Credit: © 2008 PNAS

The traditional method of producing phospholipid membranes with patchy stiffness — that is, different stiffness from point to point — is by phase separation of lipids with different intrinsic stiffness. It is thought that the patchiness arises from the non-uniform distribution of the different lipids and other components in the membranes. Researchers at the University of Illinois now show that the adsorption of charged nanoparticles on to lipids can form similar membranes with irregular stiffness.

Steven Granick and colleagues1 mixed positively and negatively charged polystyrene nanoparticles (˜20 nm in diameter) with different suspensions of liposomes — spherical lipid bilayer membranes with an aqueous compartment — and measured the type of phase of the resulting membrane using fluorescence and calorimetry methods. Negatively charged particles caused fluid bilayers to become gels, and positively charged particles turned gelled bilayers into a fluid state. Experiments with the nanoparticles and liposomes made from different types of lipid showed that the local patchiness does not depend on the choice of lipids, the size of the liposomes nor the size of the particles, but mainly on their charge. Control experiments using DNA, which is a charged, flexible molecule, further suggest that the rigidly held charges on the nanoparticles enable them to change the state of the lipids to which they bind.

Using nanoparticles to locally reconstruct lipids offers a new way to modulate membrane stiffness. Also, the phase changes reported may be useful when considering the effect of nanoparticles on living systems.