Structural analysis of a novel sialic‐acid‐containing trisaccharide from Rhodobacter capsulatus 37b4 lipopolysaccharide

Abstract

Sialic‐acid‐containing lipopolysaccharides from Rhodobacter capsulatus 37b4 (S‐form lipopolysaccharide), KB‐1 (R‐type lipopolysaccharide) and Sp 18 (deep R‐type lipopolysaccharide) were investigated for the linkage and substitution of sialic acids. Methylation analysis and behaviour towards acid and enzymic hydrolysis indicated a non‐reducing terminal location of sialic acids in the R‐type lipopolysaccharide of strain Sp 18, whereas an internal, chain‐linked location of sialic acids was found in the lipopolysaccharides of strains 37b4 and KB‐1. For these latter strains, methylation analysis revealed a substitution of sialic acids by other sugars at position 7 for strain 37b4 and positions 4 and 7 for strain KB‐1. In accordance with the chain‐linked position of sialic acids, mild hydrolysis of R. capsulatus 37b4 lipopolysaccharide with acetic acid released a trisaccharide with sialic acid at the reducing terminus. Structural investigation of this trisaccharide by methylation analysis, 1H‐and 13C‐NMR spectroscopy revealed the presence of the disaccharide Gal1–6Glc at the non‐reducing end, probably with an α‐anomeric configuration of the galactose residue, i.e. melibiose, β‐glycosidically linked to position 7 of sialic acid. Therefore the structure Galα1–6Glcβ1–7Neu5Ac is proposed for this core oligosaccharide from R. capsulatus 37b4 lipopolysaccharide. Copyright © 1992, Wiley Blackwell. All rights reserved