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Divalent cation induced re-entrant condensation behavior for lipopolysaccharides.

Lipopolysaccharides (LPSs) are negatively charged molecules covering the surface of Gram-negative bacteria (GNB). Adding divalent cations (DCs) is important to stabilize the LPS bilayer. Thus, DCs are always only considered as membrane stabilizing ions. Here, on the basis of a coarse-grained (CG) Martini force field, we conduct molecular dynamic (MD) simulations to study the divalent cation mediated LPS interaction and the stability of the LPS membrane in a wide range of DC concentrations. By measuring the LPS binding free energy and the LPS-LPS aggregate from the association course between two LPS molecules, we find that the initial addition of DCs may significantly facilitate the aggregation of LPSs into a compact structure, while sequentially adding more DCs only unpacks the LPS aggregate and drives the dissolution of LPSs. With an increasing concentration of DCs, we find a gradual replacement of DCs to monovalent cations as condensed counterions on the LPS, which follows a sign change from negative to positive in terms of the LPS effective charge and a switch of LPSs in solution from undergoing precipitation to resolubilization on adding DCs. This interaction change in the level of two LPSs accounts for the structure variation of the LPS assembly on a larger scale, where the LPS packing rigidity in the assembly bilayer is found with a similar nonmonotonic dependence with the DC concentration. Thus, our results demonstrate for the first time the presence of a re-entrant condensation behavior for LPS molecules, which can be exploited for developing novel membrane-perturbing agents based on multivalent ions as efficient GNB antibiotics.