Year of Publication
Doctor of Philosophy (PhD)
Arts and Sciences
Dr. Bert C. Lynn
Lignin, the second most abundant naturally occurring polymer found in plant cell wall has the potential of becoming an alternative source for the production of chemical synthons for the pharmaceuticals and other chemical industries. While much gain has been made towards the development of degradation methods to break down lignin, effective analytical methods are still required to rapidly and accurately identify the products of lignin breakdown experiments. The goal of this work was to develop mass spectrometric methods for the characterization of lignin oligomers based on the study of model lignin compounds.
Unlike peptides and oligosaccharides, lignin model compounds that could serve as analytical standards for methods developments are not commercially available, and hence, the first project of this dissertation focused on the synthesis of lignin model compounds containing the β-O-4 bond. The priority was to synthesize compounds containing all important functionalities that reflect the structure of native lignin. By employing the known Aldol reaction, lignin oligomers containing the β-O-4 were synthesized. The synthesized β-O-4 lignin oligomers contained the characteristic functional groups of native β-O-4 lignin, that is, the phenolic functionality, the aryl glycerol β-O-4 aryl ether bond and the unsaturated side chain.
The second project was aimed at developing alternative ionization methods for the characterization of lignin in the negative ion mode mass spectrometry. A chloride adduct ionization method was developed and used for characterizing and sequencing lignin oligomers. This method proved to be very useful in stabilizing the adduct ion in the full scan spectrum mode and also providing useful structural information upon tandem mass spectrometry.
In the third project, a cationization technique was developed to unambiguously assign the sequence in which β-O-4 lignin oligomers are connected. A simple and easy to use sequencing chart was designed and could serve as a guide for predicting the sequence of larger lignin oligomers. This method offers an alternative approach for the characterization of lignin oligomers in the positive ion mode mass spectrometry.
The fourth project focused on the ionization response of a new class of β-O-4 lignin compounds. β-O-4 compounds having the same skeletal backbone but different non-polar groups at the a-position were synthesized, and their ESI response studied. Results from this study show that a slight change in the structure of a β-O-4 lignin model compound can change the cationization response to several order of magnitude. Most importantly, this work for the first time has shown a direct correlation between lignin ionization response and lignin structure.
The fifth project was aimed at studying the chromatographic behavior of the diastereomer pair in β-O-4 lignin model compounds. Using three commercially available HPLC columns, the chromatographic behavior and factors that affect the separation of the diastereomer pair of the β-O-4 lignin diastereomer on an HPLC column were studied. By performing tandem mass spectrometry on each of the diastereomers, a fragmentation mechanism was developed that could be used to unambiguous assign the configuration (erythro or threo) for the pair of diastereomer in a β-O-4 model.
The results presented in this dissertation adds significant knowledge to the lignin mass spectrometry literature, and it offers new ionization techniques for the characterization of lignin oligomers, most importantly, an alternative approach for lignin analysis using adduct ionization mass spectrometry. The developed methods could easily be extended for the characterization of larger lignin oligomers.
Digital Object Identifier (DOI)
This research was supported by the National Science Foundation EPSCoR Track 2 (OIA 1632854).
Asare, Shardrack O., "BOTTOM-UP LIGNOMICS: TOWARDS THE DEVELOPMENT OF ADDUCT ELECTROSPRAY IONIZATION MASS SPECTROMETRIC METHODS TO CHARACTERIZE AND SEQUENCE LIGNIN OLIGOMERS" (2019). Theses and Dissertations--Chemistry. 115.