Abstract
For the past decade, droplet interface bilayers (DIBs) have had an increased prevalence in biomolecular and biophysical literature. However, much of the underlying physics of these platforms is poorly characterized. To further our understanding of these structures, lipid membrane tension on DIB membranes is measured by analysing the equilibrium shape of asymmetric DIBs. To this end, the morphology of DIBs is explored for the first time using confocal laser scanning fluorescence microscopy. The experimental results confirm that, in accordance with theory, the bilayer interface of a volume-asymmetric DIB is curved towards the smaller droplet and a lipid-asymmetric DIB is curved towards the droplet with the higher monolayer surface tension. Moreover, the DIB shape can be exploited to measure complex bilayer surface energies. In this study, the bilayer surface energy of DIBs composed of lipid mixtures of phosphatidylgylcerol (PG) and phosphatidylcholine are shown to increase linearly with PG concentrations up to 25%. The assumption that DIB bilayer area can be geometrically approximated as a spherical cap base is also tested, and it is discovered that the bilayer curvature is negligible for most practical symmetric or asymmetric DIB systems with respect to bilayer area.
Highlights
Droplet interface bilayers (DIBs) [1,2] have typically been used to measure membrane bilayer characteristics such as permeability, membrane protein interactions or electrical behaviour
DIBs have been shown to be useful as they allow for the production of asymmetric membranes [6,7], where understanding membrane asymmetry is of high value as it is known to offset transmembrane potential [8,9], affect membrane bending rigidity [10], and control membrane protein conformation [11] as well as membrane permeability [12,13,14]
Under the assumption that the DIB retains axial symmetry, as demonstrated in figure 1, the bilayer bend of radius rb can be modelled as a section of a spherical cap of height hb and the droplets themselves can be modelled as intersecting spheres of radius r1 and r2 truncated at height h1 and h2 with spherical cap base radius a
Summary
Droplet interface bilayers (DIBs) [1,2] have typically been used to measure membrane bilayer characteristics such as permeability, membrane protein interactions or electrical behaviour. Measuring surface tension in lipid DIB membranes is challenging, and, currently, the only accepted measurement method is made via direct visualization of the surface morphology using bright field microscopy, which, along with known monolayer surface tensions, can be used to infer bilayer tension. Though it has been shown that symmetric DIB bilayer surface energies can be estimated using shape information from bright field images, bright field microscopy lacks the ability to capture precise information about membrane curvature due to lipid asymmetry, which can significantly affect the surface energy calculation. This shape information can be applied to the calculation of membrane tension in accordance with a force balance, i.e. Neumann’s triangle [32] (the sine rule). A free energy model is applied that describes the curvature behaviour with respect to lipid asymmetry and droplet volume difference
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