Examination of complex oligosaccharides by matrix‐assisted laser desorption/ionization mass spectrometry on time‐of‐flight and magnetic sector instruments

Abstract

AbstractMatrix‐assisted laser desorption/ionization (MALDI) spectra of underivatized oligosaccharides of the type attached to asparagine in glycoproteins (N‐linked oligosaccharides) were examined with linear time‐of‐flight (TOF) and magnetic sector instruments using 2,5‐dihydroxybenzoic acid (2,5‐DHB), α‐cyano‐4‐hydroxycinnamic acid, sinapinic acid, 1,4‐dihydroxynaphthalene‐2‐carboxylic acid or 2‐(4‐hydroxyphenylazo)benzoic acid (HABA) as the matrices. All compounds formed abundant [M + Na]+ ions with the strongest signals being obtained from 2,5‐DHB after recrystallization of the initially dried sample spot from ethanol. Only traces of fragmentation were detected from neutral oligosaccharides on the TOF system but more abundant fragment ions (about 5% relative abundance) were present in the spectra from the magnetic sector instrument. Fragmentation was dominated by Y‐type glycosidic cleavages (Domon and Costello nomenclature) between all sugar residues yielding sequence and branching information. Sialic acid‐containing oligosaccharides generally produced the sodium adduct of the sodium salt and gave much weaker signals than the neutral sugars in the positive‐ion mode. There was also considerable loss of the sialic acid moleties as the result of fragmentation on the magnetic sector instrument. The least fragmentation of both neutral and acidic sugars was caused by 2.5 DHB, which proved to be the most appropriate matrix for examination of oligosaccharide mixtures. Much better resolution of the oligosaccharides was obtained than by traditional methods such as the use of Bio‐Gel P‐4 gel filtration column chromatography. It is worth noting also that the measurements were considerably faster (a few minutes as opposed to about 16 h). In addition, no radiolabelling was necessary as required for detection on the P‐4 columns. Mixtures of oligosaccharides from several glycoproteins (ribonuclease B, human immunoglobulin G (IgG) transferrin, bovine fetuin and chicken ovalbumin) were examined and the patterns of the identified oligosaccharides were found to agree closely with the known compositions of the sugar mixtures. The mass spectrometric resolution on the magnetic sector instrument was very much better (up to 3000, FWHM) than could be obtained with the linear TOF systems (200–400). The technique was used as a detection system for the products of exoglycosidase digestion in experiments to determine the detailed structure of the oligosaccharide chains from human IgG.