IRMPD Spectroscopy of [PC (4:0/4:0) + M]⁺ (M = H, Na, K) and Corresponding CID Fragment Ions

Abstract

Glycerophospholipids (GPs) are highly abundant in eukaryotic cells and take part in numerous fundamental physiological processes such as molecular signaling. The GP composition of samples is often analyzed using mass spectrometry (MS), but identification of some structural features, for example, differentiation of stereospecific numbering (sn) isomers by wellestablished tandem MS (MS 2 ) methods, is challenging. In particular, the formation of 1,3-dioxolane over 1,3-dioxane intermediates proposed to be responsible for the sn-selectivity of these tandem MS strategies has not been validated by spectroscopic methods. In this work, we present infrared multiple photon dissociation (IRMPD) spectra of phosphatidylcholine (PC) ions [PC 4:0/4:0 + H/Na/K] + and [PC 4:0/4:0 + Na/K − 183] + fragments generated by electrospray ionization (ESI)-MS and collision-induced dissociation (CID), respectively. IRMPD spectra of protonated, sodiated, and potassiated PC 4:0/4:0 differ in the phosphate-and ester-related bands, which are increasingly shifted to lower wavenumbers with higher adduct masses. Comparison of calculated and experimental IR spectra indicates the presence of multiple, two and one isomer(s) for [PC 4:0/4:0 + H] + , [PC 4:0/4:0 + Na] + , and [PC 4:0/4:0 + K] + , respectively. Isomers exhibiting pronounced sn-1 ester−ion interactions are computationally predicted to be energetically preferred for all species and are in line with experimental results. IRMPD spectra of [PC 4:0/4:0 + Na/K − 183] + are presented and shed the first light on the fragment ion structures, rationalizing MS-based lipidomics strategies that aim to characterize the sn-isomerism of GPs.

Cryogenic infrared spectroscopy provides mechanistic insight into the fragmentation of phospholipid silver adducts

“…S2 ). In recent studies, MS 2 fragments generated by neutral headgroup loss from protonated [ 13 ], sodiated [ 4 , 10 , 11 ], and potassiated [ 12 ] glycerophospholipids were shown to feature a cyclic dioxolane structure. In order to determine whether silver adduction also yields dioxolane structures and to estimate the respective influence of the headgroup and the lipid chains on the fragment structure, silver-adducted PE and PC precursor ions were subjected to in-source fragmentation (Fig.…”

“…Neutral headgroup loss in positive ion mode was postulated to proceed via an intramolecular cyclization resulting in a six-membered dioxane ring or five-membered dioxolane ring [ 9 ]. Subsequent studies confirmed the dioxolane structure for alkali metal adducts [ 4 , 10 – 12 ] and protonated glycerolipid fragments [ 13 ]. Spectroscopic investigations of phospholipid precursor ions explained the formation of dioxolanes by strong interactions between the sn -1 ester and the cation, promoting intramolecular cyclization of the less strongly bound sn -2 ester into a five-membered ring [ 12 ].…”

“…Subsequent studies confirmed the dioxolane structure for alkali metal adducts [ 4 , 10 – 12 ] and protonated glycerolipid fragments [ 13 ]. Spectroscopic investigations of phospholipid precursor ions explained the formation of dioxolanes by strong interactions between the sn -1 ester and the cation, promoting intramolecular cyclization of the less strongly bound sn -2 ester into a five-membered ring [ 12 ]. In the cyclic dioxolane structure, the fatty acyls occupy chemically different positions, either being part of the ring or next to the ring.…”

Collision-assisted stripping for determination of microsolvation-dependent protonation sites in hydrated clusters by cryogenic ion trap infrared spectroscopy: the case of benzocaineH+(H2O)n

“…, N-protomer in this case) and are not readily produced directly in the ion source. Although IRPD spectroscopy of fragment ions generated by collision-induced dissociation has been applied to various systems, 42,48,49 we extend herein the methodology to hydrated clusters to reliably determine the protonation site. The cooling efficiency in the QIT is high enough to freeze the conformations of a molecule.…”

Formation and Tandem Mass Spectrometry of Doubly Charged Lipid-Metal Ion Complexes

“…Therefore, attachment of Ba 2+ can best stabilized by additional interactions with lipid ester groups, followed by Ca 2+ and Mg 2+ . These interactions between metal cations and lipids play a role in the formation of these complexes has recently been shown for [PC 4:0/4:0 + Na] + by IR multiple photon dissociation spectroscopy …”