Mechanisms governing the fragmentation of glycerophospholipids containing choline and ethanolamine polar head groups

Abstract

Glycerophospholipids are the major amphiphilic molecules found in the plasma membrane bilayer of all vertebrate cells. Involved in many biological processes, their huge structural diversity and large concentration scale make their thorough characterization extremely difficult in complex biological matrices. Mass spectrometry techniques are now recognized as being among the most powerful methods for the sensitive and comprehensive characterization of lipids. Depending on the experimental conditions used during electrospray ionization mass spectrometry experiments, glycerophospholipids can be detected as different molecular species (e.g. protonated, sodiated species) when analyzed either in positive or negative ionization modes or by direct introduction or hyphenated mass spectrometry-based methods. The observed ionized forms are characteristic of the corresponding phospholipid structures, and their formation is highly influenced by the polar head group. Although the fragmentation behavior of each phospholipid class has already been widely studied under low collision energy, there are no established rules based on charge-induced dissociation mechanisms for explaining the generation of fragment ions. In the present paper, we emphasize the crucial roles played by ion-dipole complexes and salt bridges within charge-induced dissociation processes. Under these conditions, we were able to readily explain almost all the fragment ions obtained under low-energy collision-induced dissociation for particular glycerophospholipids and lysoglycerophospholipids species including glycerophosphatidylcholines and glycerophosphatidylethanolamines. Thus, in addition to providing a basis for a better comprehension of phospholipid fragmentation processes, our work also highlighted some potentially new relevant diagnostic ions to signal the presence of particular lipid species.

Proposal for a chemically consistent way to annotate ions arising from the analysis of reference compounds under ESI conditions: A prerequisite to proper mass spectral database constitution in metabolomics

“…This phenomenon may occur when, in addition to featuring an inner salt, the zwitterionic representation of M displays either a strongly basic or acidic site available for protonation or deprotonation, respectively. This can be illustrated by the typical behavior of phospholipids when submitted to low energy CID processes …”

“…In positive ionization mode, adduct ions formed with cations may have different origins and may result from: the protonation of a basic site of an entity already existing as a salt; the charge solvation of a metal cation by the lone‐pair(s) of heteroatom(s) of either (A) a molecule or (B) a salt. Representative examples of such adduct ions possibly formed with glucose and sodium cations are depicted in Figure as well as the corresponding proposed annotation.…”

“…The m/z ratio of this fragment ion is shifted 37.0552 m/z lower than that of the [M + H] + ion which would correspond to an incoherent loss if mistakenly assigned to a fragment of the protonated molecule. Reversely, in negative mode, anionized betaine of diacyl glycerophosphocholine [M + HCO 2 ] − dissociates by release of HCO 2 CH 3 from the salt moiety (ie, Hoffman elimination‐like reaction) to yield [(M + HCO 2 ) − HCO 2 CH 3 ] − . The corresponding peak could not be attributed to a fragment ion of [(M − H)] − because the loss of 14.0156 Da would correspond to a methylene group (CH 2 ) that is chemically incorrect.…”

“…This can be illustrated by the typical behavior of phospholipids when submitted to low energy CID processes. 30 Similarly, protonated or deprotonated homodimeric forms of M may be observed in positive or negative mode, respectively. These Table 4.…”

“…Nonexhaustive examples ions resulting from "in-source" ion/molecule reactions observable in positive ESI mode of ionization and their corresponding proposed notation yield [(M + HCO 2 ) − HCO 2 CH 3 ] − 30,38. The corresponding peak could not be attributed to a fragment ion of [(M − H)] − because the loss of 14.0156 Da would correspond to a methylene group (CH 2 ) that is chemically incorrect.…”