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.
Keywords: Oxaloacetate, DFT, mechanism, decarboxylation, diamines, Gibbs free energy.
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