Bacterial Model Membranes Deform (resp. Persist) upon Ni²⁺ Binding to Inner Core (resp. O‑Antigen) of Lipopolysaccharides

The surface charge densities, apparent equilibrium binding constants, and free energies of binding of nickel ions to supported and suspended lipid membranes prepared from POPC and two types of lipopolysaccharide (LPS) are reported. Second- and third-order nonlinear optical mixing shows that rough LPS (rLPS)-incorporated bilayers carry the highest charge density and provide the most binding sites for nickel ions while LPS-free bilayers exhibit the lowest charge density and fewest binding sites. Ni²⁺ binding is almost fully reversible at low concentrations but less so at higher Ni²⁺ concentrations. Ni²⁺ adsorption isotherms exhibit hysteresis loops. The role of interfacial depth on the observed second harmonic generation (SHG) responses is discussed in the context of complementary dynamic light scattering, X-ray spectroscopy, and cryogenic transmission electron microscopy experiments. The latter reveal considerable Ni²⁺-induced structural deformations to the bacterial membrane models containing the short, O-antigen-free rLPS, consistent with complex formation on the vesicle surfaces that involve Ni²⁺ ions and carboxylate groups in the inner core of rLPS. In contrast, Ni²⁺ ion complexation to the charged groups (phosphates and carboxylate) of the considerably longer O-antigen units in sLPS appears to protect the phospholipid backbone against metal binding and thus preserve the vesicle structure.