Bilayer effect, chiral

The chiral-bilayer-effect hypothesis has been evoked for the rationalization of the helical fibers formed from enantiomeric or diastereomeric surfactants (Fig. 54) [373], Different packing of the chiral surfactants in the crystals (head-to-tail) and in bilayer or micellar aggregates (tail-to-tail) is the basis for this postulate. Crystallization from aggregates requires an energetically costly, 180°... 

Fig. 54a-b. The chiral bilayer effect a chiral micellar cylinders rearrange slowly to enantiopolar crystals b the hydrophobic bilayer of achiral micellar cylinders is retained in the crystal. Crystallization is fast [373]... 

The observed structures were explained by a chiral bilayer effect mechanism proposing that only the enantiomerically pure compounds can lead to the formation of helical fibers which in turn slowly rearrange to enantiopolar crystal layers (Scheme 7.1). Within the micellar fibers, the polar head groups are oriented toward the aqueous environment and must therefore go through an energetically unfavourable -slow- dehydration followed by a 180 ° to form the enantiopolar crystals. 

The chiral bilayer effect. Micellar fibres are long-lived, if crystallization is connected with a slow rearrangement from tail-to-tail to head-to-tail oriented sheets Such rearrangements occur in fibres of pure enantiomers, but not in... 

The chiral bilayer effect (see Figure 5.15) was also demonstrated in polypeptides, but not in nucleic acids, d- and L-polylysine formed perfect solutions of helical strands at pH 9, but precipitated within a few minutes as pleated sheets if mixed in a 1 1 ratio. Chiral purity is again a pre-supposition for the stability of helices (Figure 5.32). 

It stabilizes the lyotropic liquid crystalline state of biological assemblies relative to the crystalline state, due to the so-called chiral bilayer effect, which will be discussed in more detail in Section 4.2. For example, 10-nonacosanol, extruded from the lipophilic wax layer of pine needles, forms fluid lipid tubules rather than crystals. Although it is difficult to establish the enantiopurity of the natural product, the fact that synthetic pure enantiomers produce tubules while the racemate gives platelets suggests that the biologically relevant morphology is attained because of the enantiopurity of the biomolecule. °... 

Figure 4.5.9 The chiral bilayer effect. Chirality in headgroups of amphiphiles enhances the lifetime of fibers if there are strong intermolecular binding inetractions between the headgroups in the crystal and the fiber.

An interesting phenomenon concerns the co-crystallization of Phe with racemic hydrophobic amino acids, e.g., Val, Leu, or lie. In all cases it was found that a mixture of pure L-Phe and the racemate (e.g., of L-Phe and D,L-Leu) dissolved well in alkaline water and co-crystallized upon acidification. The co-crystals always consisted of L-phe and the D-enantiomer of the other amino acid, e.g., L-Phe-D-Leu (Shiraiwa, 1984). Pseudo-racemic head-to-head and tail-to-tail bilayers are formed. This is another striking example of the chiral bilayer effect (see Fig. 2.5.9 and Sec. 9.5). 

D-and L-Poly(lysine) both occur in a helical conformation in aqueous solution above pH 10. A 1 1 mixture of both helices in water precipitates in the form of regular, silk-like P-sheets (Fig. 9.5.3). This is a polypeptide example of the chiral bilayer effect. It demonstrates the importance of chiral uniformity not only for noncovalent, soft-membrane systems, but also for covalent polyamides. Helical fibers in aqueous gels do not survive the addition of enantiomeric fibers with the single exception of DNA (see Fig. 8.6.1). The precipitation of enantiomeric peptides containing more than one kind of amino acid has, however, not been reported so far. 

Figure 9.5.3 Poly(L-lysine) and poly(D-lysine) both form helices at pH 10 in water. Upon mixing, racemic P-pleated sheets precipitate (chiral bilayer effect). (From Fuhrhop, 1987.)...

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