We report unambiguous proof of the stability of a carbinol intermediate inthe case of P450 metabolism of an N-methylated natural cyclo-peptide,namely tentoxin. Under mild acidic or neutral conditions, the lifetime ofcarbinol-amide is long enough to be fully characterized. This metabolitehas been characterized using specifically labeled 14C-methyl tentoxin isotopomers,HPLC, HPLC-MS, MS-MS and NMR. Under stronger acidic conditions,the stability of this metabolite vanishes through deformylation.A theoretical mechanistic investigation reveals that the stability is governedby the accessibility of the nitrogen lone pair and its protonation state. Forcarbinol-amines, even in neutral conditions, the energy barrier for deformylationis low enough to allow rapid deformylation. Carbinol-amide behavesdifferently. Under neutral conditions, delocalization of the nitrogen lonepair increases the energy barrier of deformylation that is a slow processunder such conditions. After protonation, we were able to optimize adeformylation transition that is lower in energy and thus accounts for thelower stability of carbinol-amides observed experimentally in acidic conditions.Finally, by considering the protocol usually used for extraction andanalysis of this type of metabolite, carbinol-amide may thus be frequentlyignored in drug metabolism pathways.