Influence of Multivalent Metal Cations on Carbohydrate Separations and Fragmentation Patterns via Ion Mobility-Mass Spectrometry
Morrison, Kelsey Anne
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Carbohydrates, oligosaccharides, and glycans occupy a corner of biopolymers that can be overlooked in the shadow of proteins and nucleic acids but are nonetheless essential for the survival of biological organisms. However, unlike other biological macromolecules, known carbohydrate units have largely homogeneous elemental compositions, where species are instead distinguished by variations in stereochemistry and branching patterns. Because this creates a scenario where complex biological samples may contain isomeric mixtures which cannot be differentiated by mass isolation, incorporating ion mobility separations in front of mass spectrometry can provide another dimension of information to distinguish among isomers. Yet another angle for enhancing glycan analysis with ion mobility-mass spectrometry is by leveraging interactions between carbohydrate species and metal cations, which can form adducts that have altered gas-phase structures relative to protonated or deprotonated ions. In some situations, metal adduction exaggerates the differences in effective size among isomers and thereby facilitates isomeric glycan separation in the drift time space. This dissertation is focused on conveying the utility of metal-glycan interactions for improving the ion mobility-mass spectrometric analysis of carbohydrate species while also describing some complementary experiments that were pursued in support of glycan analysis. Of note, ultraviolet photodissociation (UVPD) with metal-enhanced fragmentation and Fourier multiplexing of ion mobility-ion trap mass spectrometry contributed well to isomeric mixture analysis. Divalent metal cations including select transition metals and alkaline earth metals were evaluated for their potential to enhance drift time separations, with transition metals proving particularly useful in this regard for mixture analysis. Preliminary oligosaccharide analysis with divalent metal species found that carbohydrates will form both singly-charged monomeric metal-glycan adducts as well as doubly-charged dimeric metal-glycan adducts where both appear at the same m/z value. These dimers were subsequently scrutinized for the potential to form heterodimeric metal-glycan dimers wherein two different isomeric glycans could form a single dimer during a mixture analysis without mobility separation and form chimeric mass spectra. Through a combination of isotopic labeling and ion mobility-mass spectrometry, it was shown that heterodimeric metal-glycan species will form, and that mobility separation is vital for analyzing glycan mixtures.