Agars from nine species of red seaweed in the genus Curdiea (Gracilariaceae, Rhodophyta)
1998
Falshaw, R. | Furneaux, R.H. | Stevenson, D.E.
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 beta-D-galactopyranosyl and 4-linked 3,6-anhydro-alpha-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 alpha-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 beta-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-alpha-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.
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