Elucidation of the chemistry of enzyme-bound thiamin diphosphate prior to substrate binding: defining internal equilibria among tautomeric and ionization states

The pH dependence of tautomeric forms providing the first data for the internal thermodynamic equlibria on thiamin diphosphate enzymes are reported. The dramatic influence of the protein on the internal equilibria points to conditions under which the long-elusive ylide intermediate could be stabilized.