Determination of the Coverage of Phosphatidylcholine Monolayers Formed at Silicone Oil–Water Interfaces by Vesicle Fusion

Methyl-β-cyclodextrin asymmetrically extracts phospholipid from bilayers, granting tunable control over differential stress in lipid vesicles

Lipid asymmetry - that is, a nonuniform lipid distribution between the leaflets of a bilayer - is a ubiquitous feature of biomembranes and is implicated in several cellular phenomena. Differential tension - that is, unequal lateral monolayer tensions comparing the leaflets of a bilayer- is closely associated with lipid asymmetry underlying these varied roles. Because differential tension is not directly measurable in combination with the fact that common methods to adjust this quantity grant only semi-quantitative control over it, a detailed understanding of lipid asymmetry and differential tension are impeded. To overcome these challenges, we leveraged reversible complexation of phospholipid by methyl-β-cyclodextrin (mbCD) to tune the direction and magnitude of lipid asymmetry in synthetic vesicles. Lipid asymmetry generated in our study induced (i) vesicle shape changes and (ii) gel-liquid phase coexistence in 1-component vesicles. By applying mass-action considerations to interpret our findings, we discuss how this approach provides access to phospholipid thermodynamic potentials in bilayers containing lipid asymmetry (which are coupled to the differential tension of a bilayer). Because lipid asymmetry yielded by our approach is (i) tunable and (ii) maintained over minute to hour timescales, we anticipate that this approach will be a valuable addition to the experimental toolbox for systematic investigation into the biophysical role(s) of lipid asymmetry (and differential tension).

Hydrocarbon Penetration into Phospholipid Monolayers Formed at Hydrocarbon-Water Interfaces

Phospholipid monolayers formed at oil-water interfaces are used for various biological applications. However, monolayer structures are not well understood. Herein, we investigated hydrocarbon partitioning in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine monolayers formed at hydrocarbon-water interfaces using fluorescence microscopy and pendant drop tensiometry. The monolayers strongly interacted with squalene, n-hexadecane, n-tetradecane, n-dodecane, n-decane, and n-butylcyclohexane. These alkane and alkylcyclohexane molecules remained within the monolayers during area compression. In contrast, the monolayers interacted weakly with n-pentylbenzene and n-butylbenzene. These alkylbenzenes were gradually removed from the monolayers upon area compression and were completely expelled at an area per lipid of ∼70 Ų. Surface pressure analysis indicated that the ability of hydrocarbons to penetrate the monolayers was enhanced in the order of n-butylbenzene < n-pentylbenzene < n-butylcyclohexane < n-hexadecane.

Determining the Dependence of Interfacial Tension on Molecular Area for Phospholipid Monolayers Formed at Silicone Oil-Water and Tricaprylin-Water Interfaces by Vesicle Fusion

Phospholipid monolayers formed at oil-water interfaces have been used to explore biological interface properties. Thus, monolayer systems need to be quantitatively understood. Previously, we investigated the formation of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) monolayers at silicone oil-water interfaces to determine the dependence of interfacial tension, γ, on the area per lipid, a, compared to that of the closely packed monolayers, a_cp. This study aims to develop a method to obtain the γ-a relationship from the γ-a/a_cp data by investigating POPC monolayers at the silicone oil-water and tricaprylin-water interfaces. Pendant drop tensiometry was used to obtain the dependence of γ on a/a_cp. Furthermore, by calculating the surface pressure, Π, from γ and multiplying a/a_cp with an estimated a_cp value, the dependence of Π on a was obtained. When a value approximately equal to the a of POPC bilayers was assigned to a_cp, the resultant Π-a profile partially or approximately completely overlapped with the Π-a isotherms obtained for the monolayers at the air-water interface using a Langmuir trough. The overlap for the silicone oil-water interface occurred at a ≤ 77 Ų, while that for the tricaprylin-water interface occurred in approximately the entire a region. The results indicate that the Π of the condensed monolayers is little affected by bulk oil. Thus, the γ-a relationship for the oil-water interface can be determined by comparing the compression isotherm with the one obtained for the air-water interface.