Polysaccharides of algae 67. Carrageenan from Pacific red alga Turnerella mertensiana (Gigartinales, Rhodophyta)

A sulfated galactan composed of nearly equimolar amounts of d-galactose, 3,6-anhydro-d-galactose, and sulfate was isolated from the red alga Turnerella mertensiana collected in the Sea of Japan. The structures of native polysaccharide and its alkaline modification products were studied by NMR spectroscopy. The polysaccharide molecules were shown to contain a linear carbohydrate chain consisting of alternating 3-linked β-d-galactopyranose 4-sulfate and 4-linked 3,6-anhydro-α-d-galactopyranose residues (known as к-carrageenan), which is typical of carrageenans, but the regularity of polymer structure is masked by the presence of some 3,6-anhydro-α-d-galactose 2-sulfate (ι-carrageenan units) and α-D-galactose 6-sulfate (µ-carrageenan units) instead of 3,6-anhydro-α-d-galactose. Upon addition of potassium chloride (up to 4%) to a solution of the native polysaccharide, about half of the substance transforms into gel. The gel-forming fraction is к-ι-µ-hybrid carrageenan with the ∼65 : 15 : 20 ratio of к-, ι-, and µ-units. The non-gelling fraction contains the к-, ι-, and µ-units at the ratio of ∼46 : 12 : 42. The gel-forming carrageenan product free of µ-units can be otained in ∼30% yield (based on the dry biomass) by alkaline treatment of the alga prior to extraction of the polysaccharide.