Structures of the O4 and O18 antigens of Stenotrophomonas maltophilia: a case of enantiomeric repeating units

The O-specific side-chain polymers of lipopolysaccharides from the referenc e strains for Stenotrophomonas maltophilia serogroups O4 and O18 are both x ylosylated rhamnans. In the O4 polymer, both sugar components are the D iso mers, whereas the O18 polymer contains only the L isomers. By means of NMR spectroscopy, methylation analysis and Smith degradation, the repeating uni t of the O4 polymer was identified as a doubly-branched pentasaccharide of the structure shown below. The O18 polymer is based on the enantiomeric pen tasaccharide, but the xylosyl substituent at the 4-position is apparently a bsent from some units. The polymers closely resemble the O antigens found i n Xanthomonas carpestris pathovars. [GRAPHICS] (C) 2001 Elsevier Science Ltd. All rights reserved.