Lipid vesicles, modeling

Jay, D. and Gilbert, W. (1987). Basic protein enhances the encapsulation of DNA into lipid vesicles model for the formation of primordial cells. Proc. Natl. Acad. Sci. 

When Mitchell first described his chemiosmotic hypothesis in 1961, little evidence existed to support it, and it was met with considerable skepticism by the scientific community. Eventually, however, considerable evidence accumulated to support this model. It is now clear that the electron transport chain generates a proton gradient, and careful measurements have shown that ATP is synthesized when a pH gradient is applied to mitochondria that cannot carry out electron transport. Even more relevant is a simple but crucial experiment reported in 1974 by Efraim Racker and Walther Stoeckenius, which provided specific confirmation of the Mitchell hypothesis. In this experiment, the bovine mitochondrial ATP synthasereconstituted in simple lipid vesicles with bac-teriorhodopsin, a light-driven proton pump from Halobaeterium halobium. As shown in Eigure 21.28, upon illumination, bacteriorhodopsin pumped protons... 

The second model of a biological membrane is the liposome (lipid vesicle), formed by dispersing a lipid in an aqueous solution by sonication. In this way, small liposomes with a single BLM are formed (Fig. 6.11), with a diameter of about 50 nm. Electrochemical measurements cannot be carried out directly on liposomes because of their small dimensions. After addition of a lipid-soluble ion (such as the tetraphenylphosphonium ion) to the bathing solution, however, its distribution between this solution and the liposome is measured, yielding the membrane potential according to Eq. 

The squaraine probe 9g was tested for its sensitivity to trace the formation of protein-lipid complexes [57]. The binding of dye 9g to model membranes composed of zwitter-ionic lipid phosphatidylcholine (PC) and its mixtures with anionic lipid cardiolipin (CL) in different molar ratios was found to be controlled mainly by hydrophobic interactions. Lysozyme (Lz) and ribonuclease A (RNase) influenced the association of 9g with lipid vesicles. The magnitude of this effect was much higher... 

All of the above-mentioned examples describe organosiloxane hybrid sheet-like structures. However, cell-mimicry requires spherical structures that can form an inner space as a container. Liposomes and lipid bilayer vesicles are known as models of a spherical cell membrane, which is a direct mimic of a unicellular membrane. However, the limited mechanical stability of conventional lipid vesicles is often disadvantageous for some kinds of practical application. 

Prior sequestration of the prebiotic reactions within the micropores of weathered feldspars or other porous rock matrices also avoids many of the other problems of catalysis and dilution encountered by models of chemical biogenesis. That is, this mechanism attains viable evolutionary chemical selection among spatially discrete systems without the need to assume an unlikely capture-and-enclosure event involving a pre-existing lipid membrane. [192] Thus autocatalysis of chiral molecules could evolve before the actual appearance of free-floating lipid vesicles. 

At the point where amphiphiles were recruited to provide the precursors to cell membranes, stable lipid vesicles could have evolved [141] to enclose autocatalytic chiral hypercycles. Credible models for the subsequent evolution of vesicles containing self-replicating chiral molecules have appeared in the literature. [193,194] These vesicles could then emerge from the feldspar spaces [134,192] as micron-sized self-reproducing, energy-metabolizing vesicular systems protobacteria ready to face the hydrothermal world on their own terms. 

Much of our chemical understanding of membrane structures has been obtained through the investigation of models, most of which are based solely on the reconstruction of the lipid bilayer part of biomembranes. In contrast to biomembranes, rebuilt bilayers such as lipid vesicles are not stable in the long term. Obviously, nature finds additional means for creating membranes of high stability ... 

Order parameters are used to interpret data on order and fluidity of a number of probes in lipid membranes obtained by measurements of fluorescence anisotropy decay 32 Ambiguities in the interpretation of time resolved fluorescence anisotropy measurements in lipid vesicle systems with DPH or TMA-DPH probes are attributed to the unsatisfactory models being used to interpret the data . The solubilisation of diphenylpolyenes in lipid bilayers has been critically examined33. It is concluded that such probes are satisfactory if used at low concentrations. 

The highly complex and variable composition of natural cell membranes makes them a difficult subject for experimental studies. Artificial lipid membranes have consequently been prepared and studied for many years as models of cell membranes [1,3-7], A diverse array of geometries has been developed, including small and large unilamellar vesicles, giant lipid vesicles, lipid membranes supported on solid and polymer-coated substrates, and BLMs. These have been used to study the physical and chemical properties of lipids and lipid mixtures as well as membrane-associated proteins, including reconstituted transmembrane receptors. 

We have previously mentioned that lipids make part of the membrane architecture, which has the typical bilayer arrangement due to the phospholipid supramo-lecular organization. This leads to biological consequences - the composition of fatty acid residues with saturated and unsaturated hydrocarbon chains is crucial to regulate membrane properties, maintaining the best balance for cellular functioning and also survival. Vesicle models made of phosphatidylcholines with saturated and unsaturated fatty acid residues are useful for studies of permeability and fluidity. Several studies have compared the effects of saturated, cis and trans unsaturated residues. An example is given in Fig. 6.4 for vesicles made up of different phospholipid compositions. 

Measurements of partitioning of drugs into lipid vesicles, liposomes or cell membranes as predictive models for drug absorption are also described in the literature (Hillgren et al. 1995 Balon et al. 1999 Stewart et al. 1997). This may be due to the similarity of these systems to biological membranes and the wish for a pure membrane system with the correct lipid and protein composition, but without enzymes and carrier proteins. 

C. Mangavel, R. Maget-Dana, P. Tauc, J-C. Brochon, D. Sy, J.A. Reynaud, Structural investigations of basic amphipatic model peptides in the presence of lipid vesicles studied by circular dichroism, fluorescence, monolayer and modeling, Biochimica et Biophysica Acta 1371, 265-283 (1998)... 

Two important aspects of Uposome formation must be emphasized here. First, in some cases we observe the formation of ordered supramolecular structures starting from a chaotic disordered mixture of surfactants (as in the ethanol injection method . As noticed before, this increase of order is attended by a simultaneous increase of water entropy and a decrease of overall free energy (Upids and solvent). Secondly, every time a liposome forms, there is the anergence of a division, with an inside world that is different from the external environment, even if the two worlds actually interact with each other. The discrimination between inside and outside, appUcable to lipid vesicles, is the first structural pre-requisite for the living cell. It is therefore clear that lipid or fatty acid vesicles may be considered relevant experimental model of simplified cells, and their role on... 

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