Plants are the most important natural resources. They can serve as energy source or provide raw materials for many industrial applications. The plant materials used mostly consist of plant cell walls. The cell walls are very rigid and heterogeneous networks of biopolymers such as proteins, lignin and most importantly polysaccharides which encapsulate every plant cell. These structures account for physical strength and defense mechanisms of the plant and play an important role in plant development. Pectins are very complex and highly branched cell wall polysaccharides. However, their distribution, function and biosynthesis have not yet been fully understood. In order to improve the economic usability of the cell wall, a better understanding of the composition, functions and biosynthesis of cell wall polysaccharides is essential. Libraries of chemically synthesized well-defined oligosaccharides and functionalized glycosyl donors can serve as important tools to advance existing experimental methods for investigating structure and biosynthesis of plant polysaccharides. In chapter 2.1, the automated solid-phase synthesis of 23 arabinogalactan (AG) oligosaccharide fragments of pectin side chains is described. Automated glycan assembly (AGA) allowed the assembly of a library of structurally related type-I and type-II AGs on a linker-functionalized solid support from a set of differently protected galactose, arabinose and glucuronic acid monosaccharide building blocks (BBs). The synthetic glycan structures obtained were used to assign the exact binding epitopes of cell wall glycan-directed mAbs using glycan microarrays. Furthermore, the synthetic type-I AGs have been used to determine the substrate specificities of three endo-galactanases which are cell wall-degrading enzymes that are used to deconstruct pectin polymers for structural analysis. A detailed knowledge of the substrate specificities of these cell wall-degrading enzymes is required to deduce the composition of the original polysaccharide. In chapter 2.2, solid-phase and solution-phase syntheses of apiose-substituted α(1,4)-homogalacturonan backbone structures, as found in rhamnoglacturonan-II (RG-II) were initiated. A set of three galactose BBs and one apiose BB was applied in AGA. However, the automated solid-phase synthesis of the backbone scaffold suffered from limited diastereoselective control during the first glycosylation reaction with the linker and the low nucleophilicity of the C4-hydroxyl group of galactose. In a solution-phase test series the diastereoselectivity during the glycosylation reaction with the linker could be improved using a fluorinated linker, but satisfactorily results were not obtained. Subsequently, two galacturonic acid lactone BBs were tested in solution-phase to assemble a (1,4)-linked galacturonic acid backbone consisting exclusively of α-linkages. XII However, the galacturonic acid lactone BBs were also not applicable to assemble oligogalacturonates beyond disaccharides. In chapter 2.3, four differently functionalized UDP-galactopyranoses (UDP-Galp) and one functionalized UDP-arabinofuranose (UDP-Araf) were chemically synthesized. The azido- or amino-functionalized sugar-nucleotides served as enzyme substrates in first proof-of-principle studies towards the development of a new high-throughput assay for the characterization of putative plant glycosyltransferases on glycan microarrays. It was shown that the tested transferases tolerate these modifications of the glycosyl donor substrates. The azido- or amino-functionalized residues were incorporated into acceptor oligosaccharides on microarray surfaces by the glycosyltransferases and the enzymatic reaction products were subsequently visualized by selective reaction of fluorescent probes with the azido- or the amino-groups.
