Lipid polymerization

These possibilities rectify the proposed subsequent appearance and amplification of chiral autocatalytic molecules and hypercydes. [190] Any autocatalytic systems would propagate [191] throughout an extensive adjoining hydrated porous network already rich in layered amphiphiles, lipids, polymeric materials, amino acids, thiols, and so forth. In addition, amphiphiles are known to be organized into lipid membranes by interaction with the inner surfaces of porous minerals. [136] It is a small organizational jump from these membranes to frilly formed lipid vesides. 

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. 

A similar study by O Brien and coworkers utilized bilayers composed of a shorter chain diacetylenicPC (9) and DSPC or DOPC [37]. Phase separation was demonstrated in bilayers by calorimetry and photopolymerization behavior. DSC of the 9/DSPC (1 1) bilayers exhibited transitions at 40 °C and 55 °C, which were attributed to domains of the individual lipids. Polymerization at 20 °C proceeded at similar rates in the mixed bilayers and pure 9 bilayers. A dramatic hysteresis effect was observed for this system, if the bilayers were first incubated at T > 55 °C then cooled back to 20 °C, the DSC peak for the diacetylenicPC at 40 °C disappeared and the bilayers could no longer be photopolymerized. The phase transition and polymerizability of the vesicles could be restored simply by cooling to ca. 10 °C. A similar hysteretic behavior was also observed for pure diacetylenicPC bilayers. Mixtures of 9 and DOPC exhibited phase transitions for both lipids (T = — 18 °C and 39 °C) plus a small peak at intermediate temperatures. Photopolymerization at 20 °C initially proceeded at a similar rate as observed for pure 9 but slowed after 10% conversion. These results were attributed to the presence of mixed lipid domains... 

Fabricating a supported lipid bilayer in which both monolayers are composed of polymerizable lipids results in formation of a polymeric network in each monolayer. Furthermore, if the reactive groups are located at the termini of the acyl chains, the monolayers can be covalently linked, which is inherently more stable than a HBM in which lipid polymerization occurs in only one monolayer. 

Large and small vesicles are more frequently studied as dispersed ensembles due to their ease of preparation and compatibility with solution phase analytical/physical methods. Lipid polymerization yields vesicles with enhanced stability to surfactants, organic solvents, dehydration, and heat [26]. Polymerization also alters membrane permeability to ions and molecules. These unique properties have spawned development of stable nanocapsules, bioreactors, and sensors. Many if not most of the liposomal architectures, methods to stabilize them, and technological applications discussed below have evolved from earlier pioneering work by many research groups. The reader is referred to previous key reviews [3,26,28]. 

Supported lipid membranes and membrane arrays functionalized with TMPs have been fabricated by numerous research groups (see for example [5,37,127-135] and references therein). Lipid polymerization could be a useful strategy to enhance the operating lifetime of these materials when incorporated into biosensing and drug screening devices. A key issue that must be addressed is the potentially adverse effects that lipid polymerization may have on TMP structure and activity, which can be separated into two major subtopics ... 

A common feature of all lipids is that biologically their hydrocarbon content is derived from the polymerization of acetyl imits followed by reduction of the chain that is formed. (However, this process also occms in the synthesis of some compoimds that are not lipids, so this property cannot be used as a definition of lipids.) Polymerization of acetyl units gives rise to the following ... 

Yeast and bacteria can produce biosurfactants, biological surfactants from various substrates including sugars, oils, alkanes and wastes [5]. Some types of biosurfactants are glycolipids, lipopeptides, phospholipids, fatty acids, neutral lipids, polymeric and particulate compounds [6]. Most are either anionic or neutral, while only a few with amine groups are cationic. The hydrophobic part of the molecule is based on long-chain fatty acids, hydroxy fatty acids or a-alkyl-jS-hydroxy fatty acids. The hydrophilic portion can be a carbohydrate, amino acid, cyclic peptide, phosphate, carboxylic acid or alcohol. 

In low-selective electrodes, assembled as sensor arrays, several membranes can be used for potentiometric sensors, namely chalcogenide glasses and lipid polymeric-membranes (Vlasov et al., 2005). These arrays constitute analytical tools whose performance capabilities depend on the selected electrodes but also on the practical purpose. Quahtative, semiquantitative and quantitative analysis are possible by using advanced chemometric techniques. 

Valencia PM, Basto PA, Zhang L, Rhee M, Langer R, Farokhzad OC, Kamik R (2010) Single-step assembly of homogenous lipid-polymeric and lipid-quantum dot nanoparticles enabled by microfluidic rapid mixing. ACS Nano 4(3) 1671-1679... 

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