Computational Studies on the Decarboxylation of 2-(1-Carboxy-1-hydroxyethyl)-3,4-dimethylthiazolium Dipolar Ion, an Analog of the Complex of Pyruvic Acid and Coenzyme of Pyruvate Decarboxylase

We have obtained the optimized geometrical structure of 2-(1-carboxy-1-hydroxyethyl)-3,4-dimethylthiazolium dipolar ion and investigated its geometric and electric changes during decarboxylation process by the MNDO-PM3 method, a molecular orbital method. The salient features of the optimized structure are that the dihedral angle of C4-C1-C2-S3 is 5.4° and the distance between thiazolium S3 and carboxyl O6 is about 1.8Å (the bond order between S3 and O6 is about 0.4). The lowest energy decarboxylation profile is the following process. First the dihedral angle of C4-C1-C2-S3 becomes about 90°, then the distance between C1-C2 increases while the dihedral angle holds about 90°, and finally the C1-C2 bond disappears. The most remarkable change caused by the 90° rotation is the disappearance of the S3-O6 bond, and this disappearance causes electric changes that prompt the decarboxylation.