Year of Publication

2017

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Arts and Sciences

Department

Chemistry

First Advisor

Dr. Bert C. Lynn

Abstract

Lignin, one of main components in the woody cell walls, is a complex heterogeneous biopolymer, which provides structural support and transportation of water in plants. It is highly recalcitrant to degradation (both chemically and environmentally) and protects cellulose from being degraded/hydrolyzed. Due to the structural complexity of native lignin, complete characterization and elucidation of lignin’s structure remains very challenging. The overarching goal of this work is to develop mass spectrometry based analytical methods to contribute to a better understanding of lignin structures.

This dissertation will focus on the development and application of High-Resolution Accurate-Mass (HRAM) Mass Spectrometry (MS) as main analytical technique for studying lignin model compounds, including understanding the ionization behavior, studying corresponding fragmentation patterns and extracting structural information for structural elucidation eventually. Analytical methods were also developed to study the post-pretreatment products of the synthetic trimeric model compound using High-Performance Liquid Chromatography (HPLC) coupled with High-Resolution Accurate Mass (HRAM) Mass Spectrometry (MS).

The first project of this dissertation focuses on mass spectral the characterization of lignin models from the in vitro oxidative coupling reactions. Three specific trimeric compounds were isolated and their ionization behaviors were investigated using HRAMMS via electrospray ionization (ESI). The reaction parameters of the in vitro oxidative coupling reaction were critical in alternating the linkage profiles of resulting dehydrogenation polymers (DHPs). Reaction parameters were tuned to obtain desired DHP linkages profile. Upon the isolation of three different trimeric compounds, a systematic comparison of ionization efficiency of three trimeric compounds was carried out using ESI-HRAM-MS under different ionization conditions.

The second project was aimed to design a synthetic route for a lignin model compound that will be a good representation for native lignin during the pretreatment process. The model compound of interest has not been obtained previously through chemical synthesis. Due to the reactivity of cinnamyl alcohol, which contains the unsaturated side chain, this new synthesis strategy was developed based on the known aldol-type reaction route. A versatile synthesis procedure for preparation of β-O-4 oligomeric compounds was designed and implemented to include the most important functional groups (phenolic alcohol, aryl glycerol β-aryl ether bond and unsaturated side chain) in the resulting model compound. This new synthesis route also allowed incorporation of different monolignols.

In the third project, Fenton chemistry was applied to a synthetic lignin model compound. Due to the non-specificity in the post pretreatment product profile, nontargeted analytical strategy was developed and applied to study the post-pretreatment products of the model compound using HPLC-HRMS.

The results from this dissertation showed a significant difference in ionization behavior between three structurally different model compounds and indicated that primary structures of lignin compounds can largely affect corresponding electrospray ionization properties as well as fragmentation pattern. The work in this dissertation provides analytical techniques for non-targeted analysis of complex lignin samples and an insightful understanding of Fenton’s reaction pretreatment upon lignin model compound.

Digital Object Identifier (DOI)

https://doi.org/10.13023/ETD.2017.149

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