Structural analysis of glycoconjugates by on-target enzymatic digestion and MALDI-TOF-MS

Exoglycosidase digestion combined with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) has been demonstrated to be an effective method for the structural characterization of glycoconjugates and oligosaccharides in picomolar amounts. A sample preparation method is described, in which 6-aza-2-thiothymine (ATT) in water is used as matrix and enzymes are dialyzed before use against a low concentration of volatile buffer such as ammonium acetate. Under these conditions, a series of sequential on-target exoglycosidase treatments was carried out in one single analyte spot in the presence of ATT matrix. Subsequent mass spectrometric analysis of the resulting products yielded information on both the completeness of the reaction and structural features of the glycoconjugates such as monosaccharide sequence, branching pattern, and anomeric configurations of the corresponding glycosidic linkages. The results show that all exoglycosidases used retain their activity in the presence of ATT matrix. Hence, structural analysis of carbohydrates or mixtures thereof can be performed very fast, without intermediate desalting steps or sample splitting. This approach is illustrated by the analysis of underivatized glycans, oligosaccharide derivatives, glycopeptides, and glycolipids. Depending on the analyte, amounts of sample required could be limited to a few picomoles.