The system of sulfated galactans from the red seaweed Gymnogongrus torulosus (Phyllophoraceae, Rhodophyta):: Location and structural analysis

Sulfated polysaccharides were localized in the cuticle, cortex and medulla of the gametophyte thallus, being more concentrated in the intercellular matrix than in the cell walls. During the water extraction sequence, a small percentage of galactan sulfates (5. 1 % of dry seaweed) with average low M-r (6-11A kDa) were extracted at room temperature without disturbing the cellular arrangement, while sulfated galactans of average medium M-r (18-45 kDa) were obtained by further hot-water extractions (52.4% of dry seaweed), with diorganization of the tissue. The residue (40.0% of dry seaweed) still contained carrageenan-type (major) and agaran-type (minor) galactans. Part of these galactans was extracted with 8.4% LiCl solution in DMSO, from which pure kappa/iota-carrageenans were isolated. Carrageenans and agarans were extracted in a ratio 1:0.5, showing the highest amount of agaran-structures for a carrageenophyte. The galactans comprise alternating 4-sulfated (major) and non-sulfated (minor) 3-linked beta-D-galactopyranose units, and 4-linked alpha-galactopyranose units with the following substitutions: (i) non-sulfated and 2-sulfated 3,6-anhydro-alpha-D-galactopyranose residues in the carrageenan-structures, which belong to the kappa-farnily (kappa/iota-carrageenans); (ii) 3-sulfated alpha-L-galactopyranose units and 2-sulfated 3,6-anhydro-alpha-L-galactopyranose residues in the agaran-structures. Alkaline treatment and alkaline dialysis of the main extracts gave pure kappa/iota-carrageenans, showing that carrageenan molecules are extracted together with low M, agarans or agaran-DL-hybrids. (C) 2008 Elsevier Ltd. All rights reserved.