Phospholipid aggregates as model systems to understand ion-specific effects: Experiments and models

Abstract

The Langmuir monolayers and bilayer stacks of zwitterionic phospholipids have been recently used as model systems to understand specific anionic effects. Simple thermodynamic experiments coupled to theoretical considerations and advanced spectroscopic methods have provided considerable insights. Inorganic monovalent anions were found to affect only the disorganised liquid-like phases of lipid monolayers. The perturbation of the surface pressure–area isotherm was treated as a problem of electrostatic ionic adsorption using a range of models, and the separate effects of sodium and various anions were assessed. A chemical potential of transfer of anions to the interface was obtained from the fits, and was found to correlate well with a function of the ionic size of the anions, suggesting that ionic specificity is largely a matter of size, and reflects the difference between ion–water and water–water interactions. Osmotic stress experiments on lipid bilayers in the presence of sodium salts yield a perpendicular and a lateral equation of state (EOS). The attempt to fit these EOS using standard theory failed in the presence of large (0.5M) concentrations of chaotropic anions, proving that the adsorption at the surface of the bilayers modifies their local structure considerably. Information about the changes at the interface propagates to the bulk, affecting the perpendicular EOS. Overall, these experiments revealed a strong need for better fundamental understanding of the effect of salts on the structure of lipid bilayers. In particular, more work is needed to understand the specific salt effect on the lipid headgroup region. © 2010 by World Scientific Publishing Co. Pte. Ltd. All rights reserved.