Structure and unimolecular decomposition pathways of pyruvic acid

Motivation. Pyruvic acid occurs naturally in the body and is the end product of the metabolism of sugar and starch. Its thermal decarboxyation reaction is important as it prevents the build-up of lactic acid in the body due to excess of pyruvic acid. However, in spite of its biochemical importance, very little is known of the structure of pyruvic acid and its decarboxylation reactions, as well as its enzymatic pathways. It was therefore considered necessary o carry out a thorough study of its reactions using a semiempirical method to test the validity of the approach, so that it could be used for studying the enzymatic pathways of pyruvic acid. Method. The PM3 method was used for these calculations. Equilibrium geometries of all possible conformers and tautomers of pyruvic acid were calculated, and the geometries and calculated vibrational spectra compared with experimental data. Various decarboxylation reactions were explored and their final products discussed. Results. The keto form of pyruvic acid is the most important tautomer on the potential energy surface. The lowest energy decarboxylation pathway involves the formation of hydroxyethylidene, which then isomerizes to the more stable acetaldehyde. Conclusions. The results from these calculations are in perfect agreement with those from accurate ab initio calculations, as well as experimental results, and this method may be profitably used for calculations of the enzymatic pathways of pyruvic acid, for which ab initio results are difficult owing to the large amount of computer time required