Multistage mass spectrometry of phospholipids using collision-induced dissociation (CID) and metastable atom-activated dissociation (MAD)

Abstract

We herein demonstrate an approach to gas phase ion manipulation that provides MS3-level CID spectra of phospholipid radical cations that are almost independent of the original charging adduct ions. In the MS2 He-MAD spectra of the protonated, sodiated and potassiated adducts of POPC, the different adducts induce different primary fragmentation pathways and provide significantly different spectra, as is commonly observed by other activation methods. In separate experiments, the even-electron adduct ions ([M+H]+, [M+Na]+, [M+K]+) of 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) were first converted to radical cations [POPC]+• by using helium metastable atom-activated dissociation (He-MAD) to eject the charging adduct ions, then exposed to low-energy collision induced dissociation (CID) to induce extensive fragmentation along the acyl chains. Such charge-remote fragmentation is generally inaccessible through low-energy CID of the even-electron precursor ions. The combination of He-MAD and CID provides radical-induced spectra that show very major similarities and only minor differences, and therefore overcomes major differences in chemistry that are otherwise observed by the original adducting species. Collisional activation of even-electron [POPC+H]+ required higher CID amplitudes than odd-electron [POPC]+• to effect fragmentation—as expected—and the latter provided fragments within the acyl chains that were influenced by the double bond position.

Structural Characterization of Phosphatidylcholines Using 193 nm Ultraviolet Photodissociation Mass Spectrometry

“…31–33 Yang et al employed a strategy in which the variation in abundances of neutral loss products were monitored as a function of double bond position to identify both the double bond position and quantify relative amounts of fatty acid isomers. 34 Additional methods have focused on novel ion activation techniques including radical-directed dissociation (RDD) via 266 nm photodissociation followed by CID 35–37 , ozone induced dissociation (OzID) 38–40 , electronic impact excitation of ions from organics (EIEIO) 41 , metastable atom-activated dissociation (MAD) 42,43 and electron transfer dissociation (ETD) 44 . In parallel studies Pham et al explored non-covalent complexation of 4-iodobenzoic acid, 4-iodoaniline or 4-iodobenzoyl 18-crown-6 to glycerophospholipids and reactions of 4-iodobenzyl alcohol with fatty acids.…”

Charge Transfer Dissociation (CTD) Mass Spectrometry of Peptide Cations: Study of Charge State Effects and Side-Chain Losses

“…Radical ions have been observed to react with residual oxygen during their confinement in electrodynamic ion traps, which was also noted for the ETD-generated z • ions [28,29] and MAD-generated [POPC] +• radical ions [30].…”

“…One of the more abundant background ions has a massto-charge ratio of 184 (see Supplementary Figure S2, for example), does not fragment using CID, and reacts with residual oxygen to form adducts at M + 16 (m/z 200) and M + 32 (m/z 216). CID of the M + 16 and M + 32 adducts re-forms the original reagent anion at m/z 184, indicating that the reagent is probably polycyclic/aromatic and almost certainly a radical (vide infra) [28][29][30]. Figure 1c shows that these reagent anions are reasonably effective at forming c and z ions from the 3+ precursor of substance P. Fortunately, this charge reduction mechanism can be minimized by raising the LMCO during CTD activation to prevent the co-accumulation of reagent anions, with the caveat that increasing the LMCO also limits the observable range of product ions for CTD.…”

Top-Down Charge Transfer Dissociation (CTD) of Gas-Phase Insulin: Evidence of a One-Step, Two-Electron Oxidation Mechanism

“…It has reported that when the radical species are stored in the electrodynamic ion trap at room temperature, they often react with residual oxygen. The addition of O or O 2 to distonic radical cations has been observed in ETD [41], fs-LID [42], He-MAD [43], and He-CTD [37]. However, oxygen-attachment was not observed for either [Chain A] 2+ or [Chain B] 3+ , indicating that the two types of individual chain adduct ions are more likely to be even-electron species, or that the radical resides on the sulfur atoms and does not undergo the typical oxygen addition.…”