Unveiling Glycerolipid Fragmentation by Cryogenic Infrared Spectroscopy.

Carla Kirschbaum,Gert Von Helden,Lukasz Polewski,Kim Greis,Kevin Pagel,Gerard Meijer,Sandy Gewinner,Wieland Schöllkopf

doi:10.1021/jacs.1c06944

Abstract

Mass spectrometry is routinely employed for structure elucidation of molecules. Structural information can be retrieved from intact molecular ions by fragmentation; however, the interpretation of fragment spectra is often hampered by poor understanding of the underlying dissociation mechanisms. For example, neutral headgroup loss from protonated glycerolipids has been postulated to proceed via an intramolecular ring closure but the mechanism and resulting ring size have never been experimentally confirmed. Here we use cryogenic gas-phase infrared (IR) spectroscopy in combination with computational chemistry to unravel the structures of fragment ions and thereby shed light on elusive dissociation mechanisms. Using the example of glycerolipid fragmentation, we study the formation of protonated five-membered dioxolane and six-membered dioxane rings and show that dioxolane rings are predominant throughout different glycerolipid classes and fragmentation channels. For comparison, pure dioxolane and dioxane ions were generated from tailor-made dehydroxyl derivatives inspired by natural 1,2- and 1,3-diacylglycerols and subsequently interrogated using IR spectroscopy. Furthermore, the cyclic structure of an intermediate fragment occurring in the phosphatidylcholine fragmentation pathway was spectroscopically confirmed. Overall, the results contribute substantially to the understanding of glycerolipid fragmentation and showcase the value of vibrational ion spectroscopy to mechanistically elucidate crucial fragmentation pathways in lipidomics.

Highlights

Mass spectrometry (MS) is one of the most-widely used analytical techniques
The protonated species was dissociated by in-source fragmentation, which is based on collisions between the molecular ions and residual gas in the source region and equivalent to collision-induced dissociation (CID) (Figure S1)
In the setup used for cryogenic IR ion spectroscopy described previously,[23,24] m/z-selected fragment ions thermalized at 90 K are encapsulated in superfluid helium droplets and their release induced by multiple photon absorption events is monitored as a function of the tunable photon energy to yield a highresolution IR spectrum

Summary

Introduction

Mass spectrometry (MS) is one of the most-widely used analytical techniques. It provides a high informational content from small amounts of sample, which makes it useful for biomolecular analysis. Of high importance for structural analysis in all omics disciplines are tandem MS techniques, in which molecular ions dissociate into smaller fragments that can yield valuable information about the original molecular structure. Structure determination is frequently thwarted by unexpected rearrangement reactions including the migration of atom groups[1,2] or intramolecular cyclization.[3] In the field of proteomics, gas-phase infrared (IR) spectroscopy has been successfully applied in the past to determine peptide fragment structures and thereby establish pivotal dissociation mechanisms.[4−6] In spite of significant progress in the understanding of peptide fragmentation, no spectroscopic studies on lipid fragmentation exist to date. The knowledge on crucial dissociation mechanisms in lipidomics is much less substantiated

Methods

Results

Conclusion