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Production and fragmentation of negative ions from neutral N ‐linked carbohydrates ionized by matrix‐assisted laser desorption/ionization
Author(s) -
Domann Paula,
Spencer Daniel I. R.,
Harvey David J.
Publication year - 2012
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.5322
Subject(s) - chemistry , adduct , fragmentation (computing) , dissociation (chemistry) , ion , electrospray ionization , ionization , mass spectrum , mass spectrometry , glycosidic bond , matrix assisted laser desorption/ionization , desorption , analytical chemistry (journal) , chromatography , organic chemistry , adsorption , computer science , enzyme , operating system
Although negative ion fragmentation mass spectra of neutral N ‐linked carbohydrates (those attached to Asn in glycoproteins) provide much more structural information than spectra recorded in positive ion mode, neutral carbohydrates are reluctant to form negative ions by matrix‐assisted laser desorption/ionization (MALDI) unless ionized from specific matrices such as nor ‐harmane or adducted with anions such as chloride. This paper reports the results of experiments to optimize negative ion formation from adducts of N ‐linked glycans with respect to ion abundance and fragment ion production. The best results were obtained with 2,4,6‐trihydroxyacetophenone (THAP) as the matrix with added ammonium nitrate as the salt providing the anion. This approach is demonstrated to be applicable for a wide range of N ‐linked glycan structures. Phosphate adducts, analogous to those that are usually encountered in electrospray spectra from N ‐glycans released by protein N ‐glycosidase F, were produced by addition of ammonium phosphate to the matrix but in relatively low yield allowing competitive ionization of endogenous anionic compounds leading to complex spectra. Fragmentation of the nitrate adducts, which were formed in higher yield, generally paralleled that seen by collision‐induced dissociation following ionization by electrospray, with the first stage of the dissociation being the elimination of the nitrate with a proton from one of the hydroxyl groups of the sugar. The spectra of the resulting [M–H] – species displayed very specific fragment ions, mainly cross‐ring and C‐type glycosidic cleavage products, that revealed more structural (linkage and branching) information of the compounds than the mainly glycosidic cleavage products that dominated the positive ion spectra. Copyright © 2012 John Wiley & Sons, Ltd.