Cell-wall polysaccharides from Australian red algae of the family Solieriaceae (Gigartinales, Rhodophyta): novel, highly pyruvated carrageenans from the genus Callophycus

Cell-wall polysaccharides from six species of red algae of the genus Callophycus were mainly galactans comprised predominantly of galactose (Gal) and 3,6-anhydrogalactose (AnGal), and were rich in pyruvate and sulfate. The Fourier Transform Infrared (FTIR) spectra of the polysaccharides superficially resembled that of α-carrageenan (composed of the repeating disaccharide carrabiose 2-sulfate), with major bands of absorption indicative of 4-linked AnGal, axial 2-sulfate on 4-linked AnGal, and unsulfated, 3-linked Gal. The FTIR spectra of solutions of Callophycus polysaccharides in D2O-phosphate buffer displayed absorption, corresponding to the carboxylate anion of the pyruvate acetal substituent. Methylation analysis showed that 3,4,6-linked Galp (interpreted as 4,6-pyruvated, 3-linked Galp) and 2,4-linked AnGalp (interpreted as 4-linked AnGalp 2-sulfate) were the dominant linkages, together with significant quantities of 3-linked Galp. Proton-decoupled 13C nuclear magnetic resonance (NMR) spectroscopy showed the polysaccharides to be composed predominantly of pyruvated carrageenans. The 13C NMR spectra were completely assigned by a J-modulated spin-echo pulse sequence and 2D experiments employing gradient Heteronuclear Multiple Bond Correlation (HMBC), 13C/1H Heteronuclear Multiple Quantum Coherence (HMQC), and HMQC Total Correlation Spectroscopy (HMQC-TOCSY). The Callophycus galactans thus consist predominantly of the novel repeating disaccharide 4′,6′-O-(1-carboxyethylidene)carrabiose 2-sulfate and minor amounts of the α-carrageenan repeating unit (carrabiose 2-sulfate), and other structural variations.