Electroformation of giant vesicles from an inverse phase precursor

Abstract

We discuss a simple modification of the well-known method of giant vesicle electroformation that allows for a direct addition of water-soluble species to the phospholipid bilayers. Using this modified method, we prepare phospholipid vesicles decorated with chitosan, a water-soluble polysaccharide currently investigated for potential pharmacological applications. We find that the method allows this polysaccharide with primary amino groups on every glucose subunit to be tightly bound to the membrane, rather than simply being encapsulated.

Figure 1

(a) Fluorescence microscopy image of a giant vesicle without fluorescently labeled phospholipid but with attached fluorescent chitosan. (b) Microscopy image of the same vesicle in the phase contrast mode and (c) a comparison of both images showing colocalization of the phospholipid membrane and fluorescently labeled chitosan by means of the direct comparison of the intensity profiles across the equator. In this graph, the y axis measures gray levels and the x axis corresponds to the pixels in the above images; the upper curve measures fluorescence gray levels and the lower curve phase measures the contrasting gray levels. Scale bars span 10 μm.

Figure 2

(a) Fluorescence microscopy image of a giant vesicle made from DOPC- and NBD-labeled phospholipids. In this case, the fluorescence is emitted by the vesicle membrane only. (b) The fluorescence intensity profile (dots) measured along the equator of image a and (line) the intensity computed from the convolution of the instrument PSF and a fluorescence distributed on the surface of a sphere. (c) Fluorescence microscopy image of a giant vesicle made from DOPC, with the hydrophilic fluorophore fluorescein that is homogeneously distributed inside the vesicle. (d) The fluorescence intensity profile (dots) measured along the equator of image c and the intensity (line) computed from the convolution of the instrument PSF and a fluorescence homogeneously distributed over the interior of a sphere.

Figure 3

(a) Fluorescence intensity (F in arbitrary units) as a function of the vesicles area (S in μm²) for three different values of the polymer weight fraction f: 0.94 (▵), 1.88 (○), and 3.75 (□)% w/w chitosan. (b) Fluorescence intensity-per-unit surface β = F/S as a function of the polymer fraction f.

Figure 4

Time variation of fluorescence intensity-per-unit surface β for giant vesicles containing the three different values of the polymer weight fraction f: 0.94 (▵), 1.88 (○), and 3.75 (□) % w/w chitosan. Average values of β are also indicated by a dashed line. The figure shows that the composite vesicles keep their polymer content over a period of 10 days.

Figure 5

Calibration curves for determining the amount of chitosan-per-unit surface on the vesicles. The curves display emission intensities at the two wavelengths λ₁ = 515 nm (○) and λ₂ = 533 nm (●) that correspond to emission peaks of NBDPC and of fluorescently labeled chitosan.

Figure 6

A schematic table comparing the classic electroformation method and the methods proposed in this article based on a precursor ordered film obtained from drying an inverse emulsion. (a) The three main steps of the classic electroformation method: 1), dissolution of the phospholipids in a volatile organic solvent, typically chloroform; 2), evaporation of the solvent and formation of an ordered film close to the surface; and 3), swelling of the film with an aqueous solution under an alternative electrical field leads to unilamellar giant vesicles. (b) The modified electroformation method proposed here for attaching water-soluble polymers to both sides of the giant vesicle membranes: 1), starting from an inverse phase of droplets of an aqueous polymer solution dispersed in the majority organic solvent; 2), drying the emulsion; and 3), swelling under an electric field leads to giant vesicles decorated with polymers that interact with the membrane by nonspecific interactions. (c) The method can also be used to decorate the membrane with water-soluble molecules that interact with the membrane by ligand receptor interactions, here schematically a fluorescent streptavidin that recognizes the biotinylated lipids of the membrane.