Introduction
The red seaweed
Iridaea undulosa is an important source of carrageenans. The structure of the ga-lactans from cystocarpic and tetrasporic thalli has been studied [
1]. The presence of galactans contain-ing
L-Gal was proved in the 2M KCl-soluble fractions from cystocarpic
Gigartina skottsbergii after alkali treatment and further KCl precipitation [
2], and also in the soluble fraction of the polysaccha-rides from tetrasporic
Iridaea undulosa [
3].
Herein we report the results of the alkali treatment of the 2M KCl-soluble and -insoluble fractions of cystocarpic Iridaea undulosa polysaccharides, and of their further fractionation by ion-exchange chromatography.
Experimental
The polysaccharide from cystocarpic
Iridaea undulosa was fractionated with 2M KCl. Both the in-soluble (
Ci) and soluble (
Cs) weres obtained after centrifugation, dialysis and liophylization.
Cs and
Ci were treated with 1M NaOH (5 h, 80°C), and their solutions, after dialysis and freeze-drying (
CsT and
CiT), were refractionated with KCl from 0.1M to 2M. The fractions soluble in 2M KCl (
CsTs-2 and
CiTs-2) were subfractionated by ion-exchange chromatography on DEAE Sephadex A-50 mixed up with Sephadex G-100 stabilized on 0.2M NaCl, by elution with increasing concentrations of NaCl (up to 4 M). The constituting monosaccharides were determined by GC, after hydrolysis with 2 M TFA and derivatization to the corresponding aldononitriles and aminoalditols acetates [
1,
4].
Results and Discussion
The first fractionation of the polysaccharide yielded 31% of Cs and 62% of Ci. After alkali treat-ment, 95% of Cs was recovered (CsT), yielding four new fractions after precipitation with increasing concentrations of KCl: CsTi-0.1, CsTi-1, CsTi-2 and CsTs-2. The first one (81%) is a carrageenan (88% D-Gal), with traces of L-Gal, Glc and Rha. On the other hand CiT was similarly fractioned with KCl, yielding also four equivalent fractions. The major one, CiTi-0.1, (92%) contained 93% of D-Gal and trace amounts of 3-O-and 6-O-Me-D-Gal. Fractionation of CsTs-2 by ion-exchange chromatogra-phy yielded 5 main fractions. The one eluted with 0.2M NaCl contained a α-1,6 glucan and significant amounts of agaroids (26% of L-Gal). Subfractions F1/s and F1.5/s contained D- and L-Gal (3:1 ratio for both). The late-eluting fraction F2/s, contained 80% of D-Gal and minor proportions of Glc, 3-O-methyl and 6-O-methyl-D-Gal.