Rapid Identification of Terminal Sialic Acid Linkage Isomers by Pseudo‐MS3 Mass Spectrometry

Abstract

AbstractWe used pseudo‐triple‐stage tandem MS to rapidly differentiate terminal α(2,3)‐ and α(2,6)‐linked sialic acids of glycopeptides and native glycans. The precursor ions generate important diagnostic fragments to distinguish different terminal linked isomeric glycans and glycopeptides based on collision‐induced dissociation (CID). The diagnostic ions include hexose linked with N‐acetylhexosamine residues, sialic acid with the loss of water, and sialic acid residues. Their values of mass‐to‐charge ratio (m/z) are at 366.14, 274.09, and 292.10, respectively. The relative intensity ratios (RIR) were calculated by the intensity of signals at m/z 274.09 and 292.10 divided by the intensity of the signal at m/z 366.14. Under optimal conditions of an in‐source fragmentation voltage of 70 V and a normalized collision energy of 30 %, the difference in RIR of diagnostic ions between terminal α(2,3)‐linked sialic acid of glycopeptide (1.3) and terminal α(2,6)‐linked sialic acid of glycopeptide (<0.18) is the largest. Moreover, quantitative calibration curves with different percentages of α(2,3)‐ and α(2,6)‐linked sialoglycopeptides demonstrated that this method applied not only to biantennary N‐linked glycan, but also to complex and hybrid N‐linked glycans and O‐linked glycans. From measurements of the RIR of diagnostic ions, erythropoietin, fetuin, and herceptin have 98, 70, and 100 %, respectively, α(2,3)‐linked N‐sialic acid of total terminal sugar moieties. N‐Glycosylation of the antibody‐binding fragment (Fab) moiety of cetuximab has 100 % α(2,6)‐linked N‐glycolylneuraminic acid terminal glycan linkages. This method can be applied to identify compounds from chemical synthesis. It can also be used for quality evaluation of bio‐similar protein drugs.