Abstract
DURING the past decade, the concept (see, for example, ref. 1) of a carbanion intermediate (ylid formation, II) for the action of thiamine pyrophosphate (TPP, Ib) in its enzymic reaction has greatly facilitated the understanding of the chemical role of this coenzyme. The pKa of the CH acid in question, however, has been estimated in the range from 14 (ref. 2) up to 20 (ref. 3). Assuming a diffusion controlled recombination rate constant, which is about 107 I mol−1 s−1 for protolysis of CH acids4 the dissociation rate is of the order of 10−1 s−1 or less. This is in conflict with results obtained by the study of deuterium exchange by NMR measurements5. The results have yielded an exchange rate constant of the base catalysed process of the order of 106 s−1.
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