Physical properties of short- and long-O-antigen-containing fractions of lipopolysaccharide from Escherichia coli 0111:B4.

Aggregates of short- and long-chain O-antigen-containing fractions of lipopolysaccharide were analyzed by electron spin resonance probing to reveal differences in their physical properties. The fluidities of the lipid regions of the two fractions were quite similar, although the long-chain lipopolysaccharide aggregates appeared to be more hydrated as reflected by the polarity determined with a lipid probe. In contrast, the head-group region of the long-chain fraction was dramatically more mobile than that of the short-chain sample. The binding of polycations (e.g., polymyxin B, spermine) to lipopolysaccharide aggregates was measured by the partitioning of a cationic spin probe. Less probe was displaced from the long-chain fraction and unseparated lipopolysaccharide than from the short-chain fraction by the addition of cations, suggesting that the long O-antigen masks anionic sites on lipopolysaccharide. These results indicate that the aggregate shape and reactivity of lipopolysaccharide are affected by O-antigen length. Thus, the biological activity of lipopolysaccharide may be modulated directly by the presence of O-antigen and indirectly by the effects of O-antigen on the lipopolysaccharide aggregate structure.