Lessons from thiamin-watching
Thiamin is a vitamin that is a catalyst for reactions involving formation of acyl carbanion equivalents. The closely related species, thiamin diphosphate, is associated with enzymes that catalyze the decarboxylation of a-ketoacids. While the enzymes attract the ketoacid, thiamin diphosphate adds to it, leading to release of carbon dioxide. Intermediates in the reactions can be synthesized. Kinetic analysis of their reactions reveals what the enzyme overcomes in each step. In the laboratory, thiamin reacts with benzaldehyde to cause a benzoin condensation. In neutral solution, benzaldehyde destroys thiamin.. Analysis of the products and kinetics of destruction reveals the steps by which it occurs: addition and proton transfers followed by a fragmentation step that is itself a mechanistic puzzle.