High-energy CID tandem TOF-MS of various types of precursor ions of selected diether phospholipids: Diagnostic known and unexpected fragmentation pathways

Abstract

The fragmentation behavior of some selected synthetic (1,2-diphytanyl- and 1,2-dihexadecyl-glycerophosphatidylethanolamine, 1,2-diphytanyl- and 1,2-dihexadecyl-glycerophosphatidylcholine) as well as of one natural diether phospholipid (2,3-diphytanyl-glycerophosphatidylinositol), the latter obtained from extracts of the archaeon Sulfolobus acidocaldaricus, was described by negative- and positive-ion MALDI high-energy CID tandem time-flight mass spectrometry for the first time. In contrast to the fragmentation pathways of classical diester glycerophospholipids, whose fragmentation behavior is already well described, the investigated diether glycerophospholipids exhibited a very different fragmentation behavior. The [M–H]⁻-precursor ions (ethanolamine, inositol) showed abundant high-mass charge-remote site fragmentation of the alkyl chains with easy determination of all methyl branching points (if present). Corresponding low mass product ions elucidated the identity of the polar head group. In contrast, [M+H]⁺-precursor ions of ethanolamine derivatives showed unusual loss of H₃PO₄ directly from the precursor ion and McLafferty-like rearrangements of selected product ions differing between sn1-and sn2-substituents, [R₁O+58]⁺ and [R₂O+42]⁺ ions, respectively. No diagnostic low mass product ions or high mass charge-remote site fragmentations are observed. A yet undescribed rearrangement reaction for protonated diether phosphocholine derivates was found by an intramolecular transesterification rearrangement of the precursor ion forming protonated O-alkyl glycerophosphatidylcholine. Besides, high mass charge-remote site fragmentation of the alkyl chains was observed. High-energy CID-spectra of [M+Na]⁺-precursor ions showed only little fragmentation (ethanolamine, inositol) with abundant partial polar head group losses and low mass head group product ions. In contrast, the [M+Na]⁺-precursor ions of corresponding choline derivatives showed significant charge-remote site fragmentation of the alkyl chains and diagnostic low mass head group ions. In case of the ethanolamine derivatives the [M+2Na–H]⁺-precursor ions exhibited abundant polar head group losses and high mass charge-remote site fragmentation with diagnostic low mass head group product ions. The inositol derivative mainly yielded disodiated dehydrated inositol phosphate as product ions. Finally, two diether phospholipid-specific product ions, the newly described K- and L-type ions, are described for the first time for all lipid derivatives and their mechanism of formation is described in detail.