Temperature Dependence of Isotope Effects

The normal temperature dependence of isotope effects (see Chart 1) 37... 

This extraordinary study shows with great clarity how far we now are from a reliable understanding of either the linkage between protein flexibility and tunneling in the most general sense, or the physical models that underlie anomalous temperature dependences of isotope effects. 

Lewis et al. (entry 11 of Table 2) examined the temperature-dependence of isotope effects in the action of both the human enzyme and the soybean enzyme, by measuring the relative amounts of per-protio and per-deuterio-13-hydroperoxy-products by HLPC. The observed effects are, therefore, composed of primary, secondary, and perhaps remote isotope-effect contributions. Isotope effects on fecat/ M for both enzymes (determined by competition between labeled substrates) are increased by high total substrate concentration, an effect previously observed but stiU ill-understood. At 100 /rM substrate, the effects are roughly independent of temperature below about 15 °C, and are about 60 (H/D) for the human enzyme and 100 (H/D) for the soybean enzyme. Above 15 °C, the effects decline to about 50 for the human enzyme and about 60 for the soybean enzyme, perhaps because non-isotope-sensitive steps become more nearly rate-limiting (see Chart 4). 

Stem MJ, Spindel W, Monse EU (1968) Temperature dependence of isotope effects. J Chem Phys 48 2908... 

THE NORMAL TEMPERATURE DEPENDENCE OF ISOTOPE EFFECTS (SEE CHART 1)... 

Finally the temperature dependence of the primary isotope effects was determined. Here the traditional expectations of Chart 3 were fully met the results translate into AH/AD = 1.1 0.1, aD — aH = 0.8 kcal/mol. Thus the amount of tunneling present, adequate to produce the observed exaltation of secondary isotope effects, violations of the Swain-Schaad relationship, and violations of the Rule of the Geometric Mean in the neighborhood of room temperature, does not lead to anomalies in either the ratio of isotopic pre-exponential factors nor the isotopic activation energy difference over the temperature range studied (approximately 0-40 °C). As will be seen later, the temperature dependence of isotope effects for reactions that include tunneling is in general a complex, unresolved issue. 

The letter A shows that an observed Arrhenius temperature dependence of isotope effect was used in the reduction to 25 °C a number is the temperature of measurement in °C with reduction to 25° by Eq. (9). T denotes use of tritium and Eq. (10). 

The results from the rudimentary model for isotope effects on a hydrogen transfer reaction shown in Fig. 11.3 are plotted in Arrhenius style as their logarithmic form versus inverse temperature. An analysis based on the temperature dependence of isotope effects is another standard method for detecting tunneling [25, 26], and again, non-tunneling models are helpful as a basis for comparison. For many ex-... 

The effects of complex mechanisms with serial or parallel shifts in rate limiting steps must also be considered in the analysis of the temperature dependence of isotope effects [28]. tn addition, tedious attention to details of temperature effects on pH, acidity constants, reaction volumes, and substrate or catalyst stabilities may be needed in some cases to avoid problematic interpretations. For these reasons, temperature studies of isotope effects may not be as convincing as the observation of very large isotope effects in providing evidence for tunneling. 

Interpreting Temperature Dependence of Isotope Effects in Terms of H-Tunneling 11343... 

Spindel W, Stem MJ, Mouse EU (1970) Further study on temperature-dependences of isotope effects. J Chem Phys 52 2022-2035... 

Nomura, M., and Suzuki, T. (2006) Temperature dependence of isotope effects in uranium chemical exchange reactions. 

The criteria that have been used - curved Arrhenius plots, large isotope effects, and the excessive temperature-dependence of isotope effects - are all internally consistent and can best be explained by important tunnel corrections. 

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