Radical delivery and fragmentation for structural analysis of glycerophospholipids

Abstract

Conventional tandem mass spectrometry relies on even-electron fragmentation that provides limited structural information for glycerophospholipids (GP), which are key constituents of all cell membranes. Different GP classes are chemically very similar and subtle variations in carbon-carbon bonding features and linkages can lead to numerous isomeric structures that are challenging to distinguish with traditional mass spectrometry. In this study, we demonstrate that the primary amine groups in many GP classes can be modified with either noncovalent attachment of crown ether derivatives containing an iodobenzoyl moiety, or by direct covalent attachment of the iodobenzoyl moiety. Radical lipids can be generated using these modifications via photoactivation of labile carbon-iodine bonds, providing rich information about headgroup and fatty acid chain structure. The method is demonstrated for lipid standards containing various carbon chain motifs and linkages, as well as phospholipids extracted from a soybean mixture. Numerous lipids were examined, including plasmalogen-, lyso-, diacyl- types of phospholipids containing mono-/poly- unsaturated fatty acid (FA) substituents, and branched-/nonbranched-FA chains. Interestingly, the presence of double bond and/or vinyl ether linkage leads to the formation of a signature fragment ion that facilitates rapid structural identification.