Identification of oligosaccharides by matrix-assisted laser desorption ionization and electrospray MS.

Introduction Structural determination of carbohydrates presents additional problems to those encountered in studies of linear biopolymers because of the multiple points of attachment between the constituent monosaccharide units and the resulting tendency of carbohydrate chains to branch. Further problems arise from the isobaric nature of many of the constituent monosaccharides and their ability to exist in different anomeric and cyclic configurations. Although these problems are now all accessible by MS, their solution requires the application of a range of different techniques. Some of these, such as GUMS, have been in use for over 30 years, whereas others such as matrix-assisted laser desorption ionization (MALDI) and electrospray MS, the main subjects of this paper, are comparatively recent innovations. GC/MS, usually with electron-impact ionization, is the best method for examining monosaccharides and is the main technique used to determine both the monosaccharide composition of a polysaccharide and the linkage between the sugar residues. Compositional analysis usually takes the form of methanolysis with methanolic HCI to give the 1-0-methyl glycosides of the monosaccharides, which are then converted into their trimethylsilyl derivatives for examination by G U M S [ 1,2]. Methanolysis is preferred to hydrolysis as it causes less decomposition of fragile structures. Linkage analysis involves a more complex derivatization process in order to label the free and bound hydroxy groups in different ways. Several versions exist but, typically, the polysaccharide will be permethylated, hydrolysed, reduced and acetylated to give a series of partially methylated alditol acetates containing an acetyl group at the site of linkage [3]. Although G U M S can be used for small