Matrix-Assisted Laser Desorption/Ionisation Time-of-Flight Mass Spectrometry. A Comparison of Fragmentation Patterns of Linear Dextran Obtained by In-Source Decay, Post-Source Decay and Collision-Induced Dissociation and the Stability of Linear and Cyclic Glucans Studied by In-Source Decay

In the first part of this study, fragmentation patterns from a range of dextran oligomers (containing 4–20 anhydroglucose units) were compared using three different methods of analysis coupled with matrix-assisted laser desorption/ionisation (MALDI) mass spectrometry. Collision-induced dissociation (CID), prompt in-source decay (ISD) and post-source decay (PSD) all caused cleavage of the glycosidic bonds. Both CID and, to a lesser extent, ISD caused further cleavage of pyranose rings of the individual sugar residues. There was very little cleavage of pyranose rings detected in the PSD spectrum. Derivatisation of the reducing end-groups of the oligodextrans with 1-phenyl-3-methyl-5-pyrazolone (PMP) restricted cleavage in the MALDI mass spectrometer to the non-reducing end and also enabled the saccharides to be separated by high-performance liquid chromatography (HPLC) so that a single chain length could be examined as a standard. Maltoheptaose was also used as a standard. In the second part of the study, prompt ISD-MALDI mass spectrometry was used to compare the fragmentation of three oligoglucans, viz. dextran, maltodextrin and gamma cyclodextrin, that have different linkages and different secondary structure. The results showed that the degree of fragmentation correlated with the degree of freedom in the saccharide chains in solution as determined by NMR. Dextran, with the most random conformation, was fragmented most whereas there was little evidence of any fragments, not even glycosidic bond breakage, from cyclodextrin, even when the laser power was increased considerably. The fragmentation pattern of maltodextrin was intermediate. The patterns of fragmentation produced by MALDI mass spectrometry, particularly where standards are available to calibrate the spectrum and the energy of the laser is controlled, can be used to predict the type of linkage present.