Chapter 9. Material transport across permeability barriers by means of lipid vesicles

G. Cevc
Medizinische Biophysik, Technische Universität München, Klinikum r.d.I.,
Ismaningerstr. 22, D-1675 München, E.U. Germany

1. Introduction

Nature is full of barriers. It was not before the formation of the first membraneous sacks that the self-replicating and catalytic molecules encapsulated in such vesicles have started to act as primitive creatures. The reason for this is that membranes present a barrier to the free diffusion of water soluble substances and thus enforce material localization. The very limited diffusion of the membraneous bodies through the partly filled space plays an important role in this as well.

Primordial membranes must have emerged from the pools of ubiquitous simple lipids which have been reported to exist even in the interstellar space. Most lipids being nearly insoluble in water, they tend to form membranes spontaneously. This is true for all basic amphiphiles (Note: From the Greek, amphi, on both ends, and philos, loving; this is, containing a hydrophilic and a hydrophobic end.) of the biological membranes at least, with the solubilities between 10^-10 and 10^-6 moles per litre. Biogenic lipids thus normally do not dissolve but rather disperse in the aqueous systems in the form of extended aggregates [1].

The most common form of the polar lipid aggregates in water are bilayers. These consist of two opposing hydrocarbon monolayers separated from the surrounding water by two layers of the polar lipid headgroups. It is this membrane core and the two interfaces which act as a diffusion barrier and/or as biochemical catalytic sites, depending on the type of the molecules added. Bilayers chiefly form closed spherules, so-called liposomes, with a radius much greater than the lipid dimensions, owing to the monolayer packing constraints [2, 3]. In the thermodynamic equilibrium vesicle radii are normally rather large, r_v >~ 40 nm.

Only the most polar lipids are soluble in water at concentrations higher than 10^-5 and up to 10^-2 moles per litre. When this solubility limit is exceeded, however, even the most hydrophilic lipids aggregate spontaneously. The resulting lipid micelles normally contain just a few dozen of molecules [5], by and large. Micellar shape depends on the precise distribution of the polar residues on each individual molecule [4]. Most lipid micelles are spherical or disk-like (cf. fig. 1) and correspondingly small. Their surface, consequently, is at least locally highly curved (r_m <~ 5 nm)and resembles the 'edge' of an open lipid bilayer.

Addition of large amounts of the highly water-soluble lipids to the suspension of less polar lipids solubilizes the closed lipid bilayer. Lipid solubilization normally pro-ceeds via the formation of mixed-micelles and other types of mixed aggregates [7]. The solubilization-inducing lipids, consequently, are also called surfactants or deter-gents, to stress this fact.

Fig. 1. Schematic representation of various types of lipid aggregates.

[Full text] (PDF 515 Kbytes)