Synthesis and structural analysis of Aspergillus fumigatus galactosaminogalactans featuring α-galactose, α-galactosamine and α-N-acetyl galactosamine linkages

Abstract

Galactosaminogalactan (GAG) is a prominent cell wall component of the opportunistic fungal pathogen Aspergillus fumigatus. GAG is a heteropolysaccharide composed of a -1,4-linked galactose, galactosamine and N -acetylgalactosamine residues. To enable biochemical studies, a library of GAG-fragments was constructed featuring specimens containing a-galactose-, a-galactosamine and a-N-acetyl galactosamine linkages. Key features of the synthetic strategy include the use of di- tert -butylsilylidene directed a-galactosylation methodology and regioselective benzoylation reactions using benzoyl-hydroxybenzotriazole (Bz-OBt). Structural analysis of the Gal, GalN and GalNAc oligomers by a combination of NMR and MD approaches revealed that the oligomers adopt an elongated, almost straight, structure, stabilized by inter-residue H-bonds, one of which is a non-conventional C-H····O hydrogen bond between H5 of the residue (i+1) and O3 of the residue (i). The structures position the C-2 substituents almost perpendicular to the oligosaccharide main chain axis, pointing to the bulk solvent and available for interactions with antibodies or other binding partners.