Measuring Bilayer Surface Energy and Curvature in Asymmetric Droplet Interface Bilayers

TL;DR

This study uses confocal microscopy to analyze the shape and curvature of asymmetric droplet interface bilayers, revealing how lipid composition influences bilayer surface energy and confirming theoretical predictions about membrane curvature.

Contribution

First experimental analysis of DIB morphology using CLSM, linking bilayer shape to surface tension and providing a method to measure bilayer surface energy.

Findings

Bilayer curvature is toward the smaller droplet in asymmetric DIBs.

Bilayer surface energy increases linearly with PG concentration up to 25%.

Bilayer curvature is negligible for most symmetric or asymmetric DIBs.

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 are poorly characterized. To further the understanding of these structures, a study of 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 (CLSM). The experimental results confirm that, in accordance with theory, the bilayer interface of a volume asymmetric DIB is curved toward the smaller droplet and a lipid asymmetric DIB is curved toward 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 (PC) 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.