Mechanistic Aspects of Charge-remote Fragmentation in Saturated and Mono-unsaturated Fatty Acid Derivatives. Evidence for Homolytic Cleavage

Abstract

The high-energy collision-induced dissociation (CID) of [M+Li]+ ions of n-butyl ester derivatives of palmitic acid and oleic acid as well as 9,9-2H2-palmitic acid and 11,11-2H2-oleic acid has been studied in order to obtain information on the charge-remote fragmentation mechanism of saturated and mono-unsaturated fatty acid ions containing a stable charge centre. The results obtained in the present study indicate that homolytic cleavage reactions, involving C—H cleavage as an initial rate-determining step, operate during the charge-remote fragmentation observed for high-energy CID of [M+Li]+ ions of n-butyl palmitate and correspond to a major fragmentation route. With respect to the charge-remote fragmentation of n-butyl oleate, our 2H-labelling results point to the same mechanism, involving an initial C—H cleavage at allylic positions, for the formations of ions corresponding to a formal homo-allylic cleavage, and are also consistent with a direct allylic C—C cleavage for the formation of ions due to a formal allylic C—C cleavage. These results, however, do not exclude the possibility of other minor homolytic fragmentation pathways for the formation of ions involving formal allylic and homo-allylic cleavages.