Kinetics and thermodynamics of the structural transformations of thiamine in aqueous media. Part 5. Interaction of thiamine with SDS micelles

Abstract

In basic micellar sodium dodecyl sulphate (SDS) media, the transformation of thiamine into its yellow form is slowed down, but still occurs via the intramolecular attack of the 4′-amino at the 2-position of A⁺ to generate the σ-adduct D, with apparent second-order rate constants varying from 25.50 to 39.50 dm³ mol^–1 s^–1 at SDS concentrations ranging from 0.1 to 0.05 mol dm^–3, respectively. This allows the measurement of the A⁺–micelle dissociation constant K_A= 1.6 × 10^–4 mol dm^–3 which indicates that one micelle interacts with one thiamine molecule and that the σ-adduct formation occurs in extramicellar water with a second-order rate constant (k₁₄= 99 dm³ mol^–1 s^–1) and in the micelle with a rate constant k′₁₄= 10.9 dm³ mol^–1 s^–1. The fast transformation of intermediate D into _J^–(second-order rate constant 5.15 × 10⁶ dm³ mol^–1 s^–1) competes with the interaction of D with the micelle. D seems to be instantly and partially solubilized by the SDS micelle, yielding J^– after the micelle dissociates into its monomers with a relaxation time of 150 × 10^–3 s. Moreover, the hydroxylation of thiamine to its pseudobase in the micellar medium is slower than in water. This transformation occurs mostly in extramicellar bulk water with a second-order rate constant k₁₂= 19.6 dm³ mol^–1 s^–1. It also occurs in the micelle where it is considerably retarded with an endomicellar second-order rate constant k′₁₂= 0.85 dm³ mol^–1 s^–1. D is the most hydrophobic thiamine species, its interaction with the SDS micelle may be of importance in the transport of the thiamine molecule through natural membranes.