Alkali Treatment of the Polysaccharides from the Cystocarpic Stage from Iridaea Undulosa

Abstract

The polysaccharides from cystocarpic Iridaea undulosa, soluble and insoluble in 2M potassium chloride, Cs and Ci, respectively, were treated with alkali and fractionated by precipitation with increasing concentrations of KCl. They were later separated by ion-exchange chromatography, to yield fractions enriched in an α-(1→6)-glucan, agaroids and carrageenans.

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.