Aqueous core

Liposomes are members of a family of vesicular structures which can vary widely in their physicochemical properties. Basically, a liposome is built of one or more lipid bilayers surrounding an aqueous core. The backbone of the bilayer consists of phospholipids the major phospholipid is usually phosphatidylcholine (PC), a neutral lipid. Size, number of bilayers, bilayer charge, and bilayer rigidity are critical parameters controlling the fate of liposomes in vitro and in vivo. Dependent on the preparation procedure unilamellar or multilamellar vesicles can be produced. The diameter of these vesicles can range from 25 nm up to 50 ym—a 2000-fold size difference. 

Indeed, the degree of binding of the counterions to the micellar surface, even in the largest aqueous core, is found to be 12% [2,94]. This means that virtually all counterions are confined in a thin shell near the surface (about 4 A), the concentration of ions in this domain is very high, and a nearly ordered bidimensional spherical lattice of charges is formed at the water/surfactant interface of ionic surfactants. 

Sometimes, the physicochemical properties of ionic species solubilized in the aqueous core of reversed micelles are different from those in bulk water. Changes in the electronic absorption spectra of ionic species (1 , Co ", Cu " ) entrapped in AOT-reversed micelles have been observed, attributed to changes in the amount of water available for solvation [2,92,134], In particular, it has been observed that at low water concentrations cobalt ions are solubihzed in the micellar core as a tetrahedral complex, whereas with increasing water concentration there is a gradual conversion to an octahedral complex [135],... 

By flourescence techniques, it was observed that the fluorescence yield and lifetime of 1,8-anilinonaphthalenesulfonate decrease with an increase in the aqueous core of AOT-reversed micelles, while the position of the emission maximum shifts to longer wavelengths [64], These changes in the electronic properties were attributed to the peculiar effective polarity and viscosity of the micellar core and to their evolution with R. 

Even entrapment of entire cells within reversed micelles without loss of their functionality has been achieved. For example, mitochondria and bacteria (Actinobacter cal-coaceticus, Escherichia coli, Corynebacterium equi) have been successfully solubilized in a microemulsion consisting of isopropyl pahnitate, polyoxyethylene sorbitan trioleate [162], Enhanced hydrogen photoproduction by the bacterium Rhodopseudomonas sphaeroides or by the coupled system Halobacterium halobium and chloroplasts organelles entrapped inside the aqueous core of reversed micelles with respect to the same cells suspended in normal aqueous medium has been reported [183,184],... 

Other electrokinetic chromatography techniques using liposomes (LEKC) and vesicles (VEKC) have also been applied for logP d estimation. Liposomes and vesicles are organized structures containing continuous bilayers of monomers enclosing an aqueous core region. 

Liposomes are vesicles with an aqueous core surrounded by lipid bilayer walls. Drugs may be encapsulated into the liposomes, either in the bilayers or into the aqueous core of the liposomes. Liposomes have been widely investigated by systemic and local drug delivery in different cavities of the body. 

PEG is hydrophilic and is widely used in biological research because it protects surfaces from interacting with cells or proteins. Thus, coated particles may result in increased blood circulation time. For their preparation, 10-mg magnetite particles were dispersed in 1.0 mU of deoxygenated water by sonication for 30 min. The aqueous dispersion of MNPs was dissolved in the aqueous cores of reverse micelles... 

Liposomes are closely related to nanocapsules in structural layout but consist of an aqueous core surrounded by a bilayer membrane composed of lipid molecules, such as phospholipids [28], as illustrated in Figure 2c. The drug can be located in the aqueous core or in the bilayer membrane. 

The aqueous cores of reverse micelles are of particular interest because of their analogy with the water pockets in bioaggregates and the active sites of enzymes. Moreover, enzymes solubilized in reverse micelles can exhibit an enhanced catalytic efficiency. Figure B4.3.1 shows a reverse micelle of bis(2-ethylhexyl)sulfosuccinate (AOT) in heptane with three naphthalenic fluorescent probes whose excited-state pK values are much lower than the ground-state pK (see Table 4.4) 2-naphthol (NOH), sodium 2-naphthol sulfonate (NSOH), potassium 2-naphthol-6,8-disulfonate (NSOH). The spectra and the rate constants for deprotonation and back-recombination (determined by time-resolved experiments) provide information on the location of the probes and the corresponding ability of their microenvironment to accept a proton , (i) NDSOH is located around the center of the water pool, and at water contents w = [H20]/[A0T] >... 

