Year of Publication

2022

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Arts and Sciences

Department/School/Program

Chemistry

First Advisor

Dr. Bert C. Lynn

Abstract

Lignocellulosic biomass is pivotal for the development of renewable energy sources and materials essential to mitigate the exploitation of fossil fuels causing climate change and environmental pollution issues. The conversion of biomass into fuel requires the hydrolysis of cellulose and a biproduct of this process is the isolation of millions of tons 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 degradation processes have been developed to study the structure of lignin, depolymerization compositions are complex and prone to repolymerization. As a result, the primary strategy to mitigate difficulties is the development of model systems based on native lignin linkages. Analytical methods for lignin and lignin model compounds are critically limited due to the lack of commercially available compounds and the complex nature of the lignin polymer. 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 fundamentally 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 that are manipulated by electric fields for mass to charge (m/z) analysis, 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 adduct cationization has recently been discovered as a promising method for ESI-MS sequencing of lignin compounds. The equilibrium of ion transfer reactions is governed by gas-phase basicity, a fundamental measure of the thermodynamics of supramolecular interactions that define ionization success. 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. Sustainable and economically viable processes are still under development since current pre-treatment methods for the removal of lignin generates toxic compounds and are unsuitable for commercial applications. 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, non-toxicity, and availability on the industrial scale. The formation constant (K) or binding strength between guest and host was evaluated for lignin model dimers with cyclodextrin by lithium adduct 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 were also extensively evaluated. Lignin compounds and metabolites have also shown biological activity and therefore the separation of diastereomers is of interest for pharmaceutical and medicinal applications. To advance biological studies, the success of chromatographic separations (HPLC) of lignin model dimers and their diastereomers were evaluated. The separative method was coupled to MS with post-column lithium adduct ionization 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.

This dissertation supports the development of analytical methods for lignin degradation products and secondary metabolites (lignans) that have shown exciting biological activities. Fundamental characterizations of ionization for mass spectrometry are important for a variety of analytical applications including the sequencing of lignin compounds, gas-phase thermodynamic studies, and the optimization of separation techniques. Continued improvements in this field will reduce our exploitation of fossil fuels and advance the sustainable conversion of lignocellulosic biomass into biofuels and platform aromatic chemicals.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2022.163

Funding Information

Funding for this research was provided by the National Science Foundation EPSCoR Track 2 (OIA 1632854) from Aug 2017 - Aug 2021

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