Figure 6. Proposed mechanism of the formation of M14 and M15.
M14 and M15 were detected in all the three LMs used in our studies. ATX was first oxidized to O-ATX (M1, M2, or M3), which was further oxidized to form the aldehyde. The aldehyde is cyclized to form the intermediate O-hemiaminal and subsequent oxidation to form the lactam M14. M15 is generated by oxidizing ATX to form M3, followed by the second oxidation to generate aldehyde. The aldehyde was intra-cyclized to form the intermediate hemiaminal, which was further oxidized to form M15.