Characterization of Inositol Phosphorylceramides from Leishmania major by Tandem Mass Spectrometry with Electrospray Ionization

We describe tandem mass spectrometric approaches, including multiple stage ion-trap and source collisionally activated dissociation (CAD) tandem mass spectrometry with electrospray ionization (ESI) to characterize inositol phosphorylceramide (IPC) species seen as [M - H]^- and [M - 2H + Li]^- ions in the negative-ion mode as well as [M + H]⁺, [M + Li]⁺, and [M - H + 2Li]⁺ ions in the positive-ion mode. Following CAD in an ion-trap or a triple-stage quadrupole instrument, the [M - H]^- ions of IPC yielded fragment ions reflecting only the inositol and the fatty acyl substituent of the molecule. In contrast, the mass spectra from MS³ of [M - H - Inositol]^- ions contained abundant ions that are readily applicable for assignment of the fatty acid and long-chain base (LCB) moieties. Both the product-ion spectra from MS² and MS³ of the [M - 2H + Alk]^-, [M + H]⁺, [M + Alk]⁺, and [M - H + 2Alk]⁺ ions also contained rich fragment ions informative for unambiguous assignment of the fatty acyl substituent and the LCB. However, the sensitivity of the ions observed in the forms of [M - 2H + Alk]^-, [M + H]⁺, [M + Alk]⁺, and [M - H + 2Alk]⁺ (Alk = Li, Na) is nearly 10 times less than that observed in the [M - H]^- form. In addition to the major fragmentation pathways leading to elimination of the inositol or inositol monophosphate moiety, several structurally informative ions resulting from rearrangement processes were observed. The fragmentation processes are similar to those previously reported for ceramides. While the tandem mass spectrometric approach using MSⁿ (n = 2, 3) permits the structures of the Leishmania major IPCs consisting of two isomeric structures to be unveiled in detail, tandem mass spectra from constant neutral loss scans may provide a simple method for detecting IPC in mixtures.