Handedness formation

So far we have considered the formation of tubules in systems of fixed molecular chirality. It is also possible that tubules might form out of membranes that undergo a chiral symmetry-breaking transition, in which they spontaneously break reflection symmetry and select a handedness, even if they are composed of achiral molecules. This symmetry breaking has been seen in bent-core liquid crystals which spontaneously form a liquid conglomerate composed of macroscopic chiral domains of either handedness.194 This topic is extensively discussed in Walba s chapter elsewhere in this volume. Some indications of this effect have also been seen in experiments on self-assembled aggregates.195,196... 

There are two reasons to think this situation might occur. The first reason is experimental. As discussed in Sections 2-5, in most experiments on chiral materials, tubules and helical ribbons are observed with only one sense of handedness. However, there are a few exceptions in experiments on diacetylenic phospholipids,144 diacetylenic phosphonate lipids,145 146 and bile.162 In these exceptional cases, some helices are observed with the opposite sense of handedness from the majority. In the work on diacetylenic phospholipids, the minority handedness was observed only during the kinetic process of tubule formation at high lipid concentration,144 which is a condition that should promote metastable states. Hence, these experiments may indeed show a case of biased chiral symmetry-breaking in which the molecular chirality favors a state of one handedness and disfavors a mirror image state. 

For an electrostatic interpretation of the formation of the cholesteric phase of DNA, which does not give, however, a stereochemical correlation between the macromolecular and cholesteric handedness, see Komyshev, A. A. Leikin, S. Phys. Rev. Lett. 2000, 84. 

It is possible for chiral mesogens to produce essentially achiral mesophases. For instance, in certain ranges of concentration and molecular weight, DNA will form an achiral line hexatic phase. A curious recent observation is of the formation of chiral mesophases from achiral mesogens. Specifically, bent-core molecules (sometimes called banana LCs) have been shown to form liquid crystal phases that are chiral. In any particular sample, various domains will have opposite handedness, but within any given domain, strong chiral ordering will be present. 

Association constant. See Formation constant Astacin 627 Asthma 26, 385 Asymmetric unit of crystal 134 Asymmetry of molecules 41-43 handedness 41-43 in oligomers 344 Ataxia telangiectasia 566, 574 Atomic force microscope 131 diagram of 131... 

Figure 26-13 Synaptonemal complexes. (A) Aligned pairs of homologous chromatids lying 0.4 pm apart in Allium cepa. Arrows indicate "recombination nodules" which may be involved in initiating formation of crossovers. Portions of meiotic chromosomes of lily are shown at successive stages (B) Pachytene. (C) Portion of diplotene nucleus. (D) A bivalent at diplo-tene. (E) Two bivalents at diakinesis. Pairs of sister chromatids are coiled with appropriate handedness. (F) Sister chromatid cores are far apart in preparation for separation. A chiasma is present between the two central strands. (B) through (F) courtesy of Stephen Stack.279,279d (G) Pair of sister chromatids coiled with opposite handedness at metaphase. These are immun-ostained with anti-topoisomerase II antibodies. From Boy de la Tour and Laemmli.280 Courtesy of U. K. Laemmli.

At high concentrations, the strands aggregate into large polymeric entities, initially via filament formation, followed by lateral, tree like growth. Figure 10.82a -c shows electron microscope images of the various mixtures under these conditions. Note, especially, the opposite handedness of the L- and D-triple helices (right- and left-handed helices, respectively). 

A related phenomenon can also occur when the crystal lattice packing is chiral. This intrinsic handedness can result in formation of a 1 1 mixture of enantiomeric crystals. In this case, although there has been self-resolution into (+)- and (—)-crystals, both molecular enantiomers remain unseparated in each crystal. The fundamental distinction is that a conglomerate single crystal contains only one molecular enantiomer and therefore would be optically active in solution, while, for the latter, a single crystal contains both molecular enantiomers and its solution would be optically inactive. 

As mentioned in the previous paragraphs, the striking relationship between the chirality of the individual molecular components and the corresponding helical handedness of the DC8,9PC tubules, has led scientists to believe that the tubule formation is driven by the molecular chirality. However, according to more recent studies the process of tubule formation is more complex than previously thought. It initially involves the formation of enantiopure La -phase vesicles which are then transformed to -phase helical ribbons composed of a nearly racemic mixture of left and right handed helices.90 In the few minutes following the sphere-to-tubule transition, monomeric lipids from the saturated... 

Studies on the tubule formation by DCs, 9 PC racemates are also supporting this new theory. The previously proposed packing theories predicted that tubules formed from non-enantiopure DC8,9PC preparations would exhibit increased tubule diameters, while race-mates would only form flat sheets (tubule diameter was predicted to be infinite in the latter case). However, the experimental results did not verify the predictions, showing tubules with unchanged diameters and both helical senses of handedness in the DC8,9PC racemate.93,94,95... 

The opposite supramolecular chiral motif is always observed for opposite enantiomers, but which handedness will be expressed in the supramolecular assembly of a particular enantiomer is difficult to predict. An example for opposite-handed supramolecular structures built by equally handed monomers is the formation of chiral rosettes with oligp(p-phenylene vinylene) derivatives on hopg (Fig. 17). Again, the interplay of lateral hy-... 

It is reasonable to assume that the molecules in these domains have opposite handedness. Doping the SU layer with one TA enantiomer suppresses completely the formation of one mirror domain and installs global homochirality [28]. The opposite TA enantiomer suppresses the opposite SU enan-tiomorph (Fig. 33). Since hydrogen bonds between the bisuccinate molecules cannot be expected to play a role, one must consider a substrate-mediated mechanism. That is, a chiral footprint onto the surface acts as a chiral bias and suppresses opposite handedness in the adjacent adsorbate complex. A chiral footprint reconstruction has also been proposed for the TA/Ni(110) system [110]. The same type of homochirality inductions have been shown for (S, S)- or (.R,.R)-TA-doped (R, S)-TA monolayers on Cu(110) [29]. 

One of the most classic examples of chiral expression in thermotropic liquid crystals is that of the stereospecific formation of helical fibres by di-astereomers of tartaric acid derivatised either with uracil or 2,6-diacylamino pyridine (Fig. 9) [88]. Upon mixing the complementary components, which are not liquid crystals in their pure state, mesophases form which exist over very broad temperature ranges, whose magnitude depend on whether the tartaric acid core is either d, l or meso [89]. Electron microscopy studies of samples deposited from chloroform solutions showed that aggregates formed by combination of the meso compounds gave no discernable texture, while those formed by combinations of the d or l components produced fibres of a determined handedness [90]. The observation of these fibres and their dimensions makes it possible that the structural hypothesis drawn schematically in Fig. 9 is valid. This example shows elegantly the transfer of chirality from the molecular to the supramolecular level in the nanometer to micrometer regime. 

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