The mechanism of oleic acid nitration by •NO₂

Fatty acid nitration is a recently discovered process that generates biologically active nitro lipids; however, its mechanism has not been fully characterized. For example, some structural details such as vinyl and allyl isomers of the nitro fatty acids have not been established. To characterize lipids that originated from a biomimetic reaction of •NO₂ with oleic acid, we synthesized several isomers of nitro oleic acids and studied their chromatography and mass spectra by various techniques of mass spectrometry. LC/MS analysis performed on a high resolution micro column detected molecular carboxylic anions of various oleic acid nitro isomers (NO₂OA). Esterification of NO₂OA with pentafluorobenzyl bromide and diisopropylethylamine as a catalyst produced a unique isoxazole ester derivative exclusively from allyl NO₂OA isomers via dehydration of the nitro group at ambient temperatures. This new analytical procedure revealed that •NO₂ generated two vinyl and two allyl isomers of NO₂OA. The vinyl isomers showed high regioselectivity with the 1.8:1 preference for the 10-NO₂OA isomer that was absent among allylic isomers. The nitration also generated elaidic acid via cis-trans isomerization and diatereoisomers of vicinal nitro hydroxy, nitro keto and α-nitro epoxy stearic acids with high stereo and regioselectivity. Nitration of small unilamelar phospholipid vesicles resulted in several phospholipids containing nitro lipids and elaidic acid amenable to hydrolysis by phospholipase A₂.