Amphiphilic molecules, membrane lipids

Liposomes, Vesicles and Cast Films - Supramolecular Assemblies Based on Lipid Bilayers Amphiphilic molecules or lipid molecules sometimes form double-layer structures. This structure is called a bilayer structure, and it can be used to model a cell membrane. 

As shown by Pockels, Langmuir [24 - 26] and others, amphiphilic molecules, including lipids, can be stabilised as monomolecular layers at the air-water interface. Also, some proteins can self-assemble - on their own or with lipids - at the air-water interface to form monolayers, e.g., lipase [27], bacterial surface-layer proteins [28, 29], or trans-membrane proteins such as bacteriorhodopsin [30], Transfer of monomolecular layers to solid support by the Langmuir-... 

The relation between the architecture of the molecules and the spatial morphology into which they assemble has attracted longstanding interest because of their importance in daily life. Lipid molecules are important constituents of the cell membrane. Amphiphilic molecules are of major importance for teclmological applications (e.g., in detergents and the food industry). 

As schematically shown in Fig. 1C, a carrier is an amphiphilic molecule capable of residing at the membrane aqueous interface with its lipophilic side interacting with the lipid of the membrane, with polar moieties directed outward into the aqueous phase, and with the polar moieties of a chemical nature to induce an ion into interaction. In the process of a carrier interacting and complexing with the ion, it... 

Two principal routes of passive diffusion are recognized transcellular (la —> lb —> lc in Fig. 2.7) and paracellular (2a > 2b > 2c). Lateral exchange of phospholipid components of the inner leaflet of the epithelial bilayer seems possible, mixing simple lipids between the apical and basolateral side. However, whether the membrane lipids in the outer leaflet can diffuse across the tight junction is a point of controversy, and there may be some evidence in favor of it (for some lipids) [63]. In this book, a third passive mechanism, based on lateral diffusion of drug molecules in the outer leaflet of the bilayer (3a > 3b > 3c), wih be hypothesized as a possible mode of transport for polar or charged amphiphilic molecules. 

Lipids have multiple functions in brain 33 Membrane lipids are amphiphilic molecules 34... 

Lipids encompass a wide class of amphiphilic molecules which, along with proteins, form the biological membranes necessary to support cellular function. While the simplest lipids, fatty acids and triglycerides, are not... 

The most common lipid components of cell membranes are phospholipids. The structures of typical representatives of these amphiphilic molecules (with hydrophobic alkyl chains and hydrophilic head group) are illustrated in Fig. 3. 

The lipid part of the membrane is essentially a two-dimensional liquid in which the other materials are immersed and to which the cytoskeleton is anchored. This last statement is not totally correct, as some membrane bound enzymes require the proximity of particular lipids to function properly and are thus closely bound to them. Simple bilayers formed from lipids in which both hydrocarbon chains are fully saturated can have a highly ordered structure, but for this reason tend to be rigid rather than fluid at physiological temperatures. Natural selection has produced membranes which consist of a mixture of different lipids together with other amphiphilic molecules such as cholesterol and some carboxylic acids. Furthermore, in many naturally occurring lipids, one hydrocarbon chain contains a double bond and is thus kinked. Membranes formed from a mixture of such materials can retain a fluid structure. The temperature at which such membranes operate determines a suitable mixture of lipids so that a fluid but stable structure results at this temperature. It will be seen that the lipid part of a membrane must, apart from its two-dimensional character, be disordered to do its job. However, the membrane bound proteins have a degree of order, as will be discussed below. 

Phospholipids are found in all living cells and typically constitute about half of the mass of animal cell plasma membranes (Cevc, 1992). The reason forthe variety of membrane lipids might simply be that these amphiphilic structures have in common the ability to arrange as bilayers in an aqueous environment (Paltauf and Hermetter, 1990). Thus, the use of endogenous phospholipids to form vesicles as drug carriers may have much less adverse effects in patients compared to synthetic drui carrier molecules. 

