Agars from nine species of red seaweed in the genus Curdiea (Gracilariaceae, Rhodophyta)

Agars have been isolated from the nine currently recognised species of red seaweed in the genus Curdiea. Characterisation of their structures by chemical and spectroscopic methods showed all had a basic repeating structure of alternating 3-linked β-d-galactopyranosyl and 4-linked 3,6-anhydro-α-l-galactopyranosyl units but substituted with high levels of methyl ether groups. The native agars, isolated with hot aqueous buffer solution, had only weak gelling abilities owing to some of the 4-linked units being present as precursor α-l-galactopyranosyl-6-sulfate units. Conversion of these precursor units to the corresponding 3,6-anhydrides by treatment with hot alkali generally led to increased gel strength. Agars from Curdiea angustata, C. codioides, C. crassa and C. flabellata were predominantly methylated on position 6 of the 3-linked β-d-galactopyranosyl units, while agars from C. irwinii, C. sp. nov. (Three Kings), and C. racovitzae were almost completely methylated on position 2 of the 4-linked 3,6-anhydro-α-l-galactopyranosyl units. The agars from C. coriacea and C. obesa, however, were nearly completely methylated at both these positions. The alkali-modified agars from these latter two algae had gel-melting temperatures significantly above the boiling point of water. Small amounts of unusual 4-O-methylxylopyranosyl branching units were detected in the agars from C. irwinii and C. obesa.