Exit Seminar "Mass Spectrometric Analysis of Lignin Model Compounds: Fundamental Investigations of Ionization and Supramolecular Interactions for Lignocellulosic Biomass Applications
Abstract: Lignocellulosic biomass is pivotal in the development of renewable energy sources and materials essential to mitigate the exploitation of fossil fuels causing environmental pollution issues. The conversion of biomass into fuel requires the hydrolysis of cellulose and a biproduct of this process is the isolation of lignin as biorefinery waste. Lignin is a complex high molecular weight polymer whose structure remains undefined and critically limits potential industrial applications of lignocellulosic biomass. The advancement of analytical methods for structural elucidation of lignin and its ensemble of phenolic compounds is therefore essential to advance this field. While a variety of analytical methods play an integral role in developing our understanding of lignin, only mass spectrometry can provide exact information on the substructure of lignin, the sequence of monolignols, and linkage types. In this dissertation, the supramolecular interactions of a variety of model lignin monomers and dimers are characterized to improve mass spectrometric analysis and potential applications of lignin as a renewable source of valuable phenolics. Mass spectrometry (MS) requires the conversion of analytes into detectable gas-phase ions, and the most widely used ionization technique for biological compounds is electrospray ionization (ESI). The primary challenge facing ESI-MS analysis of lignin is ionization because lignin compounds do not readily accept protons for positive mode analysis and negative mode analysis causes destabilization and in-source fragmentation. While protonation is unsuccessful, lithium adduction has recently been discovered as a promising method for ESI-MS sequencing of lignin compounds. Consequently, the gas-phase lithium cation basicity of synthetic monolignols and dimers were characterized by ESI-MS to improve sequencing techniques and future applications of lithium adduction. Lignin also presents a challenge in biomass processing due to its inhibition of the enzymatic hydrolysis of cellulose for biofuel production. Supramolecular guest-host interactions have the potential to isolate lignin compounds from biomass fractions through the formation of inclusion complexes and the development of selective materials. In this work a cyclodextrin host was selected based on its remarkable ability to encapsulate guest molecules and availability on the industrial scale. The binding strength between guest and host was evaluated for lignin model dimers with cyclodextrin by ESI-MS for comparison with our collaborators ITC and computational results. The retention of electrostatically bound complexes during the ESI-MS process and lithium adduct impacts are also extensively evaluated. Lignin compounds and metabolites additionally show biological activity, and therefore the separation of diastereomers is of interest for pharmaceutical applications. To advance biological studies, the success of chromatographic separations (HPLC) of lignin model dimers and their diastereomers is evaluated. The separative method is coupled to MS with post-column lithium adduction to identify lignin dimers. Novel determinations of lignin dimer partition coefficients are also presented, a measure of hydrophobicity important for biological studies and chromatographic method development. These fundamental characterizations of lignin model compounds are essential for the continued advancement of renewable energy and materials derived from lignocellulosic biomass.
