RESUMO
The hydrogen-deuterium exchange of protonated melatonin and its in vitro oxidation end-products have been examined by liquid chromatography coupled with ion-trap mass spectrometry. Specific H/D scrambling of protons in the C2 and C4 positions of the indole ring during gas-phase fragmentation process was observed for both melatonin and its oxidation products. Collision-induced dissociation spectra showed losses of variably deuterated NH(3), H(2)O and CH(3)CONH(2). In addition, a similar H/D scrambling behaviour was observed for the oxidation products, obtained from the opening of the indole ring by oxidative attack. Fragmentation pathways are proposed and H/D scrambling has been employed as a fingerprint, allowing identification of N(1)-acetyl-5-methoxykynurenin (AMK), N(1)-acetyl-N(2)-formyl-5-methoxykynurenin (AFMK), dehydro-AFMK and hydroxymelatonin as the oxidation products of melatonin in vitro.