Jacques Emile Dubois

Kinetics and thermodynamics of the structural transformations of thiamine in basic aqueous media. Part 3. Interpretation of the lability of the 2-proton via an intramolecular σ-adduct

The structural transformations of thiamine into its yellow form J– in very basic media (pH > 11) have been investigated by pH stopped-flow jump and temperature-jump techniques. This transformation via the deprotonation of the thiazolium 2-position to give the resonance-stabilized carbene A–H, hitherto considered as the biocatalyst of vitamin B1 metabolic activity, is questioned, since deprotonation of the thiamine 2-position is unlikely for pK < 15. Formation of the J– species occurs through the σ-adduct D resulting from intramolecular NH2 base-promoted nucleophilic addition at the thiazolium 2-position. This ring formation occurs with a second-order rate constant k14of 99 l mol–1 s–1. σ-Adduct D generates J– by a base-promoted reaction: k45 5.15 × 106 l mol–1 S–1. The deuterium isotope effect k45(H2O)/k45(D2O) 2.05 and ΔpK4 0.75 for this D ⇄ J– reaction imply the involvement of several proton transfers. It is surmised that this D ⇄ J– transformation takes place through the deprotonation of the 2-position of D into a carbanion intermediate D– which by prototropic ring-opening yields J–. This hypothesis accounts for the half-life of the 2-proton, known from n.m.r. measurements in neutral aqueous media. This tentative mechanism answers a long standing question on the role played by the amino pyrimidine moiety in the biocatalytic activity of thiamine. The existence of such a carbanion D– species as a candidate among the catalysts which monitor the metabolic activity of vitamin B1 is, therefore, proposed with caution.

Influence of carboxylic acid association upon the lactim–lactam tautomeric equilibrium of 2-hydroxypyridines

I.r. and u.v. absorption spectroscopy in CCl4 at room temperature provides evidence for lactim–acid and lactam–acid heterodimer formation in 6-chloro-2-hydroxypyridine–acetic acid mixtures. Measurements of association constants for two types of 1:1 hydrogen-bonded complexes reveal preferential association with the lactam tautomer, leading to a shift in the apparent tautomeric equilibrium constant. These results suggest that specific interactions are of great importance in understanding solvent effects on protomeric equilibria.