AGARS FROM 9 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 structure s by chemical and spectroscopic methods showed all had a basic repeati ng structure of alternating 3-linked beta-D-galactopyranosyl and 4-lin ked 3,6-anhydro-alpha-L-galactopyranosyl units but substituted with hi gh 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-galactopyran osyl-6-sulfate units. Conversion of these precursor units to the corre sponding 3,6-anhydrides by treatment with hot alkali generally led to increased gel strength. Agars from Curdiea angustata, C. codioides, C. crassa and C. flabellatta were predominantly methylated on position 6 of the 3-linked beta-D-galactopyranosyl units, while agars from C. ir winii, C. sp. nov. (Three Kings), and C. racovitzae were almost comple tely methylated on position 2 of the 4-linked 3,6-anhydro-alpha-L-gala ctopyranosyl 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 temperature s significantly above the boiling point of water. Small amounts of unu sual 4-O-methylxylopyranosyl branching units were detected in the agar s from C. irwinii and C. obesa. (C) 1998 Elsevier Science Ltd. All rig hts reserved.