Tilted chains

Figure 1 Examples of various lipid phases I. Lamellar phases (A) subgel, Lc (B) gel, untilted chains, Lp (C) gel, tilted chains, L / (D) rippled gel, Pp ...

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). 

The alternative, if there is no backfolding, as I said at Cooperstown in 1958, is to have an interface oblique to the chain axes. The required obliquity is at least arc sec (3/0.9) = 72.5°. Lamellae with tilted chains are familiar, but never with a tilt nearly as large as this. 

A new crystal structure with tilted chains has recently been found in pentamer of nylon-6 and nylon-8, termed A-phase.108 Molecules are in the all-trans conformation and hydrogen bond to antiparallel neighbors to form the usual nylon 6 hydrogen-bonded sheets. However, in this structure, the sheets stack with progressive c-axis shear, and consequently, the molecular layer thickness is noticeably reduced. 

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)). 

The packing rules of crystalline chains described in Section 8.1, however, with only certain discrete values of the tilt of the chains, cannot be fulfilled in a rippled structure where the tilt of the chains must vary continuously along the ripple. An alternative structure has therefore been proposed which is consistent with experiences on chain close-packing geometry (Larsson, 1977). This structure consists of folded bilayers with alternation of units with tilted chains and units with perpendicular chains. 

An extensive analysis of the polymorphism in phospholipid monolayers has been reported by Albrecht et al. (1978) who established a phase diagram of dipal-mitoyl-lecithin monolayers involving four phases. Two of these have the liquid-type of disordered chains (liquid-expanded films). It was proposed that one has disordered chains oriented perpendicular to the water surface and the other has disordered chains in a tilted arrangement. Furthermore two crystalline phases were described, one with tilted chains and the other with perpendicular chains. 

The formation of tilted chain lamellae at higher temperature is qualitatively consistent with the experiments of el Maaty and Bassett. They reported that the morphology of polyethylene crystals changed when the crystallization occurred above a certain temperature, 400 K in their case [62]. At high temperature, flat lamellae with (201) interfaces (i.e. 34 degree tilted chains) formed directly from the melt, whereas at low temperatures, the lamellae consisted initially of untilted chains with (001) interfaces, which only later relaxed to form tilted-chain and curved lamellae. The (201) interface is thermodynamically... 

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