Thiamine pyrophosphate hydrochloride: stereochemical aspects from an x-ray diffraction study.

The crystal structure of thiamine pyrophosphate has been determined by a three-dimensional x-ray analysis. The conformation of the molecule in the crystalline state is determined by the formal charge distribution within the molecule which exists as a zwitterion with the negative pyrophosphate chain folded back over the positive, ring portion of the molecule. The oxygen atoms in the pyrophosphate group are in the staggered conformation when viewed along the phosphorus-phosphorus axis. Even though the pyrophosphate is present in this compound as the monoionized monoester, the configuration is the same as that present in the inorganic pyrophosphate ion. From a comparison of three different crystal structures containing the thiamine moiety and from studies with atomic models, it seems plausible that the basic molecular conformation observed in this crystal is maintained in the catalytically active molecule. Knowledge of the detailed crystal structure provides new insight into the biochemical mechanism of reactions catalyzed by thiamine pyrophosphate.