Low-mass product ions of glycerolipids (TAGs and GPLs): ion structures and putative mechanisms of their formation - a 2025 update

Abstract

Introduction: Structural elucidation of glycerolipids (TAGs and GPLs) has been extensively performed by low- as well as high-energy collision-induced dissociation (CID) in combination with tandem mass spectrometry. Particularly high-energy CID typically yields product ion spectra rich in structurally diagnostic ions across the entire mass range. Whereas the high mass region is frequently dominated by charge-remote site fragmentations of the fatty acid substituents, the low mass region exclusively shows product ions representing individual substituents of the glycerol backbone and polar head group, respectively. The recent work focuses on the putative mechanisms and resulting ion structures of these low mass product ions elucidating also some previously undescribed ion structures. Materials and methods: Selected natural or synthetic triacylglycerols, diester and diether phospholipids were chosen for detailed structural elucidation. All tandem mass spectrometric experiments were performed either on a Shimadzu TOF2 or on a Shimadzu MALDI 7090 at a collision energy of 20 keV. Appropriate precursor ions were selected for high-energy CID in combination with tandem time-of-flight mass spectrometry. Results and discussion: Sodiated triacylglycerol precursor ions exhibit various low mass product ions differentiating between the sn1-/ sn3- (E1/3-, F1/3-, J1/3+16- and G1/3-ions) versus the sn2-substituent (J2-ion) thus allowing positional determination of the substituents linked to the glycerol backbone. In contrast, various precursor ions of diester phospholipids yield these ions only for the sn3-linked polar head group (E3-, F3-, J3+16- and G3-ions) and no such ions for the corresponding fatty acid-related ions at sn1 or sn2. A similar situation is observed for various precursor ions of diether phospholipids with the exception that E- and F-type product ions are not formed. Additionally, several phospholipid precursor ions exhibit unusual rearrangement reactions in the gas phase. Finally, the mechanism of formation of major low mass products is discussed in detail elucidating homolytic cleavages as important fragmentation mechanisms. Summary/Conclusion: Most low mass product ions of glycerolipids (TAGs, GPLs) are directly formed by two simultaneous homolytic cleavages without any medium mass intermediates. Despite their rather simple mechanism of formation at least some of them are structurally of highly diagnostic value