Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry of N-Linked Carbohydrates and Related Compounds

Abstract

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry of N-linked carbohydrates (those linked to asparagine in glycoproteins) was first reported in 1991 by Mock et al. [1] and provides a simple and sensitive method for producing spectra from underivatized N-linked glycans. It has been estimated that MALDI is about 10 to 100 times more sensitive than fast-atom bombardment FAB) for glycan analysis [2], and does not require derivatization by permethylation or peracetylation, as required by FAB, to obtain a signal. When recorded with linear time-of-flight (TOF) mass spectrometers, MALDI mass spectra of neutral N-linked glycans contain few, if any, fragment ions and so the technique provides a rapid method for glycan profiling. The spectrum of N-linked glycans released from chicken ovalbumin, shown in Fig. 1 illustrates the type of result that can be obtained. Structures of the glycans producing the peaks are shown in Table 1. The masses of the peaks can lead directly to the compositions ofthe constituent glycans in terms oftheir isobaric monosaccharide compositions, but determination of further structural details requires production of fragment ions or coupling to other methods such as exoglycosidase digestion as described below. Acidic N-linked glycans produce more fragmentation than neutral glycans and are generally more difficult to handle. Nevertheless, they provide excellent spectra under the right conditions. This paper summarizes work in this area and presents examples of data recorded in our laboratory. Positive ion MALDI mass spectrum of N-linked glycans released from chicken ovalbumin (Sigma grade VI, containing other glycoproteins) by hydrazinolysis and recorded from 2,5-DHB with a delayed extraction, reflectron-TOF mass spectrometer. Structures of the constituent glycans are listed in Table 1. The inset shows an enlargement of one of the peaks with its accompanying MK+, [M + 41]+ and [M + 42]+ ions. The [M + 41]+ ion is produced by the N-acetyl-glycosylamine, whereas the [M + 42]+ ion is caused by a mono-O-acetyl derivative. Peaks in the mIz 1400 region correspond to acetyl-hydrazides and the potassium adducts of the [M + 41]+ and [M + 42]+ ions. KeywordsSialic AcidMALDI AnalysisGlycan ProfileChicken OvalbuminMagnetic Sector Mass SpectrometerThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.