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

Abstract

The water extracts from red seaweeds Laurencia obtusa and Laurencia filiformis comprise complex sulfatedagarans. Those from L. obtusa have 3-linked -d-galactose units in part sulfated on 2-position or meth-ylated on 6-position, while the 4-linked units are mostly 3,6-anhydro- -l-galactose and -l-galactose6-sulfate, some of the latter units are substituted with -d-xylose on 3-position, precluding alkalinecyclization. The 3-linked -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. The4-linked 3,6-anhydro- -l-galactose units are methylated or substituted in part with single stubs of -d-xylose on 2-position. This is the first time that substitution with xylose of 3,6-anhydro- -l-galactoseis reported. Besides, -l-galactose 2-sulfate carrying single stubs of -d-xylose on 3-position was alsodetected. These galactans have some common structural characteristics with those of other species ofthis genus, but also others that are specific for these species.