Lipids amphiphiles, generalized

This review emphasizes an intriguing and potentially useful aspect of the polymerization of lipid assemblies, i.e. polymerization and domain formation within an ensemble of molecules that is usually composed of more than one amphiphile. General aspects of domain formation in binary lipid mixtures and the polymerization of lipid bilayers are discussed in Sects. 1.1 and 1.2, respectively. More detailed reviews of these topics are available as noted. The mutual interactions of lipid domains and lipid polymerization are described in the subsequent sections. Given the proper circumstances the polymerization of lipid monolayers or bilayers can lock in the phase separation of lipids, i.e. pre-existing lipid domains within the ensemble as described in Sect. 2. Section 3 reviews the evidence for the polymerization-initiated phase separation of polymeric domains from the unpolymerized lipids. 

In analogy to lipids, amphiphilic block copolymers, i.e., macromolecules composed of at least one hydrophilic and one hydrophobic, covalently linked, polymer chains can form in aqueous solutions vesicles the so-called polymersomes. Generally, in self-assembling copolymer solutions, a rich diversity of morphologies is possible. An overview of the various factors important for vesicle formation, including copolymer architecture, presence of additives, solvent composition, and temperature, is given in [19]. To illustrate polymersome structures we reproduce from [21] on the top row of Fig. 2 cryo-TEM images of vesicles formed by 1.0 wt % aqueous solution of PEO- -PBD (PEO, polyethylene oxide PBD, polybutadiene) diblock copolymer for three different sizes of the PEO and PBD blocks. 

General anesthetics are usually small solutes with relatively simple molecular structure. As overviewed before, Meyer and Overton have proposed that the potency of general anesthetics correlates with their solubility in organic solvents (the Meyer-Overton theory) almost a century ago. On the other hand, local anesthetics widely used are positively charged amphiphiles in solution and reversibly block the nerve conduction. We expect that the partition of both general and local anesthetics into lipid bilayer membranes plays a key role in controlling the anesthetic potency. Bilayer interfaces are crucial for the delivery of the anesthetics. 

Symmetric amphiphilic molecules, in which two hydrophilic residues are linked by hydrophobic segments, are generally known as bola-lipids based on their resemblance to an old South American hunting weapon. Well-characterized bola amphiphiles are archaebacterial lipids, which usually consist of two glycerol backbones connected by two hydrophobic... 

The measurements of n versus A isotherms generally exhibit, when compressed, a sharp break in the isotherms that has been connected to the collapse of the mono-layer under given experimental conditions. The monolayer of some lipids, such as cholesterol, is found to exhibit an unusual isotherm (Figure 4.7). The magnitude of FI increases very little as compression takes place. In fact, the collapse state or point is the most useful molecular information from such studies. It has been found that this is the only method that can provide information about the structure and orientation of amphiphile molecules at the surface of water (Birdi, 1989). 

The linker group that bridges the cationic lipid headgroup with the hydrocarbon moiety controls the biodegradability of a cationic amphiphile. Most of the linker bonds are ether, ester, or amide bonds (Fig. 1). Compounds with ether links generally render better transfection efficiency. However, they are more stable and may cause higher toxicity, while cationic lipids with ester links such as DOTAP are more biodegradable and less cytotoxic in cultured cells [28, 39]. Noteworthy,... 

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]. 

Lipids (fats) are generally insoluble in water but are soluble in organic solvents. Solubility characteristics depend on structural features those fats that have no polar residues are least water soluble, and they exist away from the aqueous environments of the organism (e.g., cholesterol esters and triglycerides). Others have hydrophilic along with hydrophobic residues. These are called amphipathic substances (also amphiphiles), and they may exist at the interface between aqueous and nonaqueous environments (e.g., phosphoglycerides). 

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