Crystal Structure of Thiamin Thiazolone: A Possible Transition-State Analogue with an Intramolecular N-H...O Hydrogen Bond in the V Form

Article abstract of DOI:10.1021/ja00282a038

The pyrophosphate ester of thiamin thiazolone (TT) has been proposed to be a transition-state analogue for the thiamin pyrophosphate dependent enzymes due to its high affinity for the apoenzyme and its structural resemblance to the metastable enamine which is assumed to be the immediate product of decarboxylation of the pyruvate adduct of thiamin.TT, C12H16N4O2S, crystallizes in the monoclinic space group P2₁/n with a = 4.634 (2) Angstroem, b = 12.591 (6) Anstroem, c = 22.291 (10) Angstroem, β = 95.20 (4) grad, and Z = 4.The structure was solved by direct methods and refined to an R value of 0.041 for 987 observed reflections measured with Cu Kα radiation on a diffractometer.The molecular comformation of TT is quite different from the S comformation which is characteristic of other C(2)-substituted thiamins.Instead, TT assumes a V comformation (Φ_T = 104 grad, Φ_P = -74 grad) that has previously been observed only in oxythiamin which is a strong antagonist of thiamin.An intramolecular hydrogen bond between the 4'-amino group and C(2) oxygen stabilizes the V form.This is the first crystal structure showing that the 4'-amino group, whose functional role in thiamin catalysis is not well established, is involved in an intramolecular interaction.The structure of TT suggests that the active conformation of free thiamin in the holoenzyme may be the V form.