The agar-type polysaccharide from the red alga Ceramium rubrum.

Aqueous extraction of the red alga C. rubrum gave a galactan sulphate and, possibly, a separate glucan and xylan. The galactan sulphate has an alternating structure of the agar-type with D-galactose or 6-O-methyl-D-galactose as one alternating unit, and L-galactose, 3,6-anhydro-L-galactose, and their respective 2-methyl ethers as the other unit. Sulphate hemi-ester groups are present on position 6 of both D- and L-galactose residues, with smaller amounts on positions 2 and 4 of, probably, D-galactose residues. The polysaccharide differs from others previously examined in that most of the L-galactose residues are non-sulphated.