Carrageenans. Part VII. Polysaccharides from Eucheuma spinosum and Eucheuma cottonii. The covalent structure of ι-carrageenan

Abstract

ι-Carrageenan has been characterized by chemical methods to provide a firm basis for interpretation of data from X-ray diffraction and optical rotation, and so to confirm the carrageenan double helix. This polysaccharide was isolated from the red seaweed Eucheuma spinosum and shown by methylation analysis and partial fragmentation, and by other evidence, to have a masked repeating structure in which D-galactose 4-sulphate and 3,6-anhydro-D-galactose 2-sulphate residues are arranged alternately in linear chains, with formal replacement of approximately one in every ten anhydride residues by D-galactose 2,6-disulphate. Treatment with alkaline borohydride converts this structure to a genuinely alternating copolymer. A polysaccharide from E. cottonii has similarly alternating 1,4- and 1,3-linked residues but is distinctive in that it has not detectable 2-sulphate. It therefore corresponds more closely than any other known natural polysaccharide to an idealized κ-carrageenan.