Rational approach to fractionation, isolation, and characterization of polysaccharides from the lichen Cetraria islandica.

Polysaccharides, isolated from the lichen Cetraria islandica, have antimicrobial effectiveness. For pharmaceutical applications the two glucan components lichenan and isolichenan as well as the galactomannan component are of actual interest. Especially the a-glucan isolichenan ist used as an active ingredient in cough lozenges. The conditions for the extraction of the raw material, mainly pH and temperature, have a strong influence on the yield of lichenan, isolichenan, and galactomannan, and also on the amount of tannins in the extract. Target products and also by-products give higher extraction yields with increasing extraction temperatures. Hot water extraction with subsequent fractionation of the extracted polysaccharides by multiple freezing/thawing steps and water removal applying ethanol and ether permitted the isolation of the target polysaccharides in preparative quantities. Tannins were removed by reversed phase chromatography. IR and NMR spectroscopy were used for structural characterization of lichenan and isolichenan. After optimization of the hot water extraction process no significant lower extraction and fractionation yields have been obtained compared to the established tricky DMSO extraction procedure.