Determination of the origin of doubly-cationized monosialylated fragments in MS/MS spectra of singly-cationized LSTb and GM1 using ion mobility spectrometry-mass spectrometry

Abstract

Sialic acid (NeuAc)-containing fragments [NeuAc-H+2Na]+ were surprisingly detected in MALDI-LID-MS/MS spectra of NeuAc-containing glycosphingolipid (ganglioside) [M+Na]+ ions in a recent study (Rajanayake et al., 2016). To further investigate the origin of these fragments, [M+A]+ (where A is Li+, Na+, K+, or Cs+) ions of a monosialylated glycan (LSTb) and a monosialylated ganglioside (GM1) were analyzed using a variety of MS/MS techniques. [NeuAc-H+2A]+ fragments were observed during MALDI-LID-MS/MS as well as ESI-CID-, ion mobility spectrometry (IMS)-CID-, and HCD-MS/MS of LSTb and GM1 precursor ions whose m/z values corresponded to [M+A]+ ions. Additionally, [NeuAc-H+A1+A2]+ (A1 is Li+, K+, or Cs+ and A2 is Na+) B1 fragments were detected in MALDI-LID-MS/MS spectra of [M+A1]+ ions. Based on ESI-IMS-CID-MS/MS performed at variable collision energies after IMS separation, the [NeuAc-H+2Na]+ B1 ions were detected predominantly due to fragmentation of multiply-charged multimer ions ([nM+nNa]n+, n=2–3 for LSTb and n=2–4 for GM1) that were isobaric with [M+Na]+ ions of LSTb and GM1, but were separated at different IMS drift times. However, [NeuAc-H+2Na]+ B1 ions were also detected with low intensities in ESI-IMS-CID-MS/MS spectra of [M+Na]+ ions of LSTb and GM1 at higher collision energies. [NeuAc-H+2A]+ and [NeuAc-H+A1+A2]+ B1 ions observed during MALDI-LID-MS/MS of LSTb and GM1 may also originate from multiply charged multimers of [M+A]+ or [M−H+A1+A2]+ ions. Overall, this study demonstrates that the formation of NeuAc-containing fragments, such as [NeuAc-H+2A]+, originating from sialylated glycan and glycosphingolipid ions and their multimers can be elucidated by combination of IMS separation of precursor ions and MS/MS.