Lipooligosaccharide (LOS) is a cell surface component of certain gram-negative bacteria. It lacks O-antigen in lipopolysaccharide (LPS) and consists of active principle lipid A and the core oligosaccharide moiety, which connects to lipid A via the bacterial-specific Kdo and heptose (Hep) (Figure 1-a).
We previously revealed that core oligosaccharide, especially Kdo moiety, deeply affects the biological activity of lipid A. Escherichia coli lipid A with Kdo shows stronger immunostimulatory activity than lipid A itself . On the other hand, it is still unclear how Hep moiety affects lipid A’s activity.
We found that LOS isolated from symbiotic bacteria Alcaligenes faecalis showed the higher immunostimulatory activity and the different concentration dependency in comparison to chemically synthesized A. faecalis lipid A. These results suggested that the core oligosaccharide moiety affects the biological activity of A. faecalis lipid A. Therefore, we started synthetic study of lipid A conjugated with Hep and Kdo to investigate the immunological function of the core oligosaccharide. According to the report by Davis et al. , we synthesized Hep disaccharide via mannose disaccharide 1. Simultaneous oxidation and homologation of two hydroxy groups of 1 gave 2. Dihydroxylation of 2 afforded Hep disaccharide 3 (Figure 1-b). The absolute configuration of the chiral center is now under investigation.
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