DTU Chemistry - PhD 2016

Synthesis of key Biomass Fragments

The stepwise glycosylation method based on preactivation is a viable path towards arabinoxylans as well as glucuronoxylans – two important groups of biomass fragments.

Hemicellulose is one of the three main classes of compounds in the plant cell wall, and therefore an important resource for biomass utilization. Some utilization of hemicellulose is seen, primarily in the food industry. However, the abundance of hemicelluloses makes it attractive to look for wider utilization, but so far this has been difficult. Lack of access to this class of molecules prevents the use of enzymatic studies which could increase our understanding of the biochemical processes relevant to the synthesis and degradation of hemicellulose. In the project, two important groups of hemicellulose fragments have been synthesized.

Hemicellulose contributes to the strength of the plant cell walls. Some hemicelluloses have been shown to be critical in the growth of plants by functioning as the vessel walls necessary to transport water. The vessel walls need to be strong to withstand the high negative pressure generated by transpirational pull. In seeds, hemicelluloses can function as storage for carbohydrates, analogous to starch.

In the food industry, hemicelluloses are currently used for improvement of the quality of cereal flours and dough or as a starting material for xylitol, a popular substitute for sugar. However, several other industrial applications can be imagined, e.g. in the production of bioethanol.

Hemicelluloses are a group of polysaccharides having β-(1→4)-linked backbones of glucose, mannose or xylose. The latter are the most common, constituting the second most abundant group of polymers in plants.

In the project, the synthesis of arabinoxylans as well as glucuronoxylans is demonstrated.

Two alternative strategies to synthesize a variety of xylan backbones were investigated. The first strategy attempted to use an unprotected xylose acceptor in a tin-mediated glycosylation, but did not produce satisfactory results. The second strategy was based on the preactivation of thioglycosides to be glycosylated with thioglycoside acceptors which in turn can be preactivated again in a second step. Optimization of this latter strategy led to a viable pathway towards a variety of protected xylan backbones. The use of protecting groups allows for the specific introduction of branching units to the backbone. Subsequently arabinose as well as glucuronic acid were attached to the xylan backbone.

In conclusion, it has been shown that the stepwise glycosylation method based on preactivation is a viable path towards arabinoxylans as well as glucuronoxylans. The building blocks are easily accessible and the method allows for a rapid assembly of at least pentaxylans. Furthermore, it has been shown that the chosen protecting group strategy allows for an easy deprotection of the xylan backbone. It also gives both options on where to install the branching sugars. The 0-2 as well as the 0-3 position are easily accessible and conceivably both positions could be accessed at the same time.


Illustration:
Synthetic strategy for assembling arabino- and glucuronoxylans

DTU Chemistry - PhD 2016
Supervisor:
Robert Madsen
rm@kemi.dtu.dk

Funded by:
The project was funded by the Danish Council for Strategic Research.
http://www.kemi.dtu.dk/english/education/phdstudy/phd_defences2016/maximilianfelixbohm
23 MAY 2018