Recent experiments have shown that the non-specific, physical chemical interactions between small hydrophobic, water-insoluble molecules and the hydrocarbon chains of lipid membranes are important determinants of the rate at which these molecules enter cells and are metabolized (3.34). Cholesterol has the capability of modifying these interactions and also increases the affinity of vesicle surfaces for amphiphillic molecules (4) separating the lipid polar groups (35). 

Fluorescence techniques have also been used to determine the localization of molecules in membranes. Using this technique, the localization of the linear dye molecule 3,3 -diethyloxadicarboxyamine iodide (DODCI) in lipid bilayer vesides was determined as a function of lipid chain length and unsaturation. It was found that the fraction of the dye in the interior region of the membrane was decreased as a function of chain length in the order C12 > C14 > C16 > C18. In unsaturated lipids it was Ci4 i > C14 0 > C16 1 > C16 0, which is in agreement with the general observation that the penetration of amphiphilic molecules into the interior of membranes increases with an increase in the fluidity of the membrane structure [59]. 

Although it is clear that complex lipids can be synthesized under laboratory simulations using pure reagents, the list of required ingredients does not seem plausible under prebiotic conditions. Therefore, it is unlikely that early membranes were composed of complex lipids such as phospholipids and cholesterol. Instead, there must have been a source of simpler amphiphilic molecules capable of self-assembly into membranes. One possibility is lipidlike fatty acids and fatty alcohols, which are products of FTT simulations of prebiotic geochemistry [12] and are also present in carbonaceous meteorites. Furthermore, as will be discussed later, these compounds form reasonably stable lipid bilayer membranes by self-assembly from mixtures (Fig. 4a). 

Because the lipid components of membranes must be in a fluid state to function as membranes in living cells, it is reasonable to assume that primitive membranes in the first forms of cellular life must also have had this property. Straight-chain hydrocarbons have relatively high melting points due to the ease with which van der Waals interactions can occur along the chains. Any discontinuity in the chains interrupts these interactions and markedly decreases the melting point. As an example, stearic acid contains 18 carbons in its alkane chain and melts at 68 °C, while oleic acid, with a cis-double bond between carbons 9 and 10, has a melting point near 14 °C. If cellular life today requires fluid membranes, it is reasonable to assume that the earliest cell membranes were also composed of amphiphilic molecules in a fluid state. 

It is well known that water dispersions of amphiphile molecules may undergo different phase transitions when the temperature or composition are varied [e.g. 430,431]. These phase transitions have been studied systematically for some of the systems [e.g. 432,433]. Occurrence of phase transitions in monolayers of amphiphile molecules at the air/water interface [434] and in bilayer lipid membranes [435] has also been reported. The chainmelting phase transition [430,431,434,436] found both for water dispersions and insoluble monolayers of amphiphile molecules is of special interest for biology and medicine. It was shown that foam bilayers (NBF) consist of two mutually adsorbed densely packed monolayers of amphiphile molecules which are in contact with a gas phase. Balmbra et. al. [437J and Sidorova et. al. [438] were among the first to notice the structural correspondence between foam bilayers and lamellar mesomorphic phases. In this respect it is of interest to establsih the thermal transition in amphiphile bilayers. Exerowa et. al. [384] have been the first to report such transitions in foam bilayers from phospholipids and studied them in various aspects [386,387,439-442]. This was made possible by combining the microscopic foam film with the hole-nucleation theory of stability of bilayer of Kashchiev-Exerowa [300,402,403]. Thus, the most suitable dependence for phase transitions in bilayers were established. 

Lipid bilayers are formed by many amphiphilic molecules in the presence of water. Their interest derives not only from the fact that they are a major, if not the only, organizing principle of biological membranes (1), but also because they tend to form closed (usually) spherical structures (liposomes or lipid bilayer vesicles) in which inner and outer aqueous spaces are separated by the lipid bilayers (2) which thereby provides a means of encapsulation (3, 4). 

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