Pustulan and branched β-galactofuranan from the phytopathogenic fungus Guignardia citricarpa, excreted from media containing glucose and sucrose

Abstract Guignardia citricarpa is a phytopathogenic fungus and the causal agent of citrus black spot. Incubation in a semi-defined media resulted in formation of exopolysaccharides [EPS(s)]. A medium containing glucose gave rise to a (1→6)-linked β-glucan (200 kD), pustulan, which was characterized by NMR and methylation analysis. A sucrose-containing medium provided a homogalactan (376 kD) and methylation analysis showed nonreducing end- (20%), 6- O - (53%) and 5,6-di- O -substituted Gal f units (27%). An HMQC spectrum of the homogalactan showed C-1/H-1 signals at δ 108.2/4.820, 108.3/4.820 and 107.1/5.079, corresponding to three types of β- d -Gal f units. A DEPT analysis showed inverted signals ( C H 2 ) at δ 67.8 and 67.2, corresponding to 6- O -substituted β- d -Gal f units, whereas a C-5 signal at δ 77.0 suggests 5- O -substitution, confirming a novel structure for a β-galactofuranan.