Dynamics of the spontaneous formation of a planar phospholipid bilayer: A new approach by simultaneous electrical and optical measurements

Abstract

An artificial lipid bilayer in planar form, well known as bilayer lipid membrane ͑BLM͒, spontaneously forms from a lipid droplet ͑L-␣-phosphatidylcholine in n-decane and chloroform in this work͒ in an aperture of a thin partition in aqueous solution. The thinning dynamics of the lipid droplet or membrane has been studied by simultaneous capacitance and image recording, because the lipid membrane sandwiched by aqueous solutions can be considered as a parallel-plate capacitor. The simultaneous measurements have revealed the two-step thinning of the lipid membrane from its specific capacitance value: first, the initial droplet thins to yield a membrane of about 100 nm thickness (0.02 F/cm 2 ), and second, within this thin lipid membrane, a lipid bilayer of 4 nm thickness (0.42 F/cm 2 ) suddenly emerges and grows, keeping a bilayer structure. In addition, the simultaneous measurements have a time stamp, and thus can determine the trigger moment of the bilayer formation. The revealed dynamics provides the first quantitative support for a ''zipper'' mechanism ͓H. T. Tien and E. A. Dawidowicz, J. Colloid Interface Sci. 22, 438 ͑1966͔͒; in the mechanism, the first thinning results in a sandwich consisting of the organic solvent between two adsorbed lipid monolayers whose distance is the order of 100 nm, and then a chance contact of both monolayers initiates the formation and growth of a lipid bilayer in a zipper-like manner. However, because of the existence of the two solvent-water interfaces containing surface-active molecules, phospholipids, this work claims that the zipper mechanism should be modified in view of the Marangoni effect. The simultaneous measurements have also revealed the adjacency effect, namely, that only the prebilayer region just adjacent to the bilayer changes into a bilayer. The present formation and growth of a lipid bilayer, including the adjacency effect, can be explained by the classic nucleation theory of two-dimensional crystallization. BLM systems with the simultaneous measurements can be considered as a useful environment for the study of soft-matter chemical physics.

Black lipid membranes studied by x-ray phase contrast imaging

“…As it was nicely stated in references [DFH82,FFI03] there is a need of alternative, high resolution techniques to investigate sub-molecular structures of membranes and the proteins that are embedded in them. The present work could hopefully contribute to this interdisciplinary effort.…”

“…These regions are called secondary or Newton black films [TOL00,TD66]. The black spots grow at a fairly constant rate, which can be viewed as a "zipper-like" effect [FFI03,TD66]. This second step of thinning proceeds from interlayer distances of about 60 nm down to the final membrane thickness d of approximately 4 nm to 5 nm, which is nearly twice the molecular length s of the surfactant molecules [Whi70,VGIB95].…”

“…In chapter 3.2 different methods to measure C m will be presented. By combining these methods with conventional transmitted light microscopy, the bilayer area A can be precisely determined and d obtained [HHT64,HHT65,vdB65,HT68,AMH70,Whi70,WT73,FFI03].…”

“…The Marangoni effect is described as follows: An interfacial tension gradient of a molecular layer, which is assembled at the interface between two phases, e.g. water and solvent (see figure 2.6), will induce further adsorption of surfactant molecules at the interface [Wil89,FFI03]. As a consequence a Marangoni flow of solvent towards the region of highest tension is generated to provide the supply of surfactant.…”

Microfluidic platform for bilayer experimentation

“…Typically, thinning of a "lipid plug" created through "painting" or flow-based deposition in an aperture, proceeds in two steps, with first drainage of the solvent to yield a plug with a thickness of ~100 nm, followed by spontaneous zipping into a bilayer structure. 44,45 Direct spontaneous thinning occurs provided the aperture-containing substrate is thin enough, as reported by Le Pioufle et al for 20 µm thick parylene. 46 In an alternative approach, but using a vertical configuration and no aperture, a bilayer was formed via solvent evaporation through PDMS, benefiting from the porosity of the material which was used to build the microchannels.…”