Interaction of Oil and Lipids in Freestanding Lipid Bilayer Membranes Studied with Label-Free High-Throughput Wide-Field Second-Harmonic Microscopy.

The interaction of oils and lipids is relevant for membrane biochemistry since the cell uses bilayer membranes, lipid droplets, and oily substances in its metabolic cycle. In addition, a variety of model lipid membrane systems, such as freestanding horizontal membranes and droplet interface bilayers, are made using oil to facilitate membrane monolayer apposition. We characterize the behavior of excess oil inside horizontal freestanding lipid bilayers using different oils, focusing on hexadecane and squalene. Using a combination of second-harmonic (SH) and white-light imaging, we measure how oil redistributes within the membrane bilayer after formation. SH imaging shows that squalene forms a wider annulus compared with hexadecane, suggesting that there is a higher quantity of squalene remaining in the bilayer compared with hexadecane. Excess oil droplets that appear right after membrane formation are tracked with white-light microscopy. Hexadecane droplets move directionally to the edge of the membrane with diffusion constants similar to those of single lipids, whereas squalene oil droplets move randomly with lower diffusion speeds similar to lipid condensed domains and remain trapped in the center of the bilayer for ∼1-3 h. We discuss the observed differences in terms of different coupling mechanisms between the oil and lipid molecules induced by the different chemical structures of the oils.