The history of natural science contains numerousexamples of creation biomimetic systems. The challenge for scientists is tooffer the system closest to in vivoconditions. Technical progress gives various opportunities to the developmentof life-like compounds.
That is why it was and still remains the topic of interestof many researchers.From the time of publishing of “Micrograhia” of RobertHooke (1665), cellular organization is the one of the most important topic ofinvestigation in biological sciences. In the middle of 17 century using anextremely simple optical magnification system, Hooke was the first whoidentified separate “cells” in the living organisms. With no doubt, it wasstarting point for many biological sciences and study of cellular membranesstructure and properties is one of it.In the current short review, I would like to introducesome of big variety of biological model modelling systems – from most simple,like lipid micelles to complex biomimetic systems – life-like artificial cells.The most known models are based on concept of lipidbilayer. This concept was proposed by Gorter and Grendel (1925).
They studied”chromocytes” of different animals and found that “every chromocyte issurrounded by a layer of lipoids, of which the polar groups are directed to theinside and to the outside”. This concept, in a big variety of modifications, isused in order to explain natural like media to study different membrane relatedprocesses. The most known is the “fluid mosaic model” presented by Singer andNicolson (1972). This model system combine liquid crystal properties of lipidbilayer and specific interaction with membrane associated proteins. Further studies of biological membranes led to thefurther complication of the corresponding model systems. Of course, thiscontributed to the progress in the reconstruction of analogues of individualorganelles with the reproduction of their functional properties. However, asclaimed by Butovich et al. (1995), this complication made it practicallyimpossible to study the individual stages of biochemical reactions.
In such cases,the simple model systems have benefits, as it is possible to minimize sideeffects of complex models.All of existing models of biomembranes based on theamphiphilic nature of lipid molecules. Most common group of membrane lipids arephospholipids –glycerol esters of fatty acids and phosphoric acid derivatives.Presence of hydrophobic chains in the molecule make it extremely difficult todissolve in polar solvent like water so they easy create different aggregateslike different micelles and liposomes.
These aggregates can have differentform, size and dimensional properties. As it was shown by Ollivon et al. (2000)these forms are flexible and in certain conditions are able to transform simplemicelles to bilayer vesicles. Author in details analyses the ability ofamphiphilic lipid molecules to form macromolecular complexes in presence ofdifferent types of synthetic and natural detergents – non-ionic Triton X-100,octylglucoside , dodecyloctaethyleneglycol, anionic – sodium cholate andcationic cetyltrimethylammonium chloride. In his study he presented data oninfluence of detergent residues in membrane models and ways how to minimizetheir concentration dependent effect on membrane properties. Presence ofnon-natural compounds in membrane models is a factor of uncertainty withregards of interpretation of results obtained using such models.
However, inmany researches where the subject of study is, for example, role oflipid-generated mechanism of enzyme regulation, lipid/detergent mixed micellesare used. Such system are especially useful for screening of possibleeffectors, enzymes inhibitors or activators. Moreover, many authors, according to Montaland Mueller (1972), show a good correlation of results obtained using simplemodels with those made on living cell cultures. Besides micelles, bilayer lipid membranes formed in achamber of “fatty” polymer, like polyethylene or PTFE also called “black lipidmembranes”, were widely used in experimental models. Such flat membranes areformed by applying of a small amount of lipids dissolved in non-polar solventon an aperture of a small (about 1 mm in diameter) hole.
The resulting film isfirst rather thick and in reflected light has a gray color tone. Then, due tosurface tension it becomes thinner with time, and due to the interference oflight gets a bright, constantly changing iridescent color. At this point, thefilm has a thickness of 100-600 nm. Finally, it appears first some black spots,and then it all becomes “black” figure. This occurs due to theclosing of two lipid monolayers into one extended bilayer, which, due to itssmall thickness practically lost its reflectivity to visible light (Fig.
1). Fig. 1(Top) A preparation of biological membranes is treated with an organicsolvent, such as a mixture of chloroform and methanol (3:1), which selectivelysolubilizes the phospholipids and cholesterol. Proteins andcarbohydrates remain in an insoluble residue. The solvent is removed by evaporation.(Bottom left) If the lipids are mechanically dispersed in water, theyspontaneously form a liposome, shown in cross-section, with an internalaqueous compartment.
(Bottom right) A planar bilayer, also shownin cross-section, can form over a small hole in a partition separating twoaqueous phases; such bilayers are often termed”black lipid membranes” because of their appearance. (Lodish H. etal. 2000)That makes it is easy to use to study properties ofmembrane channels, components of active and passive membrane transport etc.
With this model it is easy to create concentration gradient between two sidesof membrane a measuring of electrical parameters allows to control processes ontransmembrane level.The very good similarity to native membrane representsby unilamelar liposomes. The name “liposome” consists of Greek words lipo – fat and soma – body. Liposomes are small (20 – 100 nm) spherical vesicles,which have a bilayer lipid membrane, usually lower size range, more rarelyvariable in sizes (Fig. 2). Liposomes were proposed by Bangham (1963).
Theywere discovered occasionally during testing electron microscope by addingnegative stain to dry phospholipids and named the structures”liposomes” after the lysosome, which had been studying by thislaboratory. Liposomes are different from micelles as they have an internalhydrophilic water phase and literally copy the lipid barrier of the livingcell. This model has a unique advantage among other mentioned that it ispossible to study individual biomolecules in their interaction with lipidmembrane. Fig. 2 (A) An electron micrograph of unfixed,unstained phospholipid vesicles (liposomes) in water.
The bilayer structure ofthe vesicles is readily apparent. (B) A drawing of a small spherical liposomeseen in cross-section. (Alberts et al.
1994)Liposomes are widely used when it is necessary toinvestigate enzyme catalyzed reaction taken place on the surface of membrane ordirectly in the membrane when the substrate of the reaction incorporated inlipid bilayer. An example of such application can be lipoxygenase oxidation ofpolyunsaturated fatty acids. This reaction is a first step of biosynthesis ofleukotrienes and prostaglandins – very important signaling molecules involvedin signal transmitting on the cell level.
The substrate – linoleic, linolenic,arachidonic acids) are incorporated in lipid bilayer in a form of ester inphospholipids. On the liposome model it is possible to investigate the wholecascade of leukotrienes/prostaglandins biosynthesis combine phospholipase A2,lipoxygenases and cyclooxygenases enzymes in one model system. Group ofscientists from Ukraine (Butovich et al) has started their studies in 1999-2001.They also proposed a “solid liposome” model – model system similar to liposomebut with silica particle in the core area. To prepare this model fumed silicaparticles of about 100 nm in diameter were chemically modified byoctadecylsilane grout to obtain hydrophobic monolayer of similar to fatty acidshydrophobic tales.
Then hydrophobic particles were coated with phospholipids ina way similar to liposome formation. Finally, the obtained model had similar totypical liposome pseudo bilayer, but the model represents very high stability,unlike liposomes. This model was easy to use for study lipid-lipid and lipidprotein interactions as well as enzyme kinetic study, biosynthesis etc.Nowadays, scientists made huge progress in syntheticbiology, by managing to build life-like models of particles, which demonstrate thecharacteristics of living cells. Cell modeling has now come to the crucial pointwhere the next step might be synthesis the artificial cell. However, individually recreated features of lifemay not be compatible together in current shape (Forlin et al, 2012). A greatdeal of research and work has to be done on our way to their integration intomore sufficient form.