Hydrocarbon core

In another experiment tritiated adamantane diazirine fixed to the hydrocarbon core of a membrane gave rise to carbene insertion into the catalytic subunit of ATP-ase. After protolytic degradation adjacent areas of the original structure became evident (80JBC(255)860). 

The aliphatic tails form a hydrocarbon region with properties not too different from the hydrocarbon core of bilayers. The clusters have an interfacial zone... 

Unilamellar vesicles, PC Hydrocarbon core Fluorescence polarization (AS) 2 381... 

Mason, R. P., Estermyer, J. D., Kelly, J. F., and Mason, P. E. (1996). Alzheimer s disease amyloid // peptide 25-35 is localized in the membrane hydrocarbon core X-ray diffraction analysis. Biochem. Biophys. Res. Commun. 222, 78-82. 

Another approach to characterise the site of absorption in the lipid bilayer is to analyse the dielectric properties of this site. The permittivity e in the hydro-phobic core of the membrane is very small (s of approximately 2 to 3), and rises outwards until it reaches the value in the aqueous phase (e = 78) [6], s is about 30 at the location of the carboxy groups of the fatty acid chain [150], Other measurements averaged to e of 30 to 33 at the interface between polar head groups and the hydrocarbon core [176]. The e of the absorption site of... 

Performance of surfactants is closely related to surface activity and to micelle formation. Both these are due to amphiphilic nature of the surfactant molecule. The molecule contains a nonpolar hydrophobic part, usually, a hydrocarbon chain, and a polar hydrophilic group, which may be nonionic, zwitterionic, or ionic. When the hydrophobic group is a long straight chain of hydrocarbon, the micelle has a small liquid like hydrocarbon core (1,2). The primary driving... 

For spherical micelles, one of the more commonly accepted models for the micellar structure is that proposed by Gruen (Fig. 1). This model features a rather sharp interface between a dry hydrophobic hydrocarbon core and a region filled with surfactant headgroups, part of the counterions (for ionic surfactants), backfolding surfactant tails, and water, namely, the Stern region. In the remainder of this chapter, intramicelle volumes with specific features such as the Stern region and the hydro-phobic core will be referred to as zones. ... 

Yellow OB dye is an essentially water—insoluble organic and is convenient to use as a solubi1izate. The studies discussed here use Yellow OB -for solubilization studies in mixed sur-factant systems. Yellow OB can solubilize in the hydrocarbon core o-f the micelle as well as among the polyoxyethylene chains in a micelle with non ionic sur-factant. 

Although McBain suggested over 80 years ago that soap molecules form micellar structures of lamellar and spherical shape (McBain 1913), most of the subsequent work focused on spherical micelles. The earliest concrete model for spherical micelles is attributed to Hartley (1936), whose picture of a liquidlike hydrocarbon core surrounded by a hydrophilic surface layer formed by the head groups, has been essentially verified by modern techniques, and the Hartley model still dominates our thinking. We present an overview of the structure of the micelle first and then go on to examine the details a little bit more closely. 

In ionic micelles the hydrocarbon core is surrounded by a shell that more nearly resembles a concentrated electrolyte solution. This consists of ionic surfactant heads and bound counterions in a region called the Stern layer (see Chapter 11, Section 11.8). Water is also present in this region, both as free molecules and as water of hydration. 

In typical nonionic micelles the shell surrounding the hydrocarbon core also resembles a concentrated aqueous solution, this time a solution of polyoxyethylene. The ether oxygens in these chains are heavily hydrated, and the chains are jumbled into coils to the extent that their length and hydration allow. 

Next let us return to the hydrocarbon core of the micelle. Of particular interest are the geometrical constraints imposed on the micellar structure by the length of the hydrocarbon tail and the location of the yet unexplained water of hydration. As a first approximation, the core of the micelle can be viewed as a drop of liquid hydrocarbon, the maximum radius of which equals the length of the fully extended hydrocarbon tail. To get an idea of the size of this region, let us consider a numerical example. 

Biological membranes are constructed of lipid bilayers 3 nm (30 A) thick, with proteins protruding on each side. The hydrocarbon core of the membrane, made up of the —CH2— and —CH3 of the fatty acyl groups, is about as nonpolar as decane, and liposomes formed in the laboratory from pure lipids are essentially impermeable to polar solutes, as are biological membranes (although the latter, as we shall see, are permeable to solutes for which they have specific transporters). 

