Structure of a novel oligosaccharide-mycosporine-amino acid ultraviolet A/B sunscreen pigment from the terrestrial cyanobacterium Nostoc commune.

Water-soluble UV-A/B-absorbing pigments are secreted by cells of the cosmopolitan terrestrial cyanobacterium Nostoc commune. The pigments constitute a complex mixture of monomers with molecular masses of up to 1801 Da. Two different chromophores with absorption maxima at 312 and 335 nm are linked to different amino acids and to oligosaccharides consisting of galactose, glucose, xylose, glucuronic acid, and glucosamine. The 335 nm chromophore is a 1,3-diaminocyclohexen derivative, while the chromophore with an absorption maximum at 312 nm is most likely a 3-aminocyclohexen-1-one derivative. These UV-inducible substances are the first mycosporines to be described covalently linked to oligosaccharides. The pigments are located in the extracellular glycan sheath of Nostoc colonies, where they form complexes of extremely high molecular mass that are attached noncovalently to the glycan sheath. Pigments occur in concentrations that permit the cells to attenuate a significant part of incident UV-B radiation.