Thiamine: a mechanistic model of interaction with protein

Mechanistic model of thiamine-binding protein functioning which is based on the potential role of prototropic groups and hydrophobic environment around 5-beta-hydroxyethyl substituent of ligand has been proposed. As a model the chemical transformations of thiamine and its structural O-acyl substituted analogues in the presence of ferricyanide and phosphatic buffer in pH range 7,2-7,8 were investigated. The oxidation to the thiochrome and thiochrome derivatives is first order in substrate and ferricyanide concentrations. It is found that the reciprocal of the pseudo-first-order rate constant increases in ferrocyanide concentration at the constant oxidant concentration. Rate constants and partition ratios for reaction of thiamine, O-benzoylthiamine, O-(4-nitrobenzoyl)thiamine, O-(2-norbornoyl) thiamine, O-(1-norbornoyl)thiamine, O-(1-adamantoyl) thiamine, O-(2-adamantoyl) thiamine, O-(5-methyl-1-adamantyl)acetylthiamine, O-(2-adamantyl)acetylthiamine, O-(1-adamantyl)acetylthiamine were determined. The acceleration effect of hydrophobic fragment of O-acyl substituent is attributed to the formation of neutral tricyclic form in the step followed by electron transfer to ferricyanide. Mechanistic implications for possible transformation of thiamine in neutral tricyclic form at interaction with thiamine-binding protein are discussed.