Interaction of Mg2+ and Ca2+ in In Vitro Hexagonal Assembly of R-Form Lipopolysaccharides
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.
KeywordsLattice Constant Tris Buffer Klebsiella Pneumoniae Hexagonal Lattice Center Distance
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