Interaction of Mg²⁺ and Ca²⁺ in In Vitro Hexagonal Assembly of R-Form Lipopolysaccharides

Summary

The R-form lipopolysaccharide (LPS) from Klebsiella pneumoniae strain LEN-111 (03-:K1-), from which cationic material had been removed by electro­dialysis, formed an orderly hexagonal lattice structure when suspended in 50 mM Tris buffer at pH 8.5 containing MgCl₂. The center-to-center distance (lattice constant) of the hexagonal lattice structure depended upon the concentration of MgC1₂ and reached the shortest value (15 nm) at 10 mM. In contrast, CaC1₂ could not produce the orderly hexagonal lattice structure but produced an irregular network structure with a center to center distance of 19 to 20 nm. When the LPS was suspended in Tris buffer containing 10 mM MgC1₂ mixed with 1 or 10 mM CaC1₂, formation of the orderly hexagonal lattice structure of the magnesium salt type was inhibited and the LPS showed the structure of the calcium salt type. When 1 or 10 mM CaC1₂ was mixed with 10 mM MgC1₂, the binding of Mg to the LPS was significantly inhibited compared with when 10 mM MgCl₂ was added alone. On the contrary, when 10 mM CaC1₂ was mixed with 10 mM MgC1₂, the binding of Ca to the LPS was enhanced compared with when 10 mM CaC1₂ was added alone. It was therefore concluded that the inhibition of formation of the hexagonal lattice structure of the magnesium salt type by addition of CaC1₂ is due to the inhibition of the binding of Mg to the LPS. Such a competitive interaction of Mg²⁺ and Ca²⁺ was also observed with the electrodialyzed LPS of Escherichia coli K-12.