Kinetics of Oxidation of Vitamin B_1 and Its O-Acyl Analogs with Ferricyanide. A Mechanistic Model of Thiamin-binding Protein

摘要

The rate constants of oxidation of vitamin B1 and its O-acyl derivatives with ferricyanide in phosphate buffer at pH 7.2—7.8 were determined. The kinetics of the oxidation of thiamine,O-(2-norbornyl-carbonyl)thiamine,O-(l-adamantylcarbonyl)thiamine, O-(5-methyl- 1 -adamantylcarbonyl)thiamine, O-(2-adamanty!carbonyf)thiamine,O-(2-adamantyi)acetyfthiamine, and O-(f -adamantyi)acetyithiamine, followed by spectrophotometry by the formation of the corresponding thiochrome, fits first order both in substrate and in oxidant. With excess ferricyanide, the reciprocal of the apparent preudo-first-order rate constant increases with increasing ferrocyanide concentration. The intramolecular assistance by the hydrocarbon fragment of O-acyl-substituted thiamine is explained by the formation of an intermediate neutral tricyclic form of the substrate in a stage preceding electron transfer on ferricyanide. The reaction is assumed to mimic the effect of the hydrophobic environment during the transformation of vitamin ~ into the neutral tricyclic form on the surface of thiamin-binding protein.