Micelle, lipid

LANGMUIR TROUGH AND BALANCE LIPOPROTEIN LIPASE MICELLES LIPID ACTIVATION Lipid mobilization kinetics,... 

With time and improved synthetic protocols, larger templates (fuUerenes, dendrimers, nanoparticles, colloids, micelles, lipid bilayers, self-assembled block copolymers, oligonucleotides, DNA and proteins) have been imprinted [14] and the choice of matrices has expanded to liquid crystal polysiloxanes, carbon networks, zeolites, layered aluminophosphates and colloidal crystals, though organic polymer networks remain the dominant imprint casting medium [14]. 

Interfaces between aqueous phase and the volumes confined by amphiphilic molecules [288]. In vitro, these refer to lipid vesicles and micelles, lipid lamellae, cubic and hexagonal phases, Langmuir-Blodgett (LB) films, which are important in colloid science and in extraction technology. In vivo, these are the surfaces of biological membranes. 

Simplistically stated, the hydrophobic effect may be defined as the tendency of water to reject any contact with substances of a nonpolar or hydrocarbon nature. The existence of this effect was first recognized in the study of the extremely low solubility of hydrocarbons in water. The principles involved were later successfully applied to the elucidation of the native conformation of protein molecules by Kauz-mann The application of these ideas to the study of membrane structures has been advanced by Singer. Recently, Tanford published an entire book on the hydrophobic effect, including the influence of this interaction on the formation of micelles, lipid bilayers, membranes and other ordered structures. Aside from Singer s and Tanford s" statements on the decisive role of the hydrophobic effect on lyotropics, the lyotropic liquid-crystal literature seems peculiarly unaware of this phenomenon. Winsor s extensive review with its systematic analysis (R-theory) of the many lyotropic phases does not take the hydrophobic effect into account. More recent reviews of lyotropic liquid crystals do not mention the phenomenon. We hope that the present discussion will help to advance the realization of the importance of the hydrophobic effect to lyotropics. The material of the following sections is taken chiefly from Ref. [3] with some assistance from Refs. [2] and [4]. 

Recent developments m calorimetry have focused primarily on the calorimetry of biochemical systems, with the study of complex systems such as micelles, protems and lipids using microcalorimeters. Over the last 20 years microcalorimeters of various types including flow, titration, dilution, perfiision calorimeters and calorimeters used for the study of the dissolution of gases, liquids and solids have been developed. A more recent development is pressure-controlled scamiing calorimetry [26] where the thennal effects resulting from varying the pressure on a system either step-wise or continuously is studied. 

Chain models capture the basic elements of the amphiphilic behaviour by retaining details of the molecular architecture. Ben-Shaul et aJ [ ] and others [ ] explored the organization of tlie hydrophobic portion in lipid micelles and bilayers by retaining the confonuational statistics of the hydrocarbon tail withm the RIS (rotational isomeric state) model [4, 5] while representing the hydrophilic/liydrophobic mterface merely by an... 

The structure of cholic acid helps us understand how bile salts such as sodium tauro cholate promote the transport of lipids through a water rich environment The bot tom face of the molecule bears all of the polar groups and the top face is exclusively hydrocarbon like Bile salts emulsify fats by forming micelles m which the fats are on the inside and the bile salts are on the outside The hydrophobic face of the bile salt associates with the fat that is inside the micelle the hydrophilic face is m contact with water on the outside... 

FIG. 1 Self-assembled structures in amphiphilic systems micellar structures (a) and (b) exist in aqueous solution as well as in ternary oil/water/amphiphile mixtures. In the latter case, they are swollen by the oil on the hydrophobic (tail) side. Monolayers (c) separate water from oil domains in ternary systems. Lipids in water tend to form bilayers (d) rather than micelles, since their hydrophobic block (two chains) is so compact and bulky, compared to the head group, that they cannot easily pack into a sphere [4]. At small concentrations, bilayers often close up to form vesicles (e). Some surfactants also form cyhndrical (wormlike) micelles (not shown). 

Further addition of fatty acid eventually results in the formation of micelles. Micelles formed from an amphipathic lipid in water position the hydrophobic tails in the center of the lipid aggregation with the polar head groups facing outward. Amphipathic molecules that form micelles are characterized by a unique critical micelle concentration, or CMC. Below the CMC, individual lipid molecules predominate. Nearly all the lipid added above the CMC, however, spontaneously forms micelles. Micelles are the preferred form of aggregation in water for detergents and soaps. Some typical CMC values are listed in Figure 9.3. 

Phospholipids are found widely in both plant and animal tissues and make up approximately 50% to 60% of cell membranes. Because they are like soaps in having a long, nonpolar hydrocarbon tail bound to a polar ionic head, phospholipids in the cell membrane organize into a lipid bilayer about 5.0 nm (50 A) thick. As shown in Figure 27.2, the nonpolar tails aggregate in the center of the bilayer in much the same way that soap tails aggregate in the center of a micelle. This bilayer serves as an effective barrier to the passage of water, ions, and other components into and out of cells. 

Generally speaking we consider that most micro-organisms live and grow in aqueous environments, and that the cytoplasm within cells in which enzymes function is also aqueous. On die other hand, most lipids are only sparingly soluble in aqueous media. Cholesterol, for example, has a solubility of less than 2 mg l 1 (equivalent to a concentration of less than 5 pmol l 1). Even at much lower concentrations (25-40 nmol l 1) it tends to aggregate into micelles. There is, therefore, a general problem of how to supply lipid substrates at sufficient concentration to produce reaction kinetics that are appropriate for industrial purposes. 

