University of North Carolina at Chapel Hill Graduate School
Doi
Abstract
Complete structural elucidation of carbohydrate molecules remains a prominent challenge in analytical chemistry. Much of the structural intricacy of carbohydrates stems from the various isomeric monosaccharide subunits that are linked together. To date, there are few analytical techniques capable of differentiating monosaccharide isomers. Mass spectrometry-based technologies are promising for differentiation of monosaccharides because of their high selectivity and sensitivity, and short analysis times. Mass spectrometry analysis first requires generation of gas-phase ions from solution-phase molecules, which are then separated based on their mass-to-charge ratio (m/z). Monosaccharide isomers cannot be distinguished by mass spectrometry alone because they have identical m/z. Tandem-mass spectrometry (MS/MS) techniques rely on gas-phase chemistry to differentiate isomers and stereoisomers within the mass spectrometer. Two examples of gas-phase chemistries that are useful for isomer differentiation are unimolecular dissociation or an ion/molecule reaction. The MS/MS response, or gas-phase chemistry, of an ion will depend on ion structure and the charge carrier (H+/Na+/Li+/etc.), which is affected by the mode of ionization.Electrospray ionization (ESI) is commonly employed for ionization of carbohydrates. Because monosaccharides have a high metal cation affinity and because sodium is ubiquitous in solvents, ESI of a monosaccharide solution results in sodium-cationized monosaccharides. Alternatively, lithium salts can be added into the ESI solution to generate lithium-cationized monosaccharides. Monosaccharide oxygen atoms form multidentate (bi-/tri-/tetradentate) coordinations with lithium, and multiple potential sites for cation coordination exist on a monosaccharide molecule. To differentiate monosaccharide isomers, the ion distribution that each isomer forms must have measurable differences in gas-phase chemistry. The most common MS/MS technique is collision-induced dissociation (CID), but, in general, CID response is not disparate enough between lithium-cationized monosaccharide isomers for differentiation. Another MS/MS technique that has been able to differentiate isomeric monosaccharides is the water adduction ion/molecule reaction. Using a combination of computational data and experimental water adduction data the structures of solution- and gas-phase lithium-cationized monosaccharide ions were explored, and the chemistry and mechanism of the water adduction reaction was investigated. Finally, using CID and water adduction, the gas-phase dissociation chemistry and product ion structures of lithium cationized hexoses were shown to be more complex than previously postulated.Doctor of Philosoph
