Synthesis of biomass-containing xylan fragments and evaluation of ferulic acid esterase activity

Lignocellulosic material from the plant cell wall would be an important and CO2 neutral energy resource, if it could be properly utilised. However, intrinsic recalcitrance hampers the utilisation. Especially enzymatic degradation has received extensive attention as a way to solve this problem. An essential tool for development of new enzymes is well-defined model substrates, which can provide a deeper understanding of the substrate specificity and reactivity.

Hence, chemically synthesised model substrates often outcompete isolated substrates from natural sources due to homogeneity, purity, and reproducibility. The research presented in this thesis involved the synthesis of both arabinoxylan and glucuronoxylan fragments designed as model substrates for xylanases, α-arabinofuranosidases, and α-glucuronosidases.

The syntheses involved the utilisation of thioxyloside building blocks linked in an iterative glycosylation strategy without pre-activation. The arabinose building block were synthesised through a small linear synthesis. On the other hand, the glucuronate building blocks were derived from the same thioglucopyranoside derivative through a divergent strategy (Figure 1).

One known class of the lignocellulosic enzymes are ferulic acid esterases. These enzymes are able to cleave the ester-linkages between monomeric or polymeric ferulic acids and a polysaccharide chain or lignin of the plant cell wall. The ferulic acid ester-linkages of the plant cell wall are known to hamper the degradation by other hemicellulosic enzymes.

Hence, ferulic acid esterases are interesting tools for enzymatic degradation. The ferulic esterases can be classified into 13 subfamilies based on a phylogenetic three. In this thesis, 14 enzymes belonging to six different subfamilies were tested for specificity and reactivity towards both synthetic model substrates and natural substrates, thereby, being able to relate the reactivity and specificity to the classification.

Furthermore, the catalytic abilities of different molybdenum complexes for regioselective glycosylations have been tested.