Lipid dispersed in water

We have observed such a transition in intact membranes of M. laidlawii which occurs at the same temperature as in the membrane lipids dispersed in water (77). Figure 11 shows representative endothermic transitions of membranes and lipids in water. Membranes were prepared for calorimetry by sedimenting at high speed, then 90-100 mg. of packed pellet were sealed in a stainless steel sample pan. The material was neither dried nor frozen before examination. Total membrane lipids were extracted with chloroform-methanol 2 1 v/v then dried and suspended in water. Lipids from the membranes of cells grown in the usual tryptose medium without added fatty acids are shown in a, while b and c are scans of intact membranes from the same cells. In b the membrane preparation had not been previously exposed to temperatures above 27 °C. The smaller transition at higher temperature probably arises from... 

Figure 13. NMR spectra at 35°C. of (a) low density lipoprotein lipids dispersed in D20 by sonication, 512 grams phosphorus/ml. (b) low density serum lipoproteins in 0.1 M Nad-D O, 625 grams lipid phosphorus/ml. (c) high density serum lipoproteins in 0.1 M NaCl-D/0, 832 pgrams lipid phosphorus/ml. Spectrum for high density lipoprotein lipids dispersed in water by sonication was essentially identical to (a). Spinning side bands are labeled S...

Only a small quantity of an amphiphilic lipid dispersed in water can form a monolayer (unless the water is spread as a very thin film), in which case the bulk of the lipid will form soluble micelles. Micelles can take a variety of forms, each satisfying the hydrophobic effect. Fig. 6-2 shows one such form, representing a spherical micelle, although ellipsoidal, diskoidal, and cylindrical variations are possible. 

Interactions of apolipoproteins with PL are essential for the assembly of lipoproteins, stabilization of lipoprotein structures, and expression and modulation of apolipoprotein functions. The main experimental approaches for the study of apolipoprotein interactions with PL have used isolated, exchangeable apolipoproteins in conjunction with aggregated lipids dispersed in water or spread at the air-water interface. The aggregated lipid states include lipid monolayers, various types of liposomes (small unilamellar vesicles, large unilamellar vesicles, multilamellar vesicles), and emulsions. All these lipid systems consist of or include PL, especially PC. 

When amphipathic molecules are dispersed in water, their hydrophobic parts (i.e., hydrocarbon chains) aggregate and become segregated from the solvent. This is a manifestation of the hydrophobic effect which comes about because of exclusion and hence ordering of water at the interface between these distinct types of molecule. Aggregates of amphipathic molecules can be located at a water-air boimdary (monolayers) (Fig. 3-24) however, only a small quantity of an amphipathic lipid dispersed in water can form a monolayer (unless the water is spread as a very thin film). The bulk of the lipid must then be dispersed in water as micelles (Fig. 3-24). In both of these structures the polar parts, or heads (O), of the lipid make contact with the water, while the nonpolar parts, or tails (=), are as far from the water as possible. Micelles can be spherical as shown in Fig. 3-24, but can also form ellipsoidal, discoidal, and cylindrical stmctures. 

Over 40 years since it what found that phospholipids can form closed bilayered structures in aqueous systems, liposomes have made a long way to become a popular pharmaceutical carrier for numerous practical applications. Liposomes are phospholipid vesicles, produced by various methods from lipid dispersions in water. Liposome preparation, their physicochemical properties and possible biomedical application have already been discussed in several monographs. Many different methods exist to prepare liposomes of different sizes, structure and size distribution. The most frequently used methods include ultrasonication, reverse phase evaporation and detergent removal from mixed lipid-detergent micelles by dialysis or gel-filtration. To increase liposome stability towards the physiological environment, cholesterol is incorporated into the liposomal membrane (up to 50% mol). The size of liposomes depends on their composition and preparation method and can vary from... 

Second, tiie hydrophobic effect does not explain why the lipids dispersed in water do not all eventually form the same supramolecular structures. Indeed, some lipids selfaggregate into micelles, whereas other form bilayers. Understanding the molecular mechanisms controlling the state of organization of lipids in water is one of the main goals of this chapter. 

The functions of liposomes, such as interaction, incorporation, recognition, and stabilization, are attributed to the microfluidity of a membrane and its ttansitional state [2], Lipids dispersed in water can form a variety of structures, for example, the liposome-type structure at low lipid/water ratios. As temperature increases, the lipid phase shifts from a crystalline to a condensed gel-like state and then to a fluidic, expanded state, and such a transition state at the corresponding temperature can be determined by various methods [2,16]. 

When these lipids are dispersed in water, they spontaneously form bilayer membranes (also called lamellae) which are composed of two monolayer sheets of lipid molecules with their hydrophobic surfaces facing one another and their hydrophilic surfaces contacting the aqueous medium. In the case of phospholipids such as phosphatidylcholine (10.50), the structure consists of ... 

