Homochiral lattice

These schemes have been frequently suggested [105-107] as possible mechanisms to achieve the chirally pure starting point for prebiotic molecular evolution toward our present homochiral biopolymers. Demonstrably successftd amplification mechanisms are the spontaneous resolution of enantiomeric mixtures under race-mizing conditions, [509 lattice-controlled solid-state asymmetric reactions, [108] and other autocatalytic processes. [103, 104] Other experimentally successful mechanisms that have been proposed for chirality amplification are those involving kinetic resolutions [109] enantioselective occlusions of enantiomers on opposite crystal faces, [110] and lyotropic liquid crystals. [Ill] These systems are interesting in themselves but are not of direct prebiotic relevance because of their limited scope and the specialized experimental conditions needed for their implementation. 

Several prominent types of host molecule, such as the steroidal bile acids and the cyclodextrins, are chiral natural products that are available as pure enantiomers. Chemical modification of these parent compounds provides an easy route to the preparation of large numbers of further homochiral substances. Since all these materials are present as one pure enantiomer, it automatically follows that their crystalline inclusion compounds must have chiral lattice structures. It is not currently possible to investigate racemic versions of these compounds, but the examples discussed previously in this chapter indicate that very different behaviour could result. 

One of the most recent observations in supramolecular surface chirality is the induction of homochirality on surfaces via cooperatively amplified interactions in molecular monolayers. As discussed in Sect. 2, adsorption-induced chirality leads to both mirror motifs. However, in the presence of additional chiral bias, lattice homo chirality can be installed in the entire molecular layer. Such bias comes from a chiral dopant, small ee or physical fields in combination with symmetry breaking of the surface. 

The equivalent to the majority rule, but in two dimensions, has been reported recently for [7]H on Cu(lll). In that case, a small ee induces homochiral long-range order [88]. As mentioned above, the two [7]H enantiomers crystallize in a 2D racemic lattice, which forms two enantiomorphous do-... 

Homochiral Polymers via 2-D Self-Assembly and Lattice-Controlled Polymerization... 

Reactivity within (DL)-PheNCA crystals provides a number of simple ways to de-symmetrize the racemic mixtures of the homochiral oligopeptides. For example, L-2-(thienyl)-alanineNCA (ThieNCA) molecules have been shown to enantioselectively occupy the L-sites in the DL-PheNCA host crystals. Lattice-controlled polymerization of such D-Phe/(L-Phe L-Thie)-NCA mixed crystals yields libraries of non-racemic oligopeptides of ho-... 

We conclude this survey with extended systems, where the assembled structures show the translational symmetry characteristic of the crystalline state. The packing of chiral units into a crystal lattice will inevitably involve some type of diastereo-selectivity, either homochiral or heterochiral, although this is fiequently not discussed in crystal stucture reports. If the associations are all homochiral, then an enantiomerically pure crystal will be obtained, and a solution of the racemate will yield a racemic mixture (see section 3). If, on the other hand, heterochiral association (often related by a centre of inversion or glide plane) is favoured, a racemic compound will crystallize. The double helicate [ 02(51)2] " crystallizes with a homochiral association of complexes along the helieal axis (Figure 38). The homochiral columns are then arranged in pseudohexagonal arrays with the ehirality... 

Figure 39 Left the complex [Co(52H)3] carries three H-bond donor groups (-COOH, top) and 3 H-bond acceptor groups (-COO , below). Right the [Co(52H)3] units are hydrogen bonded into a homochiral pseudorhombohedral lattice (black) which is interpenetrated by an identical lattice of opposite chirality (white). Reproduced with permission from reference 108.

Figure 41 The hexagonal lattice formed by homochiral pairing in [MnCi

A chirality classification of crystal structures that distinguishes between homochiral (type A), heterochiral (type B), and achiral (type C) lattice types has been provided by Zorkii, Razumaeva, and Belsky [11] and expounded by Mason [12], In the type A structure, the molecules occupy a homochiral system, or a system of equivalent lattice positions. Secondary symmetry elements (e.g., inversion centers, mirror or glide planes, or higher-order inversion axes) are precluded in type A lattices. In the racemic type B lattice, the molecules occupy heterochiral systems of equivalent positions, and opposite enantiomers are related by secondary lattice symmetry operations. In type C structures, the molecules occupy achiral systems of equivalent positions, and each molecule is located on an inversion center, on a mirror plane, or on a special position of a higher-order inversion axis. If there are two or more independent sets of equivalent positions in a crystal lattice, the type D lattice becomes feasible. This structure consists of one set of type B and another of type C, but it is rare. Of the 5,000 crystal structures studied, 28.4% belong to type A, 55.6% are of type B, 15.7% belong to type C, and only 0.3% are considered as type D. 

Homochiral hexagonal lattice (a) Heterochiral hexagonal lattice... 

There are many natural minerals and salts that posses optical activity in their crystalline state owing to their chiral lattices, sueh as quartz, einnabar, mica, chlorates, bromates, and iodates. Crystal chirality of other minerals, like aluminosilicates, such as zeolites, were not investigated, but these minerals are considered by many investigators as possible sourees of chirality and the origin of homochirality in our biosphere The optieal activities of clays have not revealed reliable evidence of chirality and therefore they do not play any positive role in our understanding of the origin or of the amplification of homochirality in nature. 

The crystallization of racemic molecules is very similar to the crystallization of achiral molecules. However, because of their chirality, racemic molecules can form different types of crystals with different compositions. If the crystal lattice contains equal left and right handed molecules arranged in an ordered manner, the crystal is heterochiral and referred to as a racemic compound. In the case where the crystal lattice is composed of only one enantiomer (left or right), the crystal is homochiral and referred to as a conglomerate. In nature, racemic compounds greatly outnumber conglomerates. 

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