Lipids can be characterized by the head group classification, fatty acyl chain length(s), the degree of unsaturation, the double bond position(s), and any stereochemistry. Electrospray ionization (ESI) of lipids preserves their native state and can form singly and multiply charged ion species, depending on the head group and adducting ions. Furthermore, ionic lipids can present as multimeric, or oligomeric, complexes during ESI, which can complicate mass spectra but also provide additional opportunities for determining the energetics and propensities for oligomer formation. Once formed, charged lipids are often fragmented using collision-induced dissociation (CID) or, less commonly, electron-induced dissociation (EID), to yield product ions that inform about the different levels of structural characterization. Structural characterization of the lipids tends to fall short with existing fragmentation techniques because they may not cause fragmentation that informs about the double-bond position(s), for example.
Charge transfer dissociation (CTD) mass spectrometry is a high-energy, radical-induced dissociation technique that we applied for the first time to multimeric homogenous phospholipids. Experiments were conducted with a modified Bruker AmaZon quadrupole ion trap. Phospholipid standards were ionized using ESI, and we collected both CTD and CID spectra for direct comparison. For CTD, helium cations were introduced to the modified ion trap at ~5.5 KeV using a pulse width of 20 ms.
CID of the multimeric phospholipid cations, such as [Lipid₂ + Na]⁺ and [Lipid₅+ 2Na]²⁺, causes separation of the non-covalent adducts and some cleavages of labile bonds at the SN1 and SN2 positions of the head group, which help characterize the length and saturation of the fatty acyl chains. CTD also produced the same types of fragments as CID, but CTD also generated a wide range of even-electron and radical-induced fragments, including along the acyl chains. Fragment ion abundances along the acyl chains indicated a sensitivity for the location of double bond(s). In many cases, high-energy radical-induced fragments of one lipid were observed while the fragment remained non-covalently adducted to another in-tact lipid. We are currently developing workflows to maximize the extent of structural information one can derive from fragmenting one or more lipids from a multimeric adduct. Obviously, the types of available fragments are more complicated for heteromeric adducts.

Glen Jackson

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