Determination of glycopeptide structures by multistage mass spectrometry with low-energy collision-induced dissociation: comparison of electrospray ionization quadrupole ion trap and matrix-assisted laser desorption/ionization quadrupole ion trap reflectron time-of-flight approaches

Abstract

Multistage mass spectrometry, as implemented using low-energy collision-induced dissociation (CID) analysis in three-dimensional (3D) quadrupole ion traps (QITs), has become a powerful tool for the investigation of protein glycosylation. In addition to the well-known combination of QITs with electrospray ionization (ESI), also a matrix-assisted laser desorption/ionization--quadrupole ion trap--reflectron time-of-flight (MALDI-QIT-rTOF) mass spectrometer has recently become available. This study systematically investigates the differences between these types of instrument, as applied to characterization of glycopeptides from human antithrombin. The glycopeptides were obtained by tryptic digestion followed by lectin-affinity purification. Some significant differences between the ESI-QIT and MALDI-QIT-rTOF approaches appeared, most of them are causally related to the desorption/ionization process. The combination of a vacuum MALDI source with an ion-trap analyzer accentuates some characteristic differences between MALDI and ESI due the longer time frame needed for the trapping process. In contrast to ESI, MALDI generated ions that exhibited considerable metastable fragmentation during trapping. The long time span of the QIT process (ms range) compared with that for conventional rTOF experiments (micros range) significantly magnified the extent of this metastable fragmentation. With the investigated glycopeptides, a complete depletion of the terminal sialic acids of the glycopeptides as well as a variety of other fragment ions was already found in the MS1 spectra from the MALDI-QIT-rTOF instrument. The positive ion low-energy CID spectra (MS2) of the selected glycopeptides obtained using the two different QIT equipped instruments were found to be quite similar. In both approaches, fragmentation of the glycan and peptide structures occurred sequentially, allowing unambiguous sequence determination. In the case of ESI-QIT-MS, fragmentation of the glycan structure occurred at the MS2 stage and fragmentation of the peptide structure was obtained only at the MS3 stage, which indicates the necessity of multistage CID experiments for complete structure elucidation. The MALDI-QIT-rTOF instrument yielded both kinds of fragments at the MS2 stage but without mutual interference.