Mass spectrometric structural elucidation of sporopollenin and lignin

Abstract

Sporopollenin is a natural, highly cross-linked polymer composed of carbon, hydrogen, and oxygen, which forms the outer wall of pollen grains. Sporopollenin is highly resilient to mild chemical degradation. Because of this stability, its exact chemical structure and the biochemical pathways involved in its synthesis are not yet clear. There is no mild degradation method capable of degrading sporopollenin without extreme modification and/or depolymerization. However, several harsh degradation methods were capable of degrading sporopollenin into a mixture of small and modified monomers that provides no clue to the sporopollenin complex native structure. In this thesis, Sporopollenin's molecular structure was demystified and unraveled without any prior degradation and/or modification (Top-down approach). This was accomplished using Secondary Ion Mass Spectrometry SIMS-TOF-MS/MS and Matrix-Assisted Laser Desorption MALDI-TOF-MS/MS. The latter mass spectrometric techniques allowed us to identify the two major building units of the sporopollenin network. The identified building units were supported by solid-state NMR studies. Lignin oligomers are an essential source of aromatic compounds; it is composed mainly of three monomers: p-coumaryl (H), coniferyl (G), and sinapyl (S) alcohols which are linked covalently in different fashions. It is well known that various methods for releasing and purifying lignin from wood cellulose lead to a drastic change in lignin chemical structure. In 2015, Banoub et al. introduced a new paradigm that states that accurate structural studies should be done on lignin oligomers released from the polysaccharide without further purification or chemical transformations. This type of lignin is called virgin released lignin (VRL). In the second part of this thesis, different MS ionization methods such as MALDI, APPI (Atmospheric Pressure Photoionization, and ESI (Electrospray ionization) coupled with tandem mass spectrometry was used to investigate the major structural features of the French Oak and Pine VRLs.