The occurrence of galactose-6-sulfate (a biological precursor to transform in 3,6-anhydrogalactose
) in the polysaccharide structure of C.
Carrageenan is a high-molecular weight linear hydrophilic polysaccharide that's comprised of repeating galactose units and 3,6-anhydrogalactose
The strong band at 929 [cm.sup.-1] assigned to the presence of 3,6-anhydrogalactose
residue was common to agar .
It is formed by alternate units of D-galactose and 3,6-anhydrogalactose
that are joined by [alpha]-1,3 and [beta]-1,4 glycosidic linkage.
Such variations in the structural composition of SPs, especially those with high 3,6-anhydrogalactose
content and the position of sulfation on their chains, would determine their physical and biological properties (Cardozo et al., 2007).
Therefore, in present study NPOR (isolated from red alga, Porphyra vietnamensis, yielding 28.9% of NPOR which contains 16.36% sulfate content and 21.1% of 3,6-anhydrogalactose
) was reduced at 10[degrees]C by using lowest concentration of NaBr to avoid its desulfation during this procedure (modified Ishihara et al.
Carrageenan is a high molecular weight linear polysaccharide comprising repeating galactose units and 3,6-anhydrogalactose
(3,6 AG), both sulfated and non-sulfated, joined by alternating [alpha]-(1,3) and [beta]-(1,4) glycosidic links .
(3,6-AG) content was determined colorimetrically using the resorcinol-acetal method  and fructose as the standard sugar in samples extracted in water or KOH solutions (mass ratio algae/ extracting medium 1 : 20) of various concentrations for 4 h.
Galactose-6-sulfate is a natural biological precursor which can be converted to 3,6-anhydrogalactose
(CAMPO et al., 2009), which may be found in SPs from some red seaweeds species with commercial interests (MACIEL et al., 2008; SILVA et al., 2010).
The alkali-extracted carrageenans showed lower sulphate content and a decrease in galactose to the benefit of 3,6-anhydrogalactose
. This corresponds to the conversion of the 4-linked galactose-6-sulfate in native samples to anhydrogalactose in the alkali-+extracted carrageenans.
All polysaccharide fractions showed high contents of 6-O-metylgalactose e 3,6-anhydrogalactose
In the course of this troublesome study series, which involved purely chemical methods, a unique component sugar, 3,6-anhydrogalactose
, was discovered  and an idealized extreme structure type of agarose (Fig.