Infrared Laser Desorption Mass Spectrometry of Oligosaccharides: Fragmentation Mechanisms and Isomer Analysis

Infrared laser desorption/ionizatlon of oligosaccharides and their naturally occurring derivatives leads to a distinctive fragmentation pattern, which Is useful both for the analysis of functional groups (e.g., the positions of fatty acyl groups In lipid A and other glycolipids) and for distinguishing Isomers of underivatized oligosaccharides with different glycosidic linkages. The predominant mechanism of fragmentation can be understood as a pericyclic hydrogen rearrangement of the retro-aldol) reaction type, leading to fragmentation of the sugar ring. A prerequisite for this process appears to involve opening of the hemiacetal saccharide ring to the linear saccharide form by rapid heating from the infrared laser irradiation. Saccharides derivatized at the a nom eric hydroxyl group (I.e., cyclic acetate) normally do not undergo this ring opening and thus do not exhibit ring fragmentation reactions. This fragmentation mechanism can be used to predict and Interpret fragmentation patterns of unknown oligosaccharides, glycosides, lipid A, and other glycoconjugates.