Structure of highly substituted agarans from the red seaweeds Laurencia obtusa and Laurencia filiformis

The water extracts from red seaweeds Laurencia obtusa and Laurencia filiformis comprise complex sulfated agarans. Those from L. obtusa have 3-linked β-d-galactose units in part sulfated on 2-position or methylated on 6-position, while the 4-linked units are mostly 3,6-anhydro-α-l-galactose and α-l-galactose 6-sulfate, some of the latter units are substituted with β-d-xylose on 3-position, precluding alkaline cyclization. The 3-linked β-d-galactose units of the agarans from L. filiformis are mostly sulfated on 2-position, but approximately half of these residues also carry the 4,6-O-(1-carboxyethylidene) group. The 4-linked 3,6-anhydro-α-l-galactose units are methylated or substituted in part with single stubs of β-d-xylose on 2-position. This is the first time that substitution with xylose of 3,6-anhydro-α-l-galactose is reported. Besides, α-l-galactose 2-sulfate carrying single stubs of β-d-xylose on 3-position was also detected. These galactans have some common structural characteristics with those of other species of this genus, but also others that are specific for these species.