Discovery of κ-Carrageenan-Like Agarose from a Red Seaweed, Gracilaria coronopifolia

Agarose is a major gelling constituent of agar extracted from red seaweeds and is used in the separation, purification, and characterization of bio-macromolecules (DNA, RNA, protein and polysaccharide). The purpose of this study is to identify a novel agarose molecule. The polysaccharide was isolated from a red seaweed, Gracilaria coronopifolia which is grown in Okinawa, Japan. The yield of the polysaccharide was 15.2% (W/W), and the total carbohydrate and sulfuric acid were 69.5% and 20.3%. β-D-Galactoside and 3,6-anhydro-α-D or L-galactoside were identified from the methanolyzate of the polysaccharide by thin-layer chromatography. Galactose-4-sulfate and anhydro-galactose were identified by Fourier Transform Infrared (FTIR) spectrum of the polysaccharide. The specific rotation [α]589 was -2.0°. From the 13C- and 1H-NMR spectra, 1,3-linked β-D-galactose-4-sulfate and 1,4-linked 3,6-anhydro-α-D- or L-galactose were assigned. The polysaccharide did not gel even at 0.5% (W/W) at room temperature, but very weak and transparent gel was observed in 0.2% solution with addition of 13.5 mM KCl. We concluded the polysaccharide isolated from G. coronopifolia was the κ-carrageenan-like agarose, the structure of which was proposed. This work is the first to report on the agarose involving 1,3-linked β-D-galactose-4-sulfate.