Cations Do Not Alter the Membrane Structure of POPC-A Lipid With an Intermediate Area

Combining small-angle neutron scattering (SANS), small-angle X-ray scattering (SAXS), and densitometric measurements, we have studied the interactions of the divalent cations Ca2+ and Mg2+ with the lipid vesicles prepared of a mixed-chain palmitoyl-oleoyl-phosphatidylcholine (POPC) at 25 degrees C. The structural parameters of the POPC bilayer, such as the bilayer thickness, lateral area, and volume per lipid, displayed no changes upon the ion addition at concentrations up to 30 mM and minor changes at > 30 mM Ca2+ and Mg2+, while some decrease in the vesicle radius was observed over the entire concentration range studied. This examination allows us to validate the concept of lipid-ion interactions governed by the area per lipid suggested previously and to propose the mixed mode of those interactions that emerge in the POPC vesicles. We speculate that the average area per POPC lipid that corresponds to the cutoff length of lipid-ion interactions generates an equal but opposite impact on ion bridges and separate lipid-ion pairs. As a result of the dynamic equilibrium, the overall structural properties of bilayers are not affected. As the molecular mechanism proposed is affected by the structural properties of a particular lipid, it might help us to understand the fundamentals of processes occurring in complex multicomponent membrane systems.

Automated summary

In this study, the interactions of divalent cations (Ca2+ and Mg2+) with lipid vesicles were investigated using SANS, SAXS, and densitometric measurements. The results showed that the structural parameters of the lipid bilayer remained relatively unchanged upon ion addition, while the vesicle radius decreased. This study validates the concept of lipid-ion interactions and proposes a possible mechanism for the mixed mode interactions in lipid vesicles.