OMP protonation on O4 or O2 before decarboxylation, which entails and ylide formation on N1, has also been considered. Proton donor absence near O4 or O2 in crystallographic structures is evidence against it along with the ylide generation exclusion as a limiting step in 15N experiments. Moreover, doubts have aroused as to protonated intermediate viability due to electronic stabilizers absence. As a consequence, bond rupture between C6 and C7 due to protonation of the former going through a carbanion state has been proposed.