An NMR solution study of the mega-oligosaccharide, rhamnogalacturonan II.
Rhamnogalacturonan II (RG-II) is a structurally complex pectic mega-oligosaccharide that is released enzymatically from the primary cell wall of higher plants. It contains roughly 30 monosaccharide units (MW approximately 5 kDa) including very unusual residues such as Kdo, Dha, aceric acid and apiose. Previous studies have demonstrated that these monomers are arranged into four structurally well-defined oligosaccharide side chains (A-D), linked to a homogalacturonan mainchain, but the specific attachment sites of these branches on the pectic backbone have not yet been elucidated. In the present work, fairly complete assignments of the 750 MHz 1H NMR spectra and partial assignments of the 13C NMR spectra of the sodium-borohydride-reduced RG-II monomer were obtained for a 5 mM sample isolated from red wine. On the whole, these data corroborate the primary structures of the sidechains previously established by methylation analysis, partial hydrolysis and FAB-MS spectrometry but some heterogeneity has been demonstrated (partial substitution at B5, B6, and A5). The preferred orientations of the majority of the sidechain glycosidic linkages in the RG-II monomer have been determined from the sequential nOe data and the solution structure is generally in good agreement with the stable conformers previously obtained by molecular modeling (MM3) of the disaccharide and sidechain oligosaccharide building blocks. All of a two-residue, a three-residue, and a four-residue segment of the backbone have been tentatively identified from long range interactions between sidechain protons as well as in the mainchain. Taking into account the length of the 9-mer galacturonan mainchain described in prior work, these building blocks constitute almost the complete structure of RG-II (Scheme 2).