Structural identification of triacylglycerol isomers using electron impact excitation of ions from organics (EIEIO)

Uncovering Structural Diversity of Unsaturated Fatty Acyls in Cholesteryl Esters via Photochemical Reaction and Tandem Mass Spectrometry

Mass spectrometry analysis of cholesteryl esters (CEs) faces several challenges, with one of them being the determination of the carbon-carbon double bond (C=C) locations within unsaturated fatty acyl chains. Paternὸ-Büchi (PB) reaction, a photochemical reaction based on the addition of acetone to C=C, is capable of C=C location determination when coupled with tandem mass spectrometry (MS/MS). In this study, the PB reaction conditions were tailored for CEs and subsequent nanoelectrospray ionization (nanoESI). A solvent system containing acetone/methanol/dichloromethane/water (40/30/20/10, volume ratios) and 100 μM LiOH was determined to be optimal, resulting in reasonable PB reaction yield (~30%) and good ionization efficiency (forming lithium adduct of CEs). Collision-induced dissociation (CID) of the PB reaction products produced characteristic fragment ions of CE together with those modified by the PB reactions, such as lithiated fatty acyl ([FA + Li]+) and its PB product ([FA - PB + Li]+). MS3 CID of [FA - PB + Li]+ led to abundant C=C diagnostic ion formation, which was used for C=C location determination and isomer quantitation. A PB-MS3 CID approach was developed and applied for CE analysis from human plasma. A series of unsaturated CEs was identified with specific C=C locations within fatty acyl chains. Absolute quantitation for each CE species was achieved including coexisting C=C location isomers, such as Δ9 and Δ11 isomers of CE 18:1 and ω-6 and ω-3 isomers of CE 18:3. These results show that PB-MS/MS is useful in uncovering structural diversity of CEs due to unsaturation in fatty acyls, which is often undetected from current lipid analysis approach. Graphical Abstract ᅟ.

Distinguishing Cis and Trans Isomers in Intact Complex Lipids Using Electron Impact Excitation of Ions from Organics Mass Spectrometry

We present a mass spectrometry-based method for the identification of cis and trans double-bond isomers within intact complex lipid mixtures using electron impact excitation of ions from organics (EIEIO) mass spectrometry. EIEIO involves irradiating singly charged lipid ions with electrons having kinetic energies of 5-16 eV. The resulting EIEIO spectra can be used to discern cis and trans double-bond isomers by virtue of the differences in the fragmentation patterns at the carbon-carbon single bonds neighboring the double bonds. For trans double bonds, these characteristic fragments include unique closed-shell and open-shell (radical) products. To explain this fragmentation pattern in trans double bonds, we have proposed a reaction mechanism involving excitation of the double bond's π electrons followed by hydrogen atom rearrangement. Several lipid standards were analyzed using the EIEIO method, including mixtures of these standards. Prior to EIEIO, some of the lipid species in these mixtures were separated from their isomeric forms by using differential mobility spectrometry (DMS). For example, mixed cis and trans forms of triacylglycerols and phosphatidylcholines were identified by this DMS-EIEIO workflow. With this combined gas-phase separation and subsequent fragmentation, we could eliminate the need for authentic standards for identification. When DMS could not separate cis and trans isomers completely, as was the case with sphingomyelins, we relied upon the aforementioned diagnostic EIEIO fragment peaks to determine the relative contribution of the trans double-bond isomer in the mixed samples. We also applied the DMS-EIEIO methodology to natural samples extracted from a ruminant (bovine), which serve as common origins of trans fatty acids in a typical Western diet that includes dairy products.

Development of a Branched Radio-Frequency Ion Trap for Electron Based Dissociation and Related Applications

Collision-induced dissociation (CID) is the most common tool for molecular analysis in mass spectrometry to date. However, there are difficulties associated with many applications because CID does not provide sufficient information to permit details of the molecular structures to be elucidated, including post-translational-modifications in proteomics, as well as isomer differentiation in metabolomics and lipidomics. To face these challenges, we are developing fast electron-based dissociation devices using a novel radio-frequency ion trap (i.e., a branched ion trap). These devices have the ability to perform electron capture dissociation (ECD) on multiply protonated peptide/proteins; in addition, the electron impact excitation of ions from organics (EIEIO) can be also performed on singly charged molecules using such a device. In this article, we review the development of this technology, in particular on how reaction speed for EIEIO analyses on singly charged ions can be improved. We also overview some unique, recently reported applications in both lipidomics and glycoproteomics.