Differentiation of Isomeric Bis(monoacylglycero)phosphates and Phosphatidylglycerols via Gas-Phase Ion/Ion Reactions and Ion Trap Collision-Induced Dissociation

Abstract

Bis(monoacylglycero)phosphates (BMPs) and phosphatidylglycerols (PGs) constitute a unique class of lipid regioisomers with distinct structural features and biochemical roles. However, ion trap collision-induced dissociation (IT-CID) alone is insufficient to differentiate isomeric deprotonated PG and BMP ions. Herein, we describe a gas-phase charge-inversion ion/ion reaction that generates an ion type enabling straightforward, unambiguous differentiation of BMP and PG isomers upon IT-CID. The approach involves charge inversion of isomeric, deprotonated BMP or PG anions using the dicationic magnesium 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine complex ([Mg(ttbt)2]2+), followed by IT-CID to produce diagnostic fragment ions indicative of each isomer. These fragment ions arise from even-electron charge remote fragmentation (CRF) pathways. Subsequent MSn experiments involving fatty acid fragment ions complexed with [Mg(ttbt)2]2+ proceed through odd-electron radical directed dissociation (RDD), providing detailed information on acyl chain carbon–carbon double bond position(s). Ion types generated via negative ion mode nESI and through ion/ion reactions were applied to distinguish targeted PG and BMP isomers at the carbon–carbon double bond positional level in a mouse liver extract using a shotgun lipidomics approach.