A large variety of drug delivery systems are described in the literature, such as liposomes (Torchilin, 2006), micro and nanoparticles (Kumar, 2000), polymeric micelles (Torchilin, 2006), nanocrystals (Muller et al., 2011), among others. Microparticles are usually classified as microcapsules or microspheres (Figure 8). Microspheres are matrix spherical microparticles where the drug may be located on the surface or dissolved into the matrix. Microcapsules are characterized as spherical particles more than Ipm containing a core substance (aqueous or lipid), normally lipid, and are used to deliver poor soluble molecules... 

In some polysaccharides, the reducing terminal is linked, through a phosphoric diester linkage, to O-1 of a 2,3-di-6 -acylglycerol. This structural feature has been demonstrated for some capsular polysaccharides from E. coli and Neisseria species, - but is probably more common than that. Non-covalent linkage between the lipid part and the cell membrane may explain why extracellular polysaccharides often occur as capsules, and the high (apparent) molecular weight observed for these polysaccharides may be due to micelle formation in aqueous solution. 

Figure 14-22. Formation of lipid membranes, micelles, emulsions, and liposomes from am-phipathic lipids, eg, phospholipids.

Figure 41-4. Diagrammatic cross-section of a micelle. The polar head groups are bathed in water, whereas the hydrophobic hydrocarbon tails are surrounded by other hydrocarbons and thereby protected from water. Micelles are relatively small (compared with lipid bilayers) spherical structures.

The major lipids in the diet are triacylglycerols and, to a lesser extent, phospholipids. These are hydrophobic molecules and must be hydrolyzed and emulsified to very small droplets (micelles) before they can be absorbed. The fat-soluble vitamins— A, D, E, and K— and a variety of other lipids (including cholesterol) are absorbed dissolved in the lipid micelles. Absorption of the fat-soluble vitamins is impaired on a very low fat diet. 

In the bilayer or upon interaction with detergent micelles, a structural reorganization of pardaxin aggregates takes place, in which the polar side chains interact with themselves and the hydrophobic residues are externally oriented in the pardaxin aggregate, therefore allowing interactions with the lipid backbone hydrocarbons. 

With few exceptions, small particles of vegetable foods are generally stripped of their more accessible nutrients during digestion in the GI tract. In this way starch, protein, fat and water-soluble small components (sugars, minerals) are usually well absorbed. This is not always the case, however, for larger food particles or for molecules that cannot diffuse out of the celF tissue. Neither is it the case for the lipid-soluble components. These need to be dissolved in lipid before they can be physically removed from the cell to the absorptive surface, since the cell wall is unlikely to be permeable to lipid emulsions or micelles, and the presence of lipases will strip away the solvating lipid. 

Irrespective of the physical form of the carotenoid in the plant tissue it needs to be dissolved directly into the bulk lipid phase (emulsion) and then into the mixed micelles formed from the emulsion droplets by the action of lipases and bile. Alternatively it can dissolve directly into the mixed micelles. The micelles then diffuse through the unstirred water layer covering the brush border of the enterocytes and dissociate, and the components are then absorbed. Although lipid absorption at this point is essentially complete, bile salts and sterols (cholesterol) may not be fully absorbed and are not wholly recovered more distally, some being lost into the large intestine. It is not known whether carotenoids incorporated into mixed micelles are fully or only partially absorbed. 

Historically, the absorption of lipid-soluble nutrients has been considered to be carrier-independent, with solutes diffusing into enterocytes down concentration gradients. This is true for some lipid-soluble components of plants (e.g. the hydroxytyrosol in olive oil Manna et al., 2000). However, transporters have been reported for several lipid-soluble nutrients. For example, absorption of cholesterol is partly dependent on a carrier-mediated process that is inhibited by tea polyphenols (Dawson and Rudel, 1999) and other phytochemicals (Park et al., 2002). A portion of the decreased absorption caused by tea polyphenols may be due to precipitation of the cholesterol associated with micelles (Ikeda et al., 1992). Alternatively, plant stanols and other phytochemicals may compete with cholesterol for transporter sites (Plat and Mensink, 2002). It is likely that transporters for other lipid-soluble nutrients are also affected by phytochemicals, although this has not been adequately investigated. 

In contrast, the carotenes such as p-carotene and lycopene may position themselves parallel to the membrane surfaces to remain in a more lipophilic environment in the inner cores of the bilayer membranes. To move through an aqueous environment, carotenoids can be incorporated into lipid particles such as mixed micelles in the gut lumen or lipoproteins in the blood circulation and they can also form complexes with proteins with unspecific or specific bindings. 

The bioaccessibility of a compound can be defined as the result of complex processes occurring in the lumen of the gut to transfer the compound from a non-digested form into a potentially absorbable form. For carotenoids, these different processes include the disruption of the food matrix, the disruption of molecular linkage, the uptake in lipid droplets, and finally the formation and uptake in micelles. Thus, the bioaccessibility of carotenoids and other lipophilic pigments from foods can be characterized by the efficiency of their incorporation into the micellar fraction in the gut. The fate of a compound from its presence in food to its absorbable form is affected by many factors that must be known in order to understand and predict the efficiency of a compound s bioaccessibility and bioavailability from a certain meal. ... 

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