Phosphatidylcholine packing

To produce highly purified phosphatidylcholine there are two industrial processes batch and continuous. In the batch process for producing phosphatidylcholine fractions with 70—96% PC (Pig. 4) (14,15) deoiled lecithin is blended at 30°C with 30 wt % ethanol, 90 vol %, eventually in the presence of a solubiHzer (for example, mono-, di-, or triglycerides). The ethanol-insoluble fraction is separated and dried. The ethanol-soluble fraction is mixed with aluminum oxide 1 1 and stirred for approximately one hour. After separation, the phosphatidylcholine fraction is concentrated, dried, and packed. 

Hauser, H., Pascher, I., Pearson, R.H. Sundell, S. (1981). Preferred conformation and molecular packing of phosphatidyl ethanolamine and phosphatidylcholine. Biochimica Biophysica Acta, 650, 21-51. 

Inclusion of other molecules of irregular shape within membranes also lowers Tm. However, a molecule of cholesterol can pack into a bilayer with a cross-sectional area of 0.39 nm2, just equal to that of two hydrocarbon chains.49 It tends to harden membranes above Tm but increases mobility of hydrocarbon chains below Tm.97 -100 A complex of cholesterol and phosphatidylcholine may form a separate phase within the membrane.101102 The ether-linked plasmalogens may account for over 30% of the phosphoglycerides of the white matter of the brain and of heart and ether linked phospholipids are the major lipids of many anaerobic bacteria.103 Their Tm values are a few degrees higher than those of the corresponding acyl phospholipids.104... 

Lipids also show asymmetrical distributions between the inner and outer leaflets of the bilayer. In the erythrocyte plasma membrane, most of the phosphatidylethanolamine and phosphatidylserine are in the inner leaflet, whereas the phosphatidylcholine and sphingomyelin are located mainly in the outer leaflet. A similar asymmetry is seen even in artificial liposomes prepared from mixtures of phospholipids. In liposomes containing a mixture of phosphatidylethanolamine and phosphatidylcholine, phosphatidylethanolamine localizes preferentially in the inner leaflet, and phosphatidylcholine in the outer. For the most part, the asymmetrical distributions of lipids probably reflect packing forces determined by the different curvatures of the inner and outer surfaces of the bilayer. By contrast, the disposition of membrane proteins reflects the mechanism of protein synthesis and insertion into the membrane. We return to this topic in chapter 29. 

F. Muller-Landau and D.A. Cadenhead, Molecular packing in steroid-lecithin monolayers, Part III, Mixed films of 3-doxyl-cholestane and 3-doxyl-17-hydroxyl-androstane with dipal-mitoyl-phosphatidylcholine, Chem. Phys. Lipids 25 (1979) 329-343. 

The influence of lipid phase (gel or liquid crystalline), cholesterol content, lipid composition (egg phosphatidylcholine or DPPC), and structure of benzodiazepines determine their localization in the membrane. The strength of benzodiazepine-membrane interaction increases with a decrease in molecular order, molecular packing, and hydration,. The authors point to the pharmacological relevance of theses results because the extent of partitioning of these drags into biomembranes would be coupled to local oscillation of membrane dynamics which may be induced by physiological events. ... 

Since the solidity or fluidity of the bilayer membrane is likely to depend on the alkyl chain interactions and consequently their length, an understanding of the relationship between chain order and chain length for tightly packed monolayers of phospholipids are important. As an example of how VSFS can be employed to study phospholipids at a liquid surface, a series of saturated symmetric chain phosphatidylcholines (PCs) were examined at the air/water and CCfr/water interfaces [49]. At the air/D20 interface, chain order within the monolayer was found to increase as the length of the chains increased (Figure 2.9a) under conditions of constant phospholipid head group area. 

At temperatures below the main transition, a basic equilibrium stracture is the subgel (crystalline) Lc phase. Its formation usually requires prolonged low-temperature incubation. In addition to the Lc phase, many intermediate stable, metastable, and transient lamellar gel structures are adopted by different lipid classes—with perpendicular or tilted chains with respect to the bilayer plane, with fully interdigitated, partially interdigitated, or noninterdigitated chains, rippled bilayers with various ripple periods, and so forth. (Fig. 1). Several polymorphic phase transitions between these structures have been reported. Well-known examples of polymorphic transitions are the subtransition (Lc- L ) and the pretransition (Lp/- Fp/) in phosphatidylcholines (33). Recently, a polymorphic transition that included rapid, reversible transformation of the usual gel phase into a metastable, more ordered gel phase with orthorhombic hydrocarbon chain-packing (so-called Y-transition) was reported to represent a common pathway of the bilayer transformation into a subgel (crystalline) Lc phase (62). 

Hendrich et al. [155] investigated the mechanism of incorporation of phe-nothiazines in the membrane bilayer lipids. They studied the influence of a particular phenothiazine derivative, TFZ, on the thermal properties of dimyristoyl phosphatidylcholine and dimyristoyl phosphatidylethanolamine by microcalorimetry. The main phase transition of both lipids was affected by this drug, depending on its concentration. The results suggest that TFZ was probably incorporated into both dimyristoyl phosphatidylcholine and dimyristoyl phosphatidylethanolamine bilayers. The phase separation was presumably induced by the different modes of the drug-bilayer interactions of protonated and unprotonated forms of TFZ. Only phosphatidylcholine, which possesses polar heads less densely packed in bilayers than phosphatidylethanolamine ones, was able to distinguish between the different protonated forms of TFZ. 

Immobilized Artificial Membrane (LAM) packings for HPLC provide a different environment from that of the hydrocarbon-based ODS columns [17,30—32]. In this model, IAMs are purified phospholipids that are covalently bonded to the silicon support. At this time, only IAM columns prepared from phosphatidylcholine are commercially available (Regis Technologies, Inc., Morton Grove, IL). The hypothesis is that the bonded phospholipid layer is akin to the cellular monolayer that represents a barrier to transport. Interaction of the solute with the phospholipid results in a capacity factor ( J that is proportional to the membrane partition coefficient [PCm in Eq. (1)]. In this sense, the IAM approach does not attempt to correlate with literature values of oil water partition coefficients, but seeks to establish a unique membrane interaction parameter. 

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