Effect of divalent cations on the membrane properties of the lipid A analog E5531

To obtain information on the effects of Mg2+ on the membrane properties of the lipid A analog E5531, we determined the size, structure, zeta potential, membrane fluidity, and micropolarity of the aggregates and the permeability of the E5531 membrane after the addition of Mg2+. E5531 forms a vesicle structure and within the molar ratio of [E5531]:[Mg2+] = 1:3, Mg2+ increased the zeta potential of the E5531 membrane, but did not change the size of the aggregates (approximately 20 nm). Within that molar ratio, Mg2+ decreased the membrane fluidity and micropolarity of E5531 and increased the phase transition temperature. Above the molar ratio of [E5531]:[Mg2+] = 1:5, the size of the aggregates was increased, but at [E5531]:[Mg2+] = 1:3, the size of the aggregates was similar to that in the absence of Mg2+ (approximately 20 nm), and we could stabilize the aggregates in rat plasma.