Synthesis of fragments of the B side chain of rhamnogalacturonan II as probes for glycomic studies of plant cell wall structure and function
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Rhamnogalacturonan II (RG-II) is a structurally highly complex pectic oligosaccharide of the primary cell wall of higher plants and is composed of four structurally different oligosaccharides referred to as A-D side chains, which are attached to a linear ±(1’ 4)-linked D-GalpA polysaccharide backbone. These polysaccharides are covalently cross-linked through 1:2 borate diesters between two apiofuranosyl residues of A-side chains. The resulting three-dimensional pectic network contributes to the mechanical properties of the primary wall and is required for normal plant growth and development. Indeed, changes in cell wall properties that decrease borate cross-linking of pectin lead to symptoms such as dwarfism. As part of a program to generate well-defined monoclonal antibodies against primary plant cell-wall fragments, we report here the chemical synthesis of hexasaccharide 1 (Figure 1), which is derived from the B-side chain of RG-II. In addition, the simpler tetrasaccharides 2 and 3 were prepared for the epitope study. The oligosaccharides were equipped with an artificial aminopropyl spacer to facilitate conjugation to carrier proteins, which is required for immunizations and ELISA. It was found that the conformation of the six-membered ring of the central arabinopyranosyl moiety depends on the saccharide substitution pattern. It is proposed that this conformational epitope may influence the biological functions of RGII and provide a means to influence plant development.