Pectin is known as a jellifying agent in the food industry, but may find numerous other applications. The project presents synthesis of well-defined pectic oligosaccharides.
Pectins are an important group of polysaccharides found in the plant cell wall. The plant cell wall represents almost 50 % of plant biomass. Pectins are already used as food ingredients and in pharmacy, but with the increasing focus on biomass utilization also other applications become of interest. The project presents chemical synthesis of well-defined pectic oligosaccharides.
Pectin is known as a jellifying agent in the food industry, and is also used for manufacture of encapsulated drugs and skin-care products. In plants, pectin polysaccharides have a similar but wider role contributing heavily to the mechanical strength and physical properties of the cell wall.As is the case for several other natural polysaccharides it is challenging to isolate pectins from degraded plant material. Chemical synthesis, on the other hand, is able to produce structurally diverse oligosaccharides of excellent purity, and in larger quantities.
The pectic oligosaccharides oligo-(1→5)-α-Larabinofuranosides were chosen as the main subject of the project. These oligosaccharides have been reported to possess a wide range of biological activity profiles including immunological activity and being a promising dietary supplement for improvement of human intestinal health. Recently the monosaccharide L-arabinose has been found to pose selective intestinal sucrase inhibition effect as well as protective effects in high-carbohydrate, high-fat diet-induced metabolic syndrome in rats, and usage in cancer treatment as inducer of bacterial gene expression.
Chemical synthesis of two branched structures of oligo (1→5)-α-L-arabinofuranosides was developed. Due to the high reactivity of arabinofuranosides, the most efficient route to a disaccharide donor employed a perbenzoylated monosaccharide serving as glycosyl donor. Three monosaccharides donors with a chloroacetyl group as alternative protecting group were also prepared. The following coupling of these donors in different combinations resulted in three different hexasaccharide backbone structures. The chloroacetyl group employed in different positions of the fourth monosaccharide of the hexasaccharides was removed selectively, converting the hexasaccharides into the corresponding glycosyl acceptors. Two of the hexasaccharides were further branched through reaction with a diarabinofuranoside donor to furnish two branched octasaccharides.
The results constitute the first reported synthesis of (1→5)-α-L-arabinofuranosides branched at the 2- or the 3-position within the chain. In order to harness the full potential of the target molecules and apply them in the study of for example protein-carbohydrate interaction, deprotection remains. This should be achievable using Zemplén conditions, but will require future work.
Furthermore, a linker system using small molecules to link oligosaccharides to an array surface was investigated. The small linkers were found to be superior to the use of neoglycoconjugates of bovine serum albumin, when studying enzyme activity.
Finally, an assay to screen for novel glycosyl transferase/hydrolase activities was developed. The studies showed that it was possible to detect transglycosylation activity on a microarray.
Illustration:
Representation of the plant cell wall printed with permission from Willats et al.