Unilamellar liposomes are nanoparticles made of a bilayer, most often phospholipidic, entrapping an internal aqueous core. Formed spontaneously in the presence of an excess of water and above the gel-to-liquid crystal phase tran-... 

Depending upon its properties, the paramagnetic material can be encapsulated into the aqueous core of the liposome (if hydrophilic), incorporated or intercalated in the membrane (if lipophilic or amphiphatic), or directly attached by chemical bond to the surface membrane (107-109). 

Cayie OJ, Noble PF, Paunov VN. Fabrication of novel colloidosome microcapsules with gelled aqueous cores. J Mater Chem 2004 14 3351-3355. 

Lambert G, Fattal E, Pinto-Alphandary H, Gulik A, Couvreur P. Polyisobutylcyanoacrylate nanocapsules containing an aqueous core as a novel colloidal carrier for the delivery of oligonucleotides. Pharm Res 2000 17 707-714. 

The common strategy is to entrap into the aqueous core of liposomes all the ingredients for the in vitro protein expression i.e., the gene for the GFP (a plasmid), an RNA polymerase, ribosomes, and all the low-molecular-weight components (amino acids, ATP, etc.) needed for protein expression. 

FIGURE 10.3 Schematic presentation of lipid based drug delivery systems. Micelles (right) are composed of a solid lipid core with the polar heads exposed to the aqueous environment. Liposomes (left) are particles with a hpid bilayer surrounding an aqueous core. Drug can be encapsulated in the hydrophobic regions of the lipid particle, in the aqueous environment of the liposome, or adsorbed to the surface of the lipid particle. 

Liposomes were first proposed for drug topical administration to the skin more than 25 years ago by Mezei and Gulusekharam [1,2]. The basic components of liposomes are phospholipids (phosphatidylcholine, phophatidylethanolamine, phophatidylserine, dipalmitoyl phosphatidylcholine, and others), cholesterol, and water. Liposomes may vary significantly in terms of size (from tens of nm to microns) and structure. In liposomes, one or more concentric bilayers surround an aqueous core generating small or large unilamellar vesicles (SUV, LUV) or multilamellar vesicles (MLV), respectively [3]. 

More recently, Carafa et al. showed that niosomes could be obtained from polyoxyethylene sorbitan monolaurate-cholesterol in aqueous environment. These authors investigated the delivery of lidocaine HC1 and lidocaine base from vesicles through silicone membrane and nude mice skin [44]. It was found that only the charged molecule (loading pH 5.5) could be encapsulated within the vesicles ( 30%). This behavior was explained by the entrapment ability of the hydrophilic moiety within the aqueous core of the vesicles. The lipophilic unionized form of lidocaine (loading pH 8.6) remained unattached. The amount of lidocaine permeated through nude mice skin from these niosomes was similar to liposomes and only about twofold greater than from a micellar system. 

The classical concept of pH in the aqueous core reverse micelles can be used when the center of water pools is large enough to accommodate a sufficient amount of free water, (Wo > 10). In small water pools (Wo < 10) or at the interface, the problem becomes extremely difficult. Attempts to get direct or indirect information on hydrogen ion activity are futile, because hydrogen ion activity distribution is not uniform throughout the aqueous core. A large difference exists between the interface and the center of the soft-core micelles. Acidity in small water pools or at the interface must be viewed rather in terms of the tendency of water molecules to donate or accept protons. 

Adapting techniques based on in vitro protein synthesis to the isolation of enzymes requires establishing a link between a nucleic acid-protein complex and product formation. Methods based on binding, analogous to those developed for phage displayed libraries, may be used to enrich catalysts from noncatalysts. In addition, Tawfik and Griffiths (1998) exploited the aqueous core of reverse micelles as artificial compartments... 

Liposome An artificial microscopic vesicle consisting of an aqueous core enclosed in one or more phospholipid layers... 

R. Atkin, P. Davies, J. Hardy, and B. Vincent, Preparation of aqueous core/polymer shell microcapsules by internal phase separation, Macromolecules 37, 7979-7985 (2004). 

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