Structural characterization of phospholipids and sphingolipids by in-source fragmentation MALDI/TOF mass spectrometry

Phospholipids (PLs) and sphingolipids (SLs) perform critical structural and biological functions in cells. The structure of these lipids, including the stereospecificity and double-bond position of fatty acyl (FA) chains, is critical in decoding lipid biology. In this study, we presented a simple in-source fragmentation (ISF) MALDI/TOF mass spectrometry method that affords complete structural characterization of PL and SL molecules. We analyzed several representative unsaturated lipid species including phosphatidylcholine (PC), plasmalogen PC (pPC), phosphatidylethanolamine (PE), phosphatidylinositol (PI), cardiolipin (CL), sphingomyelin (SM), and ceramide (Cer). Fragment ions reflecting the FA chains at sn-1 and sn-2 position, and those characteristics of the head groups of different PL classes, are readily identified. Specific fragment ions from cleavages of the C-C bond immediately adjacent to the cis C=C double-bond position(s) of FA chains and the trans C=C double bond of the sphingosine constituents allow precise localization of double bonds. The identities of the exemplary product ions from vinylic, allylic, and double-bond cleavages were also verified by LIFT-TOF/TOF. Identification of individual PL species in the lipid mixture was also carried out with ISF-MALDI/TOF. Together, this approach provides a simple yet effective method for structural characterization of PLs and SLs without the additional modification on the instrument hardware, and serves as a simple tool for the identification of lipids.