Liquid chromatography/mass spectrometry sequencing approach for highly sulfated heparin-derived oligosaccharides

Liquid chromatography/mass spectrometry (LC/MS) is applied to the analysis of complex mixtures of oligosaccharides obtained through the controlled, heparinase-catalyzed depolymerization of heparin. Reversed-phase ion-pairing chromatography, utilizing a volatile mobile phase, results in the high resolution separation of highly sulfated, heparin-derived oligosaccharides. Simultaneous detection by UV absorbance and electrospray ionization-mass spectrometry (ESI-MS) provides important structural information on the oligosaccharide components of this mixture. Highly sensitive and easily interpretable spectra were obtained through post-column addition of tributylamine in acetonitrile. High resolution mass spectrometry afforded elemental composition of many known and previously unknown heparin-derived oligosaccharides. UV in combination with MS detection led to the identification of oligosaccharides arising from the original non-reducing end (NRE) of the heparin chain. The structural identification of these oligosaccharides provided sequence from a reading frame that begins at the non-reducing terminus of the heparin chain. Interestingly, 16 NRE oligosaccharides are observed, having both an even and an odd number of saccharide residues, most of which are not predicted based on biosynthesis or known pathways of heparin catabolism. Quantification of these NRE oligosaccharides afforded a number-averaged molecular weight consistent with that expected for the pharmaceutical heparin used in this analysis. Molecular ions could be assigned for oligosaccharides as large as a tetradecasaccharide, having a mass of 4625 Da and a net charge of -32. Furthermore, MS detection was demonstrated for oligosaccharides with up to 30 saccharide units having a mass of > 10,000 Da and a net charge of -60.