Glycan Composition by Mass Spectrometry
Because most N- and O-linked glycans contain only a limited number of mass- different monosaccharide constituents, their measured mass can lead directly to their monosaccharide composition. Table 3.3 lists masses of the common monosaccharide constituents. Several laboratories have written software for this task, and there are applications on the web such as that in GlycoWorkbench (http://download.cnet.com/windows/glycoworkbench/3260-20_4-10238922-1. html). It is tempting to submit such compositions to a database and accept the glycan structure that is returned but, because ofthe possible existence of isomeric structures, this method is not rigorous enough. Such structures must be confirmed by other techniques. Even mass spectral fragmentation is insufficient because it only produces sequence and linkage information and does not yield any information on the nature of the constituent monosaccharides. Such information must be obtained by orthogonal methods such as exoglycosidase digestion or methylation analysis by GC/MS.