Hydrocarbon chains packing

Simon, S.A. and T.J. McIntosh. 1984. Interdigitated hydrocarbon chain packing causes the biphasic transition behavior in lipid/alcohol suspensions. Biochim Biophys Acta 773 169-172. 

One type of lipid that is dominant in biological interfaces is lecithin, and lecithin-water systems have therefore been examined extensively by different physical techniques. Small s binary system (3) for egg lecithin-water is presented in Figure 2. The lamellar phase is formed over a large composition range, and, at very low water content, the phase behavior is quite complex. Their structures as proposed by Luzzati and co-workers (4) are either lamellar with different hydrocarbon chain packings or based on rods both types are discussed below. 

Crystals of most polar lipids can swell in the presence of water. The corresponding phases, gel-phases, with lamellarly packed lipid, and water layers, are sometimes thermodynamically stable (Larsson, 1994, p. 41). Also, the hydrocarbon chain packing of gel-phases usually show some axial rotational disorder. The alkyl chain cross-sectional area is close to 20 A2 in a plane perpendicular... 

Polymorphism refers to the situation in which materials of the same chemical composition possess different sub-cell packings in the solid state (Small, 1986). Figure 7.3 compares the level of structure for the unit cell and sub-cell in crystalline fats. Polymorphism arises from both variations in the tilt of TAG molecules in a bilayer and from variations in the hydrocarbon chain packing (Larsson, 1994). 

The La-//ii transition may be considered a result of competition between the spontaneous tendency of the lipid layers to bend and the resulting hydrocarbon chain packing strain thus, membranes exist in a state of fmstrated curvature stress (17). Respectively, the La-Hn transition is believed to be driven by the relaxation of the curvature of the lipid monolayers toward their spontaneous curvature. Conversely to the Lp-Lc transition, the La-H II transition temperature decreases with the hydrocarbon chain length increase (Fig. 3a). At sufficiently long chains. 

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). 

Polymorphism is the ability of a molecule to take more than one crystalline form depending on its arrangement within the crystal lattice. In lipids, differences in hydrocarbon chain packing and variations in the angle of tilt of the hydrocarbon chain packing differentiate polymorphic forms. The crystallization behavior of TAG, including crystallization rate, crystal size, morphology, and total crystallinity, are affected by polymorphism. The molecular structure of the TAG and several external factors like temperature, pressure, rate of crystalhzation, impurities, and shear rate influence polymorphism (5). 

Cholesteryl para-substituted benzoates give mesophases with transition temperatures and thermodynamic parameters which depend upon the para-substituent. " Crystal and mesophase structures of cholesteryl myristate appear to show some similarities in molecular packing. " X-Ray studies show that cholesteryl 17-bromoheptadecanoate crystals contain alternating regions with cholesterol and hydrocarbon-chain packing. ""... 

Holte LL, Separovic F, Gawrisch K. Nuclear magnetic resonance investigation of hydrocarbon chain packing in bilayers of polyunsatiuated phospholipids. Lipids 1996 31 S199-S203. 

In aqueous solution, they aggregate with their hydrocarbon chains packed inside, surrounded by the polar head groups. Cellular membranes are constructed this way, with a pair of layers of side-byside lipids facing each other. 

The TDM of the higher frequency component (1472-1473 cm" ) of the 5scisCH2 mode of hydrocarbon chains packed in the orthorhombic subcell is directed along the a-axis of the subcell, while the lower frequency component... 

All monolayer phases with crystalline chains show linear U-A curves and there are always well-defined plateau values in the U-A isotherms at phase transitions involving tilted chains. As discussed in connection with hydrocarbon chain-packing alternatives (Section 8.1), only certain discrete tilt values are possible. For example in the case of the orthorhombic chain packing, one of the most common chain tilt alternatives occurring in simple lipids is about 66°. The corresponding hydrocarbon chain structure, described by the indices of the end group plane is Oi(lOl). Monolayer U-A isotherms of ethyl stearate and of docosyl acetate show an L2-phase, which have exactly this chain tilt, as calculated from the U-A isotherms. Furthermore these two esters crystallize in jS -form with this hydrocarbon chain structure (Oi (101)). 

Hydrocarbon chain packing in mixed lameiiar and hexagonai phosphoiipid/deteigent systems... 

The hydrocarbon chain-packing modes are usually described by means of a subcell, which gives the symmetry relations between equivalent positions in one chain and its neighbors [9,10]. Four types of subcells have been identified (1) The planes contain parallel hydrocarbon chains, (2) the chains are perpendicular to each other, (3) the chain axes are crossed, and (4) the chains are packed in a hexagonal lattice. By lateral repetition of the subcell, the entire structure of the chain region is obtained. Structural analysis of normal paraffins with more than nine carbon atoms in the chain revealed mainly four possible distinct crystal structures hexagonal, triclinic, monoclinic, and orthorhombic [11]. 

The fluidity of a membrane is controlled by the fatty acid components of the phosphoglycerides. Saturated fatty acids decrease membrane fluidity because their hydrocarbon chains pack closely together. Unsaturated fatty acids increase fluidity because they pack less closely together. Cholesterol also decreases fluidity (see page 121). Only animal membranes contain cholesterol, so they are more rigid than plant membranes. 

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