Chemical modifications of algal mannans and xylomannans: Effects on antiviral activity

► Structural analysis of sulfated xylomannans from Nemalion helminthoides was carried out. ► Sulfated mannans from Nemalion helminthoides were oversulfated using the SO3-pyridine complex in DMSO. ► Elimination of xylose side-chains from sulfated xylomannans was achieved by Smith degradation. ► Oversulfated mannans inhibit herpes and dengue viruses, and have anticoagulant activity. ► Xylose side-chains are important for the antiviral activity.