| Regional Biophysics Meeting 2005, March 16-20, Zreče, Slovenia | [OtherTopics] |
In the last two decades a number of investigations of enzymatic reactions were reported in which hydrogen transfer was involved as an essential step. It was claimed to occur by a hydrogen tunneling process, because one of the following three conditions was found to be fulfilled: (1) The value of the Swain-Schaad exponent ln(kH/kT)/ln(kD/kT) > 3.3 was found, where kH, kD and kT are the rate constants for the transfer of H, D and T atom, respectively. (2) The ln(kH) did not depend linearly on 1/T, T being the absolute temperature. Non-Arrhenius behaviour was thus found. (3) kH/kD was found very large, typically greater than 10. Each of the above conditions by itself has been generally regarded as being sufficient to indicate that tunneling is involved in the hydrogen transfer process. It is shown in the present work that the condition (1) or (2) could also be fulfilled due to dynamical effects, in absence of any tunneling. The mechanism of the hydrogen transfer (tunneling or over-the-barrier transition) in the enzymatic reactions in question thus remains unexplained.
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