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

By combining dynamic light scattering (DLS) measurements 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 poly(allylamine hydrochloride) (PAH) in the presence of low and high salt concentration. We show that the polycation interactions with the lipids generally result in two distinct outcomes that depend upon salt and PAH concentration, identified here as Scheme 1 (observed under conditions of high salt concentration) and Scheme 2 (observed under conditions of low salt concentration). The schemes differ in the extent of changes to droplet size and droplet coalescence coinciding with PAH addition. Our combined DLS and SFS 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 for the choline headgroup (P-O stretches).