Recently, we reportedle3 on the basis of structural results that three types of lipopolysaccharide core occur in Citrobacter serotypes 04,027,036, and strain PCM 1487, all of them being different from E. coli core types, R,, R,, R,, and R,. The hexose-heptose regions of Citrobacter cores were described in detail, but still some structural data concerning the heptose-3-deoxyoctulosonic acid (dOctuA) part was lacking. Growing interest is directed to the heptose-dOctuA region of lipopolysaccharides because of its possible significance in bacterial physiology and interaction with the host. We describe herein the elucidation of: (a) the exact localization of side-chain heptose, and (b) the type of linkage heptose-dOctuA in Citrobatter cores.
To localize the branch point in the heptose region, the R36 incomplete-core oligosaccharide was submitted twice to Smith degradation. In the first degradation, sodium borohydride and, in the second, sodium borodeuteride was used for reduction.
This double-Smith degradation led to two possible alternative products (1 or 2, Scheme 1). Thus, the final product was submitted to methylation analysis. Two methylated derivatives (molar ratio 0.9:l.O) were identified: (a) 1,5-di-O-acetyl-2,3,4,6-tetra-O-methyl-(6-2H,)mannitol, m.s.: m/z (relative intensities in parentheses): 47 (78), 60 (16), 71 (17), 72 (37), 75 (24) 87 (47), 88 (50), 89 (16), 101 (lOO), 102 (35), 114 (23), 117 (loo), 118 (18), 129 (25), 130 (98) 131(14), 145 (lo), 146 (80), 161 (86), 162 (78) 205 (5), 206 (31), and 207 (5); and (b) 1,3,5-tri-Oacetyl-2,4,6-tri-0-methylmannitol, m.s.: m/z 46 (70), 59 (18), 71 (25), 75 (12), 85 (18), 87 (29), 99 (37), 101 (79), 117 (loo), 129 (89), 143 (8) 159 (8), 161 (56), 173 (5), 201 (12), and 233 (30). These data showed that 1 was formed, and that, in Citrobacter core, the side-chain heptose residue is linked to the base-chain heptose residue adjacent to a glycosyl residue. An identical result was obtained for the 04 incomplete core oligosaccharide.
To determine the type of linkage between heptose and 3-deoxyoctulosonic acid residue in Citrobacter cores, the dephosphorylated PCM 1487 core oligo-