Synthetic, Zwitterionic Sp1 Oligosaccharides Adopt a Helical Structure Crucial for Antibody Interaction

The zwitterionic Streptococcus pneumoniae serotype 1 polysaccharide (Sp1) is an important anchor point for our
immune system to act against streptococcal infections. Antibodies can recognize Sp1 saccharides, and it has been postulated that
Sp1 can elicit a T-cell-dependent immune reaction as it can be presented by MHC-II molecules. To unravel the molecular mode
of action of this unique polysaccharide we here describe the chemical synthesis of a set of Sp1 fragments, ranging from 3 to 12
monosaccharides in length. We outline a unique synthetic approach to overcome the major synthetic challenges associated with
the complex Sp1 structure and provide a stereoselective route of synthesis for the oligosaccharide backbone as well as a strategy
to introduce the carboxylic acid functions. Molecular dynamics (MD) simulations together with NMR spectroscopy studies
reveal that the oligosaccharides take up helical structures with the nona- and dodecasaccharide completing a full helical turn.
The 3D structure of the oligosaccharides coincides with the topology required for good interaction with anti-Sp1 antibodies,
which has been mapped in detail using STD-NMR. Our study has revealed the Sp1 nona- and dodecasaccharides as promising
synthetic antigens, displaying all (3D) structural elements required to mimic the natural polysaccharide and required to unravel
the molecular mode of action of these unique zwitterionic polysaccharides.