Vesicles in Salt Solutions

In the previous two sections we discussed the electrodeformation and electroporation of vesicles made of single-component membranes in water. In this section, we consider the effect of salt present in the solutions. The membrane response discussed above was based on data accumulated for vesicles made of phosphatidylcholines (PCs), the most abundant fraction of lipids in mammahan cells. PC membranes are neutral and predominantly located in the outer leaflet of the plasma membrane. The inner leaflet, as well as the bilayer of bacterial membranes, is rich in charged lipids. This raises the question as to whether the presence of such charged lipids would influence the vesicle behavior in electric fields. Cholesterol is also present at a large fraction in mammalian cell membranes. It is extensively involved in the dynamics and stability of raft-hke domains in membranes [120]. In this section, apart from considering the response of vesicles in salt solutions, we describe aspects of the vesicle behavior of fluid-phase vesicles when two types of membrane inclusions are introduced, namely cholesterol and charged lipids. 

The formation of the spherocylindrical shapes is not well understood. They are observed not only on lipid vesicles but also on polymersomes (vesicles made of diblock copolymers [123, 124]) [107]. Therefore, lipid-specific effects, for example partial head group charge and membrane thickness, as a possible cause for the observed cylindrical deformations are to be excluded. One possible explanation could be that ions flatten the equatorial zone of the deformed vesicle. During the pulse there is an inhomogeneity in the membrane tension due to the fact that the 

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Figure 7.6 Bursting of charged vesicles subjected to electric pulses. The time after the beginning of the pulse is marked on each image, (a) Phase contrast microscopy snapshots from fast-camera observation of a vesicle in salt solution subjected to a pulse with field strength of 120kVm and duration of 200ps. The field direction is indicated in...

Riske, K.A. and Dimova, R. (2006) Electric pulses induce cylindrical deformations on giant vesicles in salt solutions. Biophysical Journal, 91 (5), 1778-1786. 

By varying the suction pressure, the energy of adhesion could be obtained. In principle it is possible to obtain force separation curves but this has not been achieved, probably because the cells are so compliant. The same technique has been used to measure the adhesion of lecithin vesicles in salt solution. The contact and separation in this case were reversible and the work of adhesion was 13 This compared with the work of adhesion of ordinary red cells in... 

Abstract Polyelectrolyte block copolymers form micelles and vesicles in aqueous solutions. Micelle formation and micellar structure depends on various parameters like block lengths, salt concentration, pH, and solvent quality. The synthesis and properties of more complicated block and micellar architectures such as triblock- and graft copolymers, Janus micelles, and core-shell cylinder brushes are reviewed as well. Investigations reveal details of the interactions of polyelectrolyte layers and electro-steric stabilization forces. 

The evidence that water is split on the inner side of thylakoids is convincing early experiments by Fowler and Kok [33] and more recent ones [6] have shown that the protons generated by water splitting are detected inside the thylakoid lumen. Furthermore, it has been shown that the 24 and 18 kDa polypeptides are accessible to antibodies only in so-called inside-out preparations these polypeptides can be extracted in salt solutions from the inside-out vesicles, and subsequently rebound to them [34,35]. 

Small micelles in dilute solution close to the CMC are generally beheved to be spherical. Under other conditions, micellar materials can assume stmctures such as oblate and prolate spheroids, vesicles (double layers), rods, and lamellae (36,37). AH of these stmctures have been demonstrated under certain conditions, and a single surfactant can assume a number of stmctures, depending on surfactant, salt concentration, and temperature. In mixed surfactant solutions, micelles of each species may coexist, but usually mixed micelles are formed. Anionic-nonionic mixtures are of technical importance and their properties have been studied (38,39). 

P Schurtenberger, NA Mazer, W Kanzig. Micelle-to-vesicle transition in aqueous solution of bile salt and lecithin. J Phys Chem 89 1042-1049, 1985. 

In our model study reported in this contribution, we have chosen two double-chained C-13 alkylbenzenesulphonate surfactants (SLABS) of closely-related structure, which form micelles in aqueous solution in the absence of salt. However, when small amounts of electrolyte are added (e.g., —20mM NaCl), vesicles are spontaneously formed over a time period of seconds/minutes. These vesicle structures are then reasonably stable over a period of hours/days. The onset of vesicle formation can be readily characterised by the determination of the critical salt concentration (esc), needed to induce the formation of vesicles, from smaller aggregates or monomers. This parameter is easily determined experimentally from the increase in light scattering associated with self-assembly. It has now been determined for a number of electrolyte systems. 

The polyelectrolyte multilayers employed in these studies consisted of alternating layers of poly(ethyleneimine) (PEI) and poly(4-styrenesulfonicacid) sodium salt (PSS) and were prepared via adsorption from solution as described by Decher et al. [26] on functionalized Au- or SiOx substrates. The lipids used for the preparation of the bilayers were dimyristoyl-L-a-phosphatidylglycerol (DMPG, negatively charged in aqueous solution) and DMPC. Uni-lamellar lipid vesicles were prepared via the extrusion technique. 

The next step is to add the fusion-inducing component, such as a salt solution, DNA, or another kind of liposome/membrane/cell, while continuously recording the fluorescence (Tf). The ratio of this component to the lipid vesicles or particles will vary according to the experimental goals. Similar to anionic liposomes, such as phosphatidylserine/phosphatidylcholine (PS/PC) which fuse rapidly in the... 

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