Sulfated galactans from Australian specimens of the red alga Phacelocarpus peperocarpos (Gigartinales, Rhodophyta)

• Published in: Carbohydrate research 1996-12, Vol.296 (1), p.237-247
• Main Authors: Liao, Ming-Long, Chiovitti, Anthony, Munro, Sharon L.A., Craik, David J., Kraft, Gerald T., Bacic, Antony
• Format: Article
• Language: English
• Subjects: Australia; Carbohydrate Conformation; carbohydrate structure; Carrageenan, 4′,6′-disulfated, κ-, and ω; cell wall components; chemical structure; Disaccharides - chemistry; Eukaryota - chemistry; Galactans - chemistry; Galactans, sulfated; Galactose - analogs & derivatives; Galactose - analysis; Gigartinales; Magnetic Resonance Spectroscopy; Polysaccharides - chemistry; Polysaccharides, algal; Repetitive Sequences, Nucleic Acid; Rhodophyta; Spectroscopy, Fourier Transform Infrared; Structure determination; Sulfuric Acid Esters - chemistry
• ISSN: 0008-6215; 1873-426X
• DOI: 10.1016/S0008-6215(96)00232-7

Polysaccharides from the red alga Phacelocarpus peperocarpos were extracted with hot water, clarified, and precipitated with 2-propanol. The native preparation was highly sulfated (36.2% w/w). Alkali modification decreased the sulfate content by 2.0% w/w. The alkali-modified polysaccharide is composed mostly of galactose (Gal, 51 mol%) and 3,6-anhydrogalactose (AnGal, 41 mol%), with minor amounts of a mono- O-methylgalactose (MeGal, 1 mol%), xylose (Xyl, 6 mol%), and glucose (Glc, 1 mol%). The FTIR spectrum of the alkali-modified polysaccharide resembled κ-carrageenan with absorption at 930 cm −1 (indicative of AnGal) and 850 cm −1 (Gal 4-sulfate). However, an additional, major band of absorption occurred at 820 cm −1, indicating the presence of equatorial sulfate ester substitution at O-6 of Gal residues. A combination of linkage and 13C NMR spectroscopic analyses showed that the polysaccharide was composed predominantly of a novel repeating-unit, O-β-d-galactopyranosyl 4,6-disulfate)-(1 → 4)-3,6-anhydro-α-d-galactopyranose. Minor structural variations also occurred, including alternative patterns of sulfation and the presence of terminal Xyl p. The location of the terminal Xyl p residues was not certain but evidence supported their attachment at O-3 of some 4-linked Gal p residues. The cell-wall galactans remain unchanged during the life cycle of the alga.