Inhibitory Effect of Ca²⁺on Formation of Mg²⁺-Mediated Two-Dimensional Hexagonal Lattice Structure by an R-Form Lipopolysaccharide from Klebsiella pneumoniae

When the R-form lipopolysaccharide (LPS) from Klebsiella pneumoniae strain LEN-111 (03-:Kl-), from which cationic material had been removed by electrodialysis, was suspended in 50 mM Tris buffer at pH 8.5 containing 0.1 mM or higher concentrations of MgCl₂, it formed an ordered two-dimensional hexagonal lattice structure and its center-to-center distance (lattice constant) depended upon the concentration of MgCl₂ and reached the shortest value (14 nm) at 10 mM. In contrast, in the presence of 0.1 to 10 mM CaCl₂ in place of MgCl₂, the electrodialyzed LPS did not form such an ordered hexagonal lattice structure but formed an irregular network structure with a center-to-center distance of 19 to 20 nm. We investigated interaction of Mg²⁺ and Ca²⁺ in formation of the hexagonal lattice structure by the electrodialyzed LPS suspended in 50 mM Tris buffer at pH 8.5. When 0.1 mM or higher concentrations of CaCl₂ were mixed with 1 mM MgCl₂ or when 1 mM or higher concentrations of CaCl₂ was mixed with 10 mM MgCl₂, the electrodialyzed LPS did not form the hexagonal lattice structure of the magnesium salt type but formed the irregular network structure of the calcium salt type. In the coexistence of equimolar or higher concentrations of CaCl₂ together with 1 or 10 mM MgCl₂, the binding of Mg to the electrodialyzed LPS was significantly inhibited and, conversely, the binding of Ca was enhanced as compared with when MgCl₂ or CaCl₂ was present alone. However, the coexistence of 10 times less molar concentrations of CaCl₂ did not significantly inhibit the binding of Mg to the electrodialyzed LPS. Therefore, the inhibition of formation of the Mg²⁺-mediated hexagonal lattice structure of the electrodialyzed LPS by equimolar or higher concentrations of CaCl₂ accompanied the inhibition of binding of Mg but that by 10 times less molar concentrations of CaCl₂ did not accompany it.