The zeta potential of the formulations was determined by Doppler velocimetry and PCS on a Zetasizer 4 (Malvern Instruments, U.K.), without further dilution. The zeta potential of LC-AmB under these conditions was —44 mV, slightly lower than that measured for the same lipid composition without AmB, —55 mV, but remaining consistent with colloidal stability. This reduction in the absolute value of the zeta potential could be due to the presence of AmB at the surface, because free AmB dispersed in water under the same conditions had a less negative zeta potential about —27 mV. 

Many substances as found in nature (lipids) exhibit unique properties in aqueous media. Some lipids (such as lecithins or alike), when dispersed in water, form very well-defined assemblies, in which the alkyl part of the molecule is in close proximity to each other. This leads to self-assembly formation with many important consequences. 

A certain type of lipid (or lipid-like) molecules are found that when dispersed in water tend to make self-assembly structures (Figure 4.13). Detergents were shown to aggregate to spherical or large cylindrical-shaped micelles. It is known that if egg phosphatidylethanolamine (egg lecithin) is dispersed in water at 25°C, it forms a self-assembly structure, which is called liposome or vesicle. 

Many phospholipids, including (I) through (IV) above, form lipid bilayers when dispersed in water. These bilayers may take on various shapes, depending on their treatment. When strongly sonicated in aqueous solutions, phosphatidylcholines such as (II) form quite stable singlecompartment vesicles, with outer diameters of the order of 200 to 350 A. 

Often when phospholipids such as (I) through (IV) are dispersed in water, they form so-called liposomes, which have a large number (e.g., 10 or more) of concentric bilayer shells. When phospholipids are dissolved in detergents and the detergents are removed slowly by dialysis, the phospholipids form relatively large vesicles containing one or only a few bilayer shells. Lipid bilayers have also been studied as supported by black lipid membranes [Fettiplace et al. (1975)]. 

Most of the cationic lipids arrange into bilayers and readily form liposome dispersions in water [16]. At higher electrolyte concentrations, e.g., in physiological solutions, in which electrostatic repulsion is largely screened, and at higher lipid contents bringing the bilayers together, these lipids form well-correlated lamellar... 

It is of interest to note that in spite of the fact that biological lipids are often chiral, their chirality is very rarely expressed in the resulting functional assemblies. One of the few examples of this expression of chirality has been observed in lecithin dispersions in water, where the formation of helical intermediates was monitored with polarizing microscopy.63 64... 

Lipase, Animal Obtained from the edible forestomach tissue of calves, kids, or lambs and from animal pancreatic tissue. Produced as purified edible tissue preparations or as aqueous extracts dispersible in water, but insoluble in alcohol. Major active principle lipase. Typical applications used in the manufacture of cheese and in the modification of lipids. 

Liposomes were also utilized for preparing inkjet inks. Liposomes are aqueous compartments enclosed by lipid bilayer membranes which form spontaneously when amphiphilic lipid molecules are dispersed in water (see Fig. 2). Liposomes are also known as lipid vesicles. 

Foht PJ, Quynh MT, Lewis RNAH, McElhaney RN. Quantitation of the phase preferences of the major lipids of the Acholeplasma laidlawii B membrane. Biochemistry 1995 34 13811-13817. Lewis RNAH, McElhaney RN. Acholeplasma laidlawii B membranes contain a lipid (glycxerylphosphoryldiglucosyl diacylgly-cerol) which forms micellar rather than lamellar or reversed phases when dispersed in water. Biochemistry 1995 34 13818-13824. Steim JM, Tonrtellotte ME, Reinert JC, McElhaney RN, Rader RL. Calorimetric evidence for the liquid-crystalline state of lipids in a biomembrane. Proc. Natl. Acad. Sci. U.S.A. 1969 63 104-109. Reinert JC, Steim JM. Calorimetric detection of a membrane lipid phase transition in living cells. Science 1970 168 1580-1582. Melchior DL, Morowitz HJ, Sturtevant JM, Tsong TY. Characterization of the plasma membrane of Mycolplasma laidlawii. Vni. Phase transitions of membrane lipids. Biochim. Biophys. Acta 1970 219 114-122. 

The stmeturai units of liposomes are ainphiphile molecules, mainly phospholipids. Alec Bangham and co-workers observed self-closed lipid structures after they had been dissolved in water [35]. This first observation took place after egg yolk lecithin had been dispersed in water. According to D.D.Lasic and D. Papahadjopoulos, liposomes are self-assembling colloidal particles in which a lipid bilayer encapsulates a fraction of the surrounding medium [36]. 

Thin, liquid films as such are not only systems of interest, they also occur frequently in nature and in laboratory practice, for example, in foams. In emulsions, to name another case, thin water films between oil (instead of air ) phases are present in a concentrated emulsion of small oil droplets dispersed in water. The properties of the thin water film determine the interaction forces between the oil droplets and will determine, for example, whether the emulsion in stable. Also the reverse case, oil films in water, occurs. Extremely thin ( 5 nm) oil-in-water films are prototypes of lipid bilayers occurring in biological membranes. Thin liquid films on a solid... 

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