Ionic and compositional effects on phospholipid thin film formation and stability

Membrane fusion and adhesion are ubiquitous processes in biological cells that are vital to cell function. Phospholipid membranes and thin films suspended in water provide insight into lipid membrane interactions based on the presence of mono or divalent cations (Na⁺, Ca²⁺), altered ionic concentrations, and chemical properties of the lipid constituents. Of specific interest is the role of phosphatidylethanolamine (PE), which initiates membrane fusion in the presence of Ca²⁺. In this work, we create free-standing lipid films in water to investigate interleaflet headgroup interactions as opposing lipid monolayers come close together, mimicking membrane adhesion. We perform disjoining pressure isotherm experiments to shed light on the effect of salt concentration on close-range (<10nm) interactions between leaflet headgroups and the final newton black film thickness. A stepwise increase in film thinning is also done to explore dynamic film evolution. These experiments provide insight into the stability (coalescence time, drainage characteristics) of lipid headgroups in altered environments. We present results for DOPC films with varying aqueous buffer characteristics, then investigate DOPE to better understand the effect of Ca²⁺ on monolayer headgroup-headgroup interactions.