Polycation Interactions with Zwitterionic Phospholipid Monolayers on Oil Nanodroplets Suspensions in Water (D2O) Probed by Sum Frequency Scattering

By combining dynamic light scattering (DLS) measurements and infrared (IR) spectroscopy with the interface- and bond-specificity of vibrational sum frequency generation scattering (SFS) spectroscopy, we probe several structural aspects of how zwitterionic DMPC lipids adsorbed to oil droplets suspended in water (D2O) respond to the presence of the common polycation polyallylamine hydrochloride (PAH) in the presence of low and high salt concentration. We show that the polycation interactions with the lipids generally results in two distinct outcomes that depend upon salt and PAH concentration, identified here as Scenario 1 (observed under conditions of high salt concentration) and Scenario 2 (observed under conditions of low salt concentration). The scenarios differ in the extent of changes to droplet size and Olenick et al. Page 2 droplet coalescence coinciding with PAH addition. Our results illustrate that cationic polymers do not always interact in the same fashion with lipid membranes and demonstrate the feasibility of second-order spectroscopic methods to probe those interactions with chemical bond specificity, not only for the alkyl tails (C-H stretches) but also the choline headgroup (P-O stretches).