Influence of salt on the formation and separation of droplet interface bilayers

Abstract

Phospholipid bilayers are a major component of the cell membrane that is in contact with physiological electrolyte solutions including salt ions. The effect of salt on the phospholipid bilayer mechanics is an active research area due to its implications for cellular function and viability. In this manuscript we utilize droplet interface bilayers(DIBs), a bilayer formed artificially between two aqueous droplets, to unravel the bilayer formation and separation mechanics with a combination of experiments and numerical modelling under the effects of K$^+$, Na$^+$, Li$^+$, Ca$^{2+}$ and Mg$^{2+}$. Initially, we measured the interfacial tension and the interfacial complex viscosity of lipid monolayers at a flat oil-aqueous interface and show that both properties are sensitive to salt concentration, ion size and valency. Subsequently, we measured DIB formation rates and show that the characteristic bilayer formation velocity scales with the ratio of the interfacial tension to the interfacial viscosity. Next, we subjected the system to a step strain by separating the drops in a stepwise manner. By tracking the evolution of the bilayer contact angle and radius, we show that salt influences the bilayer separation mechanics including the decay of the contact angle, the decay of the bilayer radius and the corresponding relaxation time. Finally, we explain the salt effect on the observed bilayer separation by means of a mathematical model comprising of the Young-Laplace and an evolution equation.

A simple method to make, trap and deform a vesicle in a gel

“…The membrane tension can be estimated from the contact angles of the oil cap with the aqueous and agar phases ( see Supplementary Information ) and yields mN/m (obtained from 13 images on N=8 pseudo-vesicles), which is much larger than values usually reported in electroformed lipid vesicles 35 , 36 , mN/m. On the contrary, our membrane tension value compares well with those obtained in Droplet Interface Bilayers (DIBs) which are planar lipid bilayers obtained by putting in contact two aqueous droplets bathing in an oil+lipid mixture (see for instance 37 – 39 ). In DIBs, there is also water/oil interfaces surrounding the lipid bilayer.…”

The Role of Lipid Intrinsic Curvature in the Droplet Interface Bilayer

“…The choice of 0.1 M NaCl as osmotic agent was motivated by its correspondence to physiological ionic strength, although it is recognized that the interfacial tension of the monolayer at oil–water interface will increase with salt (e.g, KCl) concentration, as would the DIB bilayer surface tension. 45 An increasing degree of contact angle (θ) within the DIB pair is clearly apparent in the photomicrographs (and is tabulated in Table 1 ), with the following trend for θ: DOPC > DOPC-Me 2 > DOPE-Me > DOPE.…”

“…Almost all lipids that form condensed monolayers at the water–oil interface should be capable of forming metastable planar bilayers, provided that the oil and lipid do not interact too strongly, as is the case for the hydrocarbon oils employed in forming DIBs. , Figure A–D shows photomicrographic images of iso-osmotic pairs of aqueous droplets (each very close to 100 μm diameter) in SqE solution containing DOPE, DOPE-Me, DOPE-Me 2 , and DOPC, respectively. The choice of 0.1 M NaCl as osmotic agent was motivated by its correspondence to physiological ionic strength, although it is recognized that the interfacial tension of the monolayer at oil–water interface will increase with salt (e.g, KCl) concentration, as would the DIB bilayer surface tension . An increasing degree of contact angle (θ) within the DIB pair is clearly apparent in the photomicrographs (and is tabulated in Table ), with the following trend for θ: DOPC > DOPC-Me 2 > DOPE-Me > DOPE.…”