Ionic Environment Affects Bacterial Lipopolysaccharide Packing and Function

The interaction of lipopolysaccharides (LPS) with metal cations strongly affects the stability and function of the Gram-negative bacterial outer membrane. The sensitivity of deep rough (Re) LPS packing and function to the ionic environment, as affected by cation valency and ionic radius, has been determined using molecular dynamics simulations and Langmuir balance experiments. The degree of LPS aggregation within the LPS models in the presence of different cations is assessed by measuring the effective mean molecular area (Â_m) of each LPS molecule projected onto the interfacial plane at the end of the equilibration. These results are compared to the LPS mean molecular area from experimental measurements in which the LPS monolayers are assembled at the air-water interface using a Langmuir film balance. We found that packing of the LPS arrays is sensitive to the ionic radius and ion valency of the cations present in solution during LPS array packing. Using enhanced sampling of the free energy for the intercalation of oligo(allylamine HCl) (OAH) into deep rough Salmonella enterica LPS bilayers, we obtained the affinity of the core section of LPS to OAH as a function of the nature of the metal cations present in solution. We found that packing of the solvated LPS bilayer models is sensitive to ionic radius and ion valency of the neutralizing cations. This further suggests that ion bridging and steric barriers rather than charge shielding are important factors in mitigating ligand intercalation under conditions with low ionic concentrations.