Mechanistic Insights into Protonated Diamines-catalyzed Decarboxylation of Oxaloacetate

The chemical mechanisms of protonated diamines-catalyzed decarboxylation of oxaloacetic acid anions in water solutions have been studied by using density functional theory. The calculated results show that the activated Gibbs free energy of the decarboxylation step is the highest in the whole diamine-catalytic processes for OA2-, and protonated ethylenediamine (ENH+) is the best catalyst of the five diamines, which is consistent with the study of Thalji et al. However, for OA-, different with OA2-, the dehydration step is the rate-determining one except 1,3-diaminopropane, and protonated 1,4- diaminobutane is the best catalyst of the five catalysts. The results also indicate that the second amino group participates in the reaction as the proton acceptor or proton donor, and it assists in decarboxylation by hydrogen bonds, decreasing the active Gibbs free energy barrier of the whole catalytic process. These results provide insight into the precise catalytic mechanism of several enzymes whose reactions are known to proceed via an imine intermediate.