Studies of oligo- and polysaccharides by MALDI-TOF and ESI-ITMSn mass spectrometry

摘要

This thesis presents methods for analyzing carbohydrates with matrix-assisted laser desorption/ionization time-of-flight, MALDI-TOF, and electrospray ionization ion trap multiple-stage, ESI-IT MSn, mass spectrometry, MS. MALDI-TOF MS was employed to determine chain length distributions of amylopectin, the main constituent of starch. The technique was compared with high-performance anion-exchange chromatography with pulsed amperometric detection, HPAEC-PAD, an established technique for this purpose. Starch from potato, wheat, and waxy maize was debranched with isoamylase and analyzed using both techniques. Similar differences between chain length distributions for amylopectin from different sources were observed with both methods, but MALDI-TOF MS was less reproducible than HPAEC-PAD and overestimated the amount of longer chains. However, MALDI-TOF MS analysis is faster, more sensitive, and provides detailed information on the mass of the unit chains. Maltoheptaose and three human milk oligosaccharides were derivatized by reductive amination followed by N,N-dimethylation. The carbohydrate derivatives were investigated by MALDI-TOF MS and MALDI post-source decay (PSD) TOF MS. The resulting derivatives have a positive charge localized to the modified reducing end. The approximate detection limit for the resulting maltoheptaose derivative was 50 fmol corresponding to a tenfold increase in sensitivity compared to underivatized oligosaccharides. When the derivatives were analyzed by MALDI-PSD TOF MS the observed fragmentation pattern was dominated by fragment ions retaining the modified reducing terminus, thus simplifying the interpretation of the MS-data. Saponins from Quillaja saponaria Molina with previously identified structures were investigated by ESI-IT MSn in both positive and negative ion modes. MS1–MS4 spectra were analyzed showing that structural information can be obtained on both oligosaccharide parts in the saponins from positive ion mode spectra whereas negative ion mode spectra mainly provided information on one of the oligosaccharide parts. Analysis of MS1–MS4 spectra identified useful key fragment ions important for structural elucidation of Quillaja saponins. A flowchart involving a stepwise procedure based on the key fragments from MS1–MS3 spectra was constructed for identification of structural elements in the saponin.