Interpretation of the Calorimetric Results. There is little doubt that the transition observed in M. laidlawii membranes arises from the lipids since it occurs at the same temperature in both intact membranes and in water dispersions of membrane lipids. It is reasonable to conclude that in both membranes and membrane lipids the lipid hydrocarbon chains have the same conformation. The lamellar bilayer is well established for phospholipids in water (I, 20, 29) at the concentration of lipids used in these experiments. In the phase change the hydrocarbon core of the bilayer undergoes melting from a crystalline to a liquid-like state. Such a transition, like the melting of bulk paraffins, involves association between hydrocarbon chains and would vanish or be greatly perturbed if the lipids were apolarly bound to protein. We can reasonably conclude that most of the lipids in M. laidlawii membranes are not apolarly bound to protein. 

All four systems illustrated in Fig. 4 exhibit properties differing from those of cell membranes. Methods a-c have no influence on the head groups and preserve physical properties, such as charge, charge density, etc. The fluidity of the hydrocarbon core, however, is drastically decreased by the polymerization process. In case d, fluidity is not affected, but there is no free choice of head groups. In comparison to biomembranes, all polymerized model membrane systems will show an increase in viscosity and a decrease in lateral mobility of the molecules. 

Whether polymerized model membrane systems are too rigid for showing a phase transition strongly depends on the type of polymerizable lipid used for the preparation of the membrane. Especially in the case of diacetylenic lipids a loss of phase transi tion can be expected due to the formation of the rigid fully conjugated polymer backbone 20) (Scheme 1). This assumption is confirmed by DSC measurements with the diacetylenic sulfolipid (22). Figure 25 illustrates the phase transition behavior of (22) as a function of the polymerization time. The pure monomeric liposomes show a transition temperature of 53 °C, where they turn from the gel state into the liquid-crystalline state 24). During polymerization a decrease in phase transition enthalpy indicates a restricted mobility of the polymerized hydrocarbon core. Moreover, the phase transition eventually disappears after complete polymerization of the monomer 24). 

Example 12.1. The CMC of C12E7 is 0.083 mM at room temperature. By SANS and dynamic light scattering the mean hydrocarbon core radius was found to be 1.70 nm at a surfactant concentration of 2 mM [532], The mean aggregation number is 64. If we divide the total surface area of the core by the number of surfactants, we get the area per molecule at the core radius. It is 47r (1.7 nm)2 /64 = 0.57 nm2. The cross-sectional area of polyethylene oxide is below 0.2 nm2. So, more than half the core area is exposed to aqueous or at least to a polar medium. 

The second important characteristic of the micellar solution that relates to solubilization is the micelle size. Poor aqueous soluble compounds are solubilized either within the hydrocarbon core of the micelle or, very commonly, within the head group layer at the surface of the micelle or in the palisade portion of the micelle. Predictions of the micelle size have relied on the use of empirical relationships employed within a thermodynamic model, for instance the law of mass action where micellization is in equilibrium with the associated and unassociated (monomer) surfactant molecules (Attwood and Florence, 1983). 

Aliphatic hydrocarbon solutes are primarily solubilized within the hydrocarbon core region of the surfactant micelles. Solubilization isotherms (activity coefLcient versus mole fraction, X) for these hydrophobic solutes exhibit curves that decrease from relatively large values at inLnite dilution to lower values as X increases toward unity (Figure 12.6). The aromatic hydrocarbons are intermediate in behavior between highly polar solutes, which are anchored in the micelle surface region, and aliphatic hydrocarbons, which preferentially solubilize in the hydrocarbon core region (Kondo et al., 1993). 

The life-time of a monomer in a micelle may be of the order of a microsecond and in view of the accepted dynamic state of a micelle this implies that less extensive motions occur on a shorter time-scale. Aniansson229 has recently examined the dynamic protrusion of methylene groups from the hydrocarbon core of a micelle... 

Cholesterol can modify both the hydrophobic attraction between lipid hydrocarbon chains and electrostatic interactions between lipid polar groups. The influence it has on the location of 9HP reflects this dual effect At low temperature, the "spacer" effect of cholesterol allows the ketone to gain access directly to the lipid-water interface. At high temperatures, a more disordered hydrocarbon core favors the solubilization of the guest molecule. 

A theoretical model has been developed by Gruen (26) which helps to harmonize the apparent experimental discontinuities. The model assumes the existence of a hydrocarbon core completely devoid of headgroups, counterions, and most importantly, water. The methylene chains are men allowed to exist in a number of conformations (trans, gauche+, and gauche states for each C-C bond), and are, on average, constrained to pack at liquid hydrocarbon density. Each chain... 

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