Identification by electrospray tandem mass spectrometry of spin-trapped free radicals from oxidized 2-oleoyl-1-palmitoyl-sn-glycero-3-phosphocholine

GPC radical species formed during oxidation of a glycerophosphocholine (16:0/18:1) under the Fenton reaction conditions were detected using a spin trap, 5,5-dimethyl-1-pyrrolidine N-oxide (DMPO). The stable spin-trapped radical adducts were identified by mass spectrometry (MS) using electrospray (ES) as ionization method and characterized by tandem mass spectrometry (MS/MS). Radical adducts of oxidized free sn-2 fatty acid and of oxidized intact GPC, containing one, two and three additional oxygen atoms, were assigned. DMPO adducts of oxidized intact GPC were observed as singly and doubly charged ions in ES-MS, while adducts of oxidized free fatty acids were observed as singly charged ions. Oxidized free sn-2 fatty acids and intact GPC-DMPO adducts correspond to carbon- and oxygen-centered radicals that were identified by MS/MS as alkyl, hydroxy-alkyl, alkoxyl, hydroxy-alkoxyl, peroxyl and hydroperoxide-alkoxyl spin adducts. The DMPO molecule was attached predominantly at C(9) of the oleic chain. The fragmentation pathway of spin adducts with two DMPO molecules strongly suggests the presence of species that were simultaneously carbon- and oxygen-centered radicals. Several fragments identified are consistent with the presence of isomeric structures contributing to the same ions.