Lamellar bilayers

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. 

Fig. 6.36 Phase diagram calculated using SCFT for a blend of a symmetric diblock with a homopolymer with fl = 1 (see Fig. 6.32 for a blend with a diblock with / = 0.45) as a function of the copolymer volume fraction

Fig. 6.40 A phase diagram calculated using SCFT for a mixture containing equal amounts of two homopolymers and a symmetric diblock, all with equal chain length (Janert and Schick 1997a). A-rich and B-rich swollen lamellar bilayer phases are denoted LA and LH respectively whilst the corresponding disordered phases are denoted A and B. The con-solute line of asymmetric bilayer phases LA and Lu, shown dotted, is schematic.The dashed line is the unbinding line. The arrows indicate the locations of the unbinding transition X jN and multicritical Lifshitz point, cMiV " 6.0.

In comparison to the skin, the buccal mucosa offers higher permeability and faster onset of drug delivery, whereas the key features which help it score over the other mucosal route, the nasal delivery system, include robustness, ease of use, and avoidance of drug metabolism and degradation. The buccal mucosa and the skin have similar structures with multiple cell layers at different degrees of maturation. The buccal mucosa, however, lacks the intercellular lamellar bilayer structure found in the stratum corneum, and hence is more permeable. An additional factor contributing to the enhanced permeability is the rich blood supply in the... 

Figure 2 Organization of the lamellar bilayer phases of DPPC in the fiuid (Lx), ripple (Pp), gel (Lp/) and pseudo-crystalline (Lc) states. A top view of the packing of the hydrocarbon chains is shown in the last column. (From Reference 12.)...

We shall deal with ionic and zwitterionic amphiphiles. These can take on a confusing variety of different shapes and sizes some aggregate into small spherical or globular micelles, others appear to form long cylindrical micelles, while others coalesce spontaneously into vesicular or lamellar bilayers. 

As regards the outer and inner surface areas of the amphiphiles, the results indicate that these should be about the same, and close (within 1 %) to the area in lamellar bilayers, as observed. - ... 

The stratum corneum is the outermost layer of the epidermis and has a thickness of 10-15 pm. It is the principal barrier for the transport of most solutes (except for very lipophilic compounds) across the skin. The stratum corneum is a continuous heterogeneous structure that consists of approximately 10-25 layers of closely packed dead keratinized cells (corneocytes) cemented together by intercellular lipids. The intercellular lipids in the stratum corneum are in the form of multiple lamellar bilayers composed mainly of ceramides, cholesterol, and fatty acids. Proteins in the stratum corneum are largely concentrated within the corneocytes as keratin fibrils. The transport of lipophilic compounds across the stratum corneum is related to the intercellular lipids (lipoidal or intercellular pathways). On the other hand, it is believed that the transport of polar and ionic compounds is related to pathways with aqueous properties (the polar or pore pathways) when the stratum corneum is under a hydrated state. ... 

Equation 2 may now be extrapolated to fully flat (1/R = 0) to estimate the bending energy locked into the monolayers of a lamellar bilayer. Of course, this is just a crude estimate because there is no expectation that eq 2 is valid over such a large range of extrapolation. The result is an energy of about kT per lipid at room temperature. This value suggests that a protein... 

It is straightforward to imagine protein conformational changes that couple to the stress of a frustrated monolayer elastic curvature. The experiments described in the preceding sections demonstrate that there is an energetically significant elastic stress locked into the leaflets of a lamellar bilayer near to a lamellar-nonlamellar phase transition. Experimentally, bilayers near to a lamellar-nonlamellar transition means that relatively... 

Liposomes with compositions that imitate the skin s lipid content (ceramides, cholesterol, fatty acids, cholesterol sulfate) have also been prepared [37] and their pharmacological response on a skin barrier disruption model was assessed. Apart from their action as lipidic carriers for the repair of skin barrier, these liposomes are expected to penetrate easily the skin barrier due to their biocompatibility with the stratum corneuin. Many attempts have being made to incorporate ceramides, the major component of stratum corneum. These lipids have a relatively large stereochemical shape, and in the presence of water they tend to form multi lamellar bilayers [50-53]. 

Of course, the two layers of the bilayer do not have areas per molecule which are exactly equal, nor are their curvatures equal and opposite the areas are self-adjusting in equilibrium. This can result in a vesicle that is more stable than a flat, lamellar bilayer since it may be that the favorably curved layer e,g., the outer layer) has a smaller value of the area per molecule and hence more molecules in it than the inner layer. However, such corrections are only important for systems with large spontaneous curvatures and the resulting vesicles would have sizes comparable to molecular lengths. It therefore may turn out that micelles may be more stable than either vesicles or lamellae. In what follows, we consider the spontaneous formation of large vesicles whose sizes are much greater than a surfactant size and hence neglect these effects. 

Most glycerolipids and sphingolipids in aqueous dispersions form closed vesicles, limited by lipids in the lamellar (bilayer) disposition. Depending on the lipid structure, different thermotropic transitions may be observed, of which the following are the most common. 

Stable oil-in-water emulsions can also be obtained by dispersing polar lipids such as phospholipids into triglycerides and then emulsifying the oil in water. The presence of charged phosphatidylcholine components of phosphohpids improves the stabilization of the emulsions. In most of these systems, the polar phospholipids form a separate phase at the interface where they form lamellar bilayers and a monolayer separated by triglyceride oil, between the outer water phase and the iimer triglyceride oil phase (Figure 10.3). 

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