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

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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 KA= 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.

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