Relationship between Mg²⁺-Induced Hexagonal Assembly of R-Form Lipopolysaccharides and Chemical Structure of Their R-Cores

The relationship between formation of the Mg²⁺-induced hexagonal lattice structure by R-form lipopolysaccharides (LPS) and chemical structure of their R-cores was investigated using different kinds of R-form LPS from a series of mutants of Salmonella minnesota or S. typhimurium. The optimal experimental condition for formation of the hexagonal lattice structure was to suspend LPS preparations, from which cationic material was removed by electrodialysis, in 50 mM tris (hydroxymethyl) aminomethane buffer at pH 8.5 containing 10 mM MgCl₂. Under this experimental condition, Rb₁ LPS formed the hexagonal lattice structure with the lattice constant of 14.0±0.2 nm. Ra LPS, which possesses the full length of R-core, also formed the hexagonal lattice structure but its lattice constant was larger (18.1 ± 0.2 nm) than that of Rbi LPS (the lattice structure by Ra LPS was looser than that by Rbi LPS). All the other R-form LPS preparations tested, RcP+, RcP⁻, Rd₁P⁻, and Re LPS, whose R-cores are shorter than that of Rbi LPS, did not form the hexagonal lattice structure, but formed membranous structures showing various shapes which consisted of multiple bilayer structures. Failure to form the hexagonal lattice structure was the common feature of these kinds of R-form LPS irrespective of temperature at which the LPS suspensions in 10 mM MgCl₂-50 mM Tris buffer were incubated. From the results of the present study it was concluded that capability of R-form LPS to form the hexagonal lattice structure has a close correlation with the chemical structure of their R-cores.