Structural elucidation of rhamnogalacturonans from flaxseed hulls.
The structure of acidic fraction gum (AFG) from flaxseed hulls was elucidated by methylation analysis and 1D/2D NMR spectroscopy. This acidic fraction was separated from water-soluble flaxseed gum using anion-exchange chromatography. AFG consisted of a rhamnogalacturonan-I (RG-I) backbone that features diglycosyl repeating units, →2)-α-l-Rhap-(1→4)-α-d-GalpA-(1→. Rhamnosyl residues (38.2%) were the most abundant neutral sugar component. It was present mainly as unbranched (16.5%) and branched (19.5%) →2)-α-l-Rhap-(1→ at O-3. Most of its branches were terminated by monosaccharides, α/β-d-Galp-(1→ (19.6%), α-l-Fucp-(1→ (4.5%) or β-d-Xylp-(1→ (3.1%). However, when this branching site was occasionally appended with →4)-α-d-GalpA-(1→ or →2)-α-l-Rhap-(1→, side chains may consist of rhamnogalacturonan-I (RG-I), homorhamnan (HR) or a mixture of both. AFG was highly branched as indicated by its high degree of branching (0.55). A possible structure of AFG was proposed:  (HR, RG-I, and HG refer to homorhamnan, rhamnogalacturonan-I, and homogalacturonan, respectively. The locations of HR, RG-I, and HG are interchangeable; (m+n)/(n+i)≈1.5. The substitution rate of R(1) is ∼54%. R(1) is mostly monosaccharide (α/β-d-Galp-(1→, α-l-Fucp-(1→ or β-d-Xylp-(1→). R(1) may also occasionally be a longer side chain with more than two residues beginning with →4)-α-GalpA-(1→ or →2)-α-l-Rhap-(1→, wherein the side-chain structure may be similar to part of the main chain.).
Department of Food Science, University of Guelph, Guelph, ON, Canada. kqian@uoguelph.ca
Ke-Ying
KY
Qian
Steve W
SW
Cui
John
J
Nikiforuk
H Douglas
HD
Goff
D000143:Acids; D002240:Carbohydrate Sequence; D002852:Chromatography, Ion Exchange; D019597:Flax / Q000737:chemistry*; D006601:Hexoses / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008745:Methylation; D008969:Molecular Sequence Data; D010368:Pectins / Q000737:chemistry* / Q000302:isolation & purification; D010429:Pentoses / Q000737:chemistry; D053149:Plant Gums / Q000737:chemistry*; D014866:Waste Products
D000143:Acids; D006601:Hexoses; D010429:Pentoses; D053149:Plant Gums; D014866:Waste Products; C042491:rhamnogalacturonan I; D010368:Pectins