Matrix-Assisted Laser Desorption/Ionization Mass Spectrometric Analysis of Uncomplexed Highly Sulfated Oligosaccharides Using Ionic Liquid Matrices

An effective matrix-assisted laser desorption/ionization mass spectrometric (MALDI-MS) analysis depends on several steps, among which the sample preparation procedure, especially the selection of suitable matrix, largely determines whether subsequent desorption/ionization and detection successfully follow.1 Significant effort has been put forth toward understanding the mechanism of MALDI2,3 and the role of matrix in the ionization process;4-6 yet matrix selection and formulation remain essentially empirical procedures. Over the past few years, a growing interest in ionic liquids has resulted in the publication of several reports describing applications of these compounds in the MALDI-MS analysis of a variety of compounds including biomolecules.7-11 Ionic liquids are organic salts generally having melting points below 100 °C.12 This class of compounds have a number of characteristics that make them attractive for use as MALDI matrices, particularly, their high thermal stability, negligible vapor pressure, and virtually universal solvent properties.13,14 In a pioneering study, Armstrong and co-workers evaluated 38 ionic liquids for use as matrices for ultraviolet (UV) MALDI-MS;13 20 of these were shown effective for more than one analyte.7 In comparison to the conventional matrices, the effective ionic liquid matrices (ILMs) afforded greater vacuum stability, lower detection limits, higher homogeneity of the matrix/analyte mixture, and thus better shot-to-shot reproducibility.13 Later, ionic liquids were successfully applied in the MALDI-MS analysis of synthetic oligonucleotides, monosialylated glycans and glycosides, neutral oligosaccharides, poly-(ethylene glycol), glycoconjugates, peptides, proteins, and phospholipids.8-11 The work presented here describes a method for UV-MALDI time-of-flight (TOF) mass spectrometric analysis of highly sulfated oligosaccharides using ILMs. Oligosaccharides are small biopolymers (dimers to decamers) of monosaccharide units joined through glycosidic linkages between the anomeric hydroxyl group of one monosaccharide and a hydroxyl group of another monosaccharide. A great variety of monosaccharide building blocks and the possibility of different positional and stereochemical (α- or β-glycosidic) linkages between these saccharide building blocks result in the tremendous structural complexity of oligosaccharides critical for their diverse biological functions.15,16 All cells are surrounded by a carbohydrate-rich anionic coating known as a glycocalix. Alterations in a cell’s glycocalix, particularly the extended hydrated mesh of negatively charged proteoglycans capable of specific interactions with a large number of proteins and enzymes, have been implicated in many pathological conditions. The central role of the oligosaccharide sequences of the glycosaminoglycan chains of these proteoglycans in cell recognition and signaling renders them a potentially important class of pharmaceutical agents. Anionic polysaccharides have also been successfully tested in vivo as vaccines against malaria, leishmaniasis, HIV, cancer, and tuberculosis.17 Two analytes used in this study are synthetic, highly sulfated oligosaccharides that are established pharmaceutical agents. An octasulfated disaccharide, sodium sucrose octasulfate (SOS), is the active ingredient of the antiulcer drug Sucralfate, and an octasulfated pentasaccharide (Arixtra) is an anticoagulant drug used in prophylaxis of deep vein thrombosis. While the biological significance of highly sulfated oligosaccharides is widely recognized, relatively little progress has been made to date in developing high-throughput MALDI mass spectrometry-based procedures for characterization of these compounds.18,19 Due to their polydispersity, structural complexity, and labile chemical nature, polysulfated oligosaccharide natural products are challenging analytes. The only currently available method for detection and characterization of highly sulfated oligosaccharides by MALDI-TOF-MS not involving chemical derivatization was developed 11 years ago by Juhasz and Biemann.20 They showed that heparin-derived polysulfated oligosaccharides produced high-quality MALDI mass spectra in a form of noncovalent complexes with basic peptides, and the analyte fragmentation through sulfate loss was almost completely suppressed. The method permitted the detection of a polysulfated oligosaccharide molecular ion in complex with the peptide. While uncomplexed disulfated and trisulfated oligosaccharides required 100 pmol/μL concentration to obtain a signal in the negative-ion mode, a noncovalent complex of octasulfated hexasaccharide and an arginine-containing synthetic peptide produced a strong positive-ion signal at 3 pmol/μL concentration of oligosaccharide. The most efficient formation of a peptide-oligosaccharide complex was observed when the number of arginine residues, n in a peptide with a sequence (Arg-Gly)n exceeded the number of sulfate groups in oligosaccharide by at least one.21 While this method has been pioneered by Sasisekharan and co-workers22 for the analysis of sulfated oligosaccharides, its relatively high sensitivity is compromised by the necessity of having a library of peptides suitable for analysis of polydisperse mixtures, which renders the method time-consuming and material-consuming. The current study describes the first successful analysis of uncomplexed, underivatized, highly sulfated oligosaccharides. UV-MALDI-TOF mass spectrometry was used to analyze the sodium salts of two octasulfated oligosaccharides, SOS and Arixtra. Five ILMs were tested for their effectiveness in facilitating the detection of the two analytes. Each analyte produced signal with multiple ionic liquids, but only two ILMs were determined to be equally efficient for the analysis of both SOS and Arixtra.