Kinetics and thermodynamics of the structural transformation of thiamine in neutral and basic media. Part 2. Ultraviolet spectrum of the intramolecular adduct

Abstract

Through chemical relaxation methods, it is shown that, in basic media, the thiamine yellow form J^– is transformed into thiolate C^–via intramolecular σ-adduct D, cation A⁺, and pseudo-base B. The formation of the pseudo-base B by the hydroxylation of the cation A⁺ is the rate-limiting step of the structural transformations of thiamine in aqueous media, with a second-order rate constant k₁₄ 19.2 l mol^–1 s^–1 which is in very good agreement with the rate constant measured earlier in mildly basic media where J^– is not involved. The reversible transformation of J^– into A⁺ was also analysed in neutral media where the yellow form J^– yields cation A⁺ by two distinctive steps. (i) The first is very fast and corresponds to the formation and accumulation of σ-adduct D.(ii) The second one is fast and corresponds to the acid-promoted ring-opening of σ-adduct D to yield cation A⁺, with a second-order rate constant k₆₁ 4.15 × 10⁵ l mol^–1 s^–1. σ-Adduct D is isolated as a kinetic product and its u.v. spectrum is reported. Thus, in neutral media (which are close to natural media) thiamine is in a very rapid equilibrium with species B and D. This means that these two tetrahedral adducts are available, for any biological reaction requiring their assistance, by shifting via A⁺ the global thiamine equilibrium in neutral media.