Preliminary investigation of a highly sulfated galactofucan fraction isolated from the brown alga Sargassum polycystum

Abstract

A fucoidan preparation was isolated from the brown alga Sargassum polycystum (Fucales, Sargassaceae). The preparation was fractionated by anion-exchange chromatography, and two highly sulfated fractions F3 and F4 were obtained. The fractions were quite similar in composition, but different in chemical structure. F4 was analyzed by chemical methods, including desulfation, methylation, Smith degradation, and partial acid hydrolysis with mass-spectrometric monitoring, as well as by NMR spectroscopy. Several 2D NMR procedures, including HMQC-TOCSY and HMQC-NOESY, were used to obtain reliable structural information from the complex spectra. Molecules of F4 were shown to contain a backbone built up mainly of 3-linked α-L-fucopyranose 4-sulfate residues, as in many other fucoidans, but rather short sequences of these residues are interspersed by single 2-linked α-D-galactopyranose residues also sulfated at position 4. This rather unusual structural feature should have a great influence on the conformation of the polymeric molecule and may be important for biological activity of the polysaccharide. Hence, F4 is an example of a new sulfated galactofucan isolated from the brown alga. According to the data obtained, the distribution of galactose residues along the polysaccharide backbone seems to be not strictly regular, but the definitive sequence of monomers in the polymeric molecules awaits additional investigation.

Keywords: Brown algae; Fucoidan; Sargassum polycystum; Sulfated galactofucan.