Generating Fatty Acid Profiles in the Gas Phase: Fatty Acid Identification and Relative Quantitation Using Ion/Ion Charge Inversion Chemistry

Representing the
most fundamental lipid class, fatty acids (FA)
play vital biological roles serving as energy sources, cellular signaling
molecules, and key architectural components of complex lipids. Direct
infusion electrospray ionization spectrometry, also known as shotgun
lipidomics, has emerged as a rapid and powerful toolbox for lipid
analysis. While shotgun lipidomics can be a sensitive approach to
FA detection, the diverse molecular structure of FA presents challenges
for unambiguous identification and the relative quantification of
isomeric contributors. In particular, pinpointing double bond position(s)
in unsaturated FA and determining the relative contribution of double
bond isomers has limited the application of the shotgun approach.
Recently, we reported the use of gas-phase ion/ion reactions to facilitate
the identification of FA. Briefly, singly deprotonated FA anions undergo
charge inversion when reacted in the gas phase with tris-phenanthroline
magnesium dications by forming [FA – H + MgPhen]+ complex ions. These charge-inverted FA complex cations fragment
upon ion-trap collision-induced dissociation (CID) to generate product
ion spectra unique to individual FA isomers. Herein, we report the
development of a mass spectral library comprised of [FA – H
+ MgPhen]+ product ion spectra. The developed FA library
permits confident FA identification, including polyunsaturated FA
isomers. Furthermore, we demonstrate the ability to determine relative
contributions of isomeric FA using multiple linear regression analysis
paired with gas-phase ion/ion reactions. We successfully applied the
presented method to generate a FA profile for bovine liver phospholipidome
based entirely on gas-phase chemistries.