per a D, while for S l reactions the values are higher. This method is especially good because it provides the minimum of perturbation of the system under study changing from a H to a D hardly affects the reaction, while other probes, such as changing a substituent or the polarity of the solvent, may have a much more complex effect. 

Hi) Oxidation of C2D4 produces only a secondary isotope effect ... 

These are reasonable KIEs for a secondary isotope effect in a 1,2-C migration, and originate from the hybridization changes at the migrant carbon (sp2,6 -> sp24) QMT is therefore not invoked.77... 

In reaction (10.11) the deuterium isotope effect is a secondary isotope effect, that is one in which the bonding to the isotopically substituted atom is not broken or formed during the course of the reaction. Secondary deuterium isotope effects are generally much smaller than primary ones. 

Equation 14.39 is relatively simple for a secondary isotope effect because neither E nor E is expected to be isotope dependent for 3-H/D isotope effects. To illustrate, Rabinovitch and Setzer (reading list) considered 2,3 C-C bond rupture of n-perprotiobutane and 1,4 ditrideutero-n-butane... 

If secondary isotope effects arise strictly from changes in force constants at the position of substitution, with none of the vibrations of the isotopic atom being coupled into the reaction coordinate, then a secondary isotope effect will vary from 1.00 when the transition state exactly resembles the reactant state (thus no change in force constants when reactant state is converted to transition state) to the value of the equilibrium isotope effect when the transition state exactly resembles the product state (so that conversion of reactant state to transition state produces the same change in force constants as conversion of reactant state to product state). For example in the hydride-transfer reaction shown under point 1 above, the equilibrium secondary isotope effect on conversion of NADH to NAD is 1.13. The kinetic secondary isotope effect is expected to vary from 1.00 (reactant-like transition state), through (1.13)° when the stmctural changes from reactant state to transition state are 50% advanced toward the product state, to 1.13 (product-like transition state). That this naive expectation... 

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. 

Ah/At = 7.4 and A /Ax = 1.8 and isotopic activation energy differences that are within the experimental error of zero. The values of the two A-ratios correspond to a Swain-Schaad exponent of 3.4, not much different from the semiclassical expectation of 3.3. The a-secondary isotope effects are 1.19 (H/T), 1.13 (H/D), and 1.05 (D/T), which are exactly at the limiting semiclassical value of the equilibrium isotope effect. The secondary isotope effects generate a Swain-Schaad exponent of 3.5, again close to the semiclassical expectation. At the same time that the isotope effects are temperature-independent, the kinetic parameter shows... 

Rp is an exponent that describes the relationship of a primary isotope effect with H in the secondary position to a primary isotope effect with D in the secondary position. Rs is an exponent that describes the relationship of a secondary isotope effect with H in the primary position to a secondary isotope effect with D in the primary position. According to the Rule of the... 

The observation of systematic deviations from the Rule of the Geometric Mean between primary and a-secondary isotope effects. The magnitude of both effects should be greater (more normal) when the other position is occupied by a lighter isotope than when it is occupied by a heavier isotope. 

An enzyme reaction intermediate (Enz—O—C(0)R or Enz—S—C(O)R), formed by a carboxyl group transfer (e.g., from a peptide bond or ester) to a hydroxyl or thiol group of an active-site amino acyl residue of the enzyme. Such intermediates are formed in reactions catalyzed by serine proteases transglutaminase, and formylglyci-namide ribonucleotide amidotransferase . Acyl-enzyme intermediates often can be isolated at low temperatures, low pH, or a combination of both. For acyl-seryl derivatives, deacylation at a pH value of 2 is about 10 -fold slower than at the optimal pH. A primary isotope effect can frequently be observed with a C-labeled substrate. If an amide substrate is used, it is possible that a secondary isotope effect may be observed as welF. See also Active Site Titration Serpins (Inhibitory Mechanism)... 

However, quantitative evaluation of the size of this preference depended on knowing the size of the secondary deuterium isotope effect on which C—C bond in 7b cleaves. With the seemingly reasonable assumption of a secondary isotope effect of 1.10 on bond cleavage, the experimental data led to the conclusion that double methylene rotation was favored over single methylene rotation by a factor of 50 in the stereomutation of 7b. Although the error limits on the measurements were large enough to allow the actual ratio to be much smaller, Berson wrote, There is no doubt that the double rotation mechanism predominates by a considerable factor. ... 

Vaska118 found a shift of v(CO) in trans- but not cis-X—M—CO arrangements of about 20-30 cm-1 to higher frequency when X = D. He ascribed this result to coupling effects. Halpern119 showed that kH/k0 for the replacement of Cl- by pyridine in frans-PtHClLj is 1.44, unexpectedly large for a secondary isotope effect. He ascribed this to Pt—H bond weakening in the transition state. 

A secondary isotope effect is one that results from isotopic substitution at a bond not being broken in the reaction. As the reaction cordinate, not being affected by the substitution, does not make any contribution, the secondary effects must arise solely from changes of zero-point energies of ordinary vibrations. Thus if an isotopically substituted C—H bond experiences a change of force constant on going from reactant to transition state, the effect is approximately... 

Solvolysis of 15 in 97% trifluoroethanol gave a secondary isotope effect of 1.17, which indicates a vertically stabilized transition state. Thus the highly unsymmetrical dihedral dependence of silicon participation can almost entirely be attributed to the hyperconjugation model with little non-vertical involvement of the silicon nucleophile. 

A value of kA is available from measurements of hydrogen isotope exchange (kx) corrected for primary and secondary isotope effects. This is illustrated in Scheme 8 for the detritiation of tritiated benzene. In the lower part of the scheme, the reacting isotope of the benzenonium ion intermediate (9-t) is indicated as a superscript on the rate constant kv and a secondary isotope effect is neglected. 

Felder, T., Schalley, C.A. Secondary isotope effects on the deslipping reaction of rotaxanes high-precision measurement of steric size, Angew. Chem. 115 (2003), 2360-2363 Angew.Chem. Int. Ed. 42, (2003), 2258-2260. 

In equation (153) the proton in SH is not in isotopic equilibrium with the solvent and therefore does not contribute to the solvent isotope effect (which is entirely a secondary isotope effect in this instance), although its replacement by deuterium prior to the kinetic experiment would result in the observation of a primary isotope effect. 

In this review, primary isotope effects are defined as concerning reactions in which, at some stage, a bond to the isotopically substituted atom is either broken or formed. A secondary isotope effect will refer to reactions in which no bond to the isotopically substituted atom is broken and none is formed. The strengths of the bonds to the substituted atoms may, however, be altered and, in general, will be to some degree. 

Just as in the preceding examples, early indications of tunneling in enzyme-catalyzed reactions depended on the failure of experiments to conform to the traditional expectations for kinetic isotope effects (Chart 3). Table 1 describes experimental determinations of a-secondary isotope effects for redox reactions of the cofactors NADH and NAD+. The two hydrogenic positions at C4 of NADH are stereochemically distinct and can be labeled individually by synthetic use of enzyme-catalyzed reactions. In reactions where the deuterium label is not transferred (see below), an... 

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