Hydrolysis of Thiamin: Evidence for Rate-Limiting Breakdown of the Tricyclic Dihydrothiachromine Intermediate in Neutral Aqueous Solution

Rate constants in the pH range 3-9 for formation of the enethiolate product upon hydrolysis of N(1′)-unprotonated ("free") thiamin (1a),N(1′)-nethylthiamin (3), oxythiamine (4), and several 3-substituted-4-methylthiazolium ions in oxygen-containing and amine buffers have been determined by irreversible iodination of the enethiolate at 25°C and ionic strength 1.0 M in aqueous solution. General acid catalysis of the ring opening of 1a has a Brønsted α value of 0.57 and a solvent deuterium isotope effect of (kH2O/kD2O)BL+ = 5.0 for catalysis by acetic acid. The Hammett ρI value for catalysis by water or phosphate monoanion of the ring-opening reaction of 3-R-4-methylthiazolium ions is 12 ± 1 for substituents on the nitrogen atom of the thiazolium ring that cannot function as intramolecular catalysts. There is a 200- or 600-fold positive deviation of the rate constant for catalysis by water or phosphate monoanion, respectively, of the ring-opening reaction of 1a from the corresponding Hammett correlation which is absent when the N(3)-aminopyrimidinyl substituent is modified to make it unreactive as an intramolecular catalyst in 3 or 4. On the basis of an estimated "effective molarity" of ≥700 M for intramolecular catalysis of the ring-opening reaction by the exocyclic 4′-amino group, it is concluded that the N(3)-aminopyrimidinyl substituent of 1a provides significant intramolecular nucleophilic catalysis of nonenzymatic enethiolate formation in neutral aqueous solution. The general acid-catalyzed reaction of 1a is formulated as concerted general acid catalysis of departure of the enethiol from the neutral tricyclic amine addition campound (TN) derived from addition of the 4′-amino group to the C(2) position of the thiazolium ion. The water-catalyzed reaction of 1a is formulated as unassisted expulsion of the enethiolate from TN.