Figure 1.

Structures of the core oligosaccharides (OS) and [ECA]-core OS in S. sonnei phase II and E. coli R1, R2, R3, and R4 lipooligosaccharides (LOS). The ECA stands for enterobacterial common antigen. The sugar residue of the ligation between core OS and ECA is colored in red. The ECA repeating unit is colored in green. R stands for nonstoichiometric substituents, such as a phosphate group (P), a pyrophosphate group (PP), pyrophosphorylethanolamine (PPEtn). The symbol * indicates heterogeneity of the core OS regarding terminal sugar residues. Symbol ** indicates ECA ligation sites assumed by the mass spectrometry only. Symbol *** indicates an inverted anomeric configuration of the D-GlcpNAc in the first ECA unit linked to the core OS, whereas an α-configuration is characteristic for the polymeric chain. The R1, R2, R3, and R4 core OS structures are presented according to published data [9,10,11].