Structural studies on agar fractions extracted sequentially from chinese red sea weeds:Gracilaria sjeostedtii,G. textorii andG. salicornia using13C-NMR and IR spectroscopy

The polysaccharides of three species of ChineseGracilaria (Rhodophyta) were extracted by sequential solvent extraction and the chemical structures of the fractions obtained were investigated by using¹³C-NMR and IR spectroscopy. The¹³C-NMR spectra showed that the main extract, cold water fraction, fromG. sjeostedtii consisted of repeating disaccharides of agarobiose and (1→3)-β-D-galactopyranosy 1-(1→4)-α-L-galactopyranose 6-sulfate, the latter being a precursor to agarobiose and converted to agarobiose after alkali modification. The component of the 60% ethanol fraction fromG. textorii was composed mainly of agarose 6-sulfate, 6-O-methyl-agarose and some amount of agarose, whse structures remained unchanged after alkali treatment. The autoclave extract fromG. salicornia revealed a complex structure composed of 6-O-methyl-agarose, 2-O-methyl-agarose, branched 4-O-methyl-L-galactose and agarose 4-sulfate. All the substituents in these agaroses were stable against alkali treatment. The alkali extracts fromG. sjeostedtii andG. textorii showed the distinct spectra of amylopectin-like floridean starch, while that fromG. salicornia showed a mixture of agaroses and floridean starch. From the IR spectra the relative contents of total sulfate, 3,6-anhydro-galactose, 4-sulfate and 6-sulfate in some fractions of polysaccharides of the algae were qualitatively estimated by the ratios of absorption bands of 1250/2920, 930/2920, 850/2920 and 820/2920, respectively. Contribution No. 1489 from Institute of Oceanology, Academia Sinica