Overall summary and outlook

Calculations have so far answered some questions related to the decarboxylation of orotic acid derivatives in different media, but these same computational studies have also opened up many additional areas of controversy. 

In the gas phase, calculations predict that decarboxylation via 04 pre-protonation should be preferred over decarboxylation via 02 pre-protonation. While this result has not been tested directly in an experimental setting, the greater proton affinity of 04 over 02 in uracil has been established experimentally in the gas phase. 

Gas-phase quantum mechanical calculations have also revealed the energetic favorability of C5 pre-protonation, which also awaits experimental verification. 

In solution the decarboxylation of 1,3-dimethylorotic acid in sulfolane has been shown through a combination of theoretically predicted and experimentally observed isotope effects to proceed via protonation of the 4-oxygen. 

The enzyme mechanism, however, remains elusive. Quantum mechanical models generally disfavor C6-protonation, but 02, 04, and C5-protonation mechanisms remain possibilities. Free energy computations also appear to indicate that C5-protonation is a feasible mechanism, as is direct decarboxylation without preprotonation O-protonation mechanisms have yet to be explored with these methods. Controversy remains, however, as to the roles of ground state destabilization, transition state stabilization, and dynamic effects. Because free energy models do take into account the entire enzyme active site, a comprehensive study of the relative energetics of pre-protonation and concerted protonation-decarboxylation at 02, 04, and C5 should be undertaken with such methods. In addition, quantum mechanical isotope effects are also likely to figure prominently in the ultimate identification of the operative ODCase mechanism. 

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