Enantiomers self-assembled molecules, chirality

As described in the Introduction, Pasteur showed beautifully that racemic molecules resolve spontaneously into chiral forms when they crystallize. We call them conglomerates, in which molecules form condensates comprised of only one enantiomer. The condensation into conglomerates can now be observed not only in crystals but in monolayers, fibers, and supramolecules self-assembled in solution [35]. The researches became possible because of the development of microscopic observation techniques at the nanometer scale. However, in crystals we still do not know what kinds of molecules show spontaneous resolution. Hence, observation of chiral resolution in soft matter may provide important information on the general question. 

Finally, we mention a remarkable example of lateral resolution reported for supramolecular nanostructures on hopg [96]. Held together by 72 hydrogen bonds, the molecular nanostructure is formed from three melamine-substituted calix[4]arene units and twelve 5,5-diethylbarbiturate molecules (Fig. 31a). The nanostructure, basically a stack of four rosettes, has chiral symmetry. With its components all being achiral, both enantiomers are formed upon self-assembly in solution. Deposition of the tetrarosettes on hopg leaves this nanostructure intact and allows surface self-assembly. AFM studies revealed close-packed 2D lattices formed by the tetrarosettes on hopg... 

Control of information in the components can maximize the thermodynamic stability, but it can do little to change the lability of the systems. This lability is indeed inherent to the self-assembly process, "in order to allow the exploration of the energy hypersurface of the system and the evolution towards the final architecture" [21]. However this lability means that the system will react rapidly to any change in the thermodynamic conditions. In the case of chiral systems such as the helicates this means that the racemate will be formed unless the ligands are themselves chiral and can induce a specific chirality [22]. We are faced therefore with the problem of a simple synthesis of helical molecules which can never be resolved into enantiomers. 

The various relations that can be established between molecular chirality and fiber handedness are worth a detailed presentation. The general rule is that the handedness of a chiral self-assembled fiber is controlled by the stereochemistry of the molecule. One enantiomer gives a right-handed fiber and the other enantiomer a left-handed fiber. However, there are some rare cases where a pure enantiomer of a chiral molecule assembles into a mixture of right- and left-handed helices. This is the case for the phosphonate analogues of diacetylenic lipid 22 (Fig. 8) [98-100], for cholesteryl anthryloxy-butanoate [83], or for a mixture of a bile salt, a phosphatidylcholine, and cholesterol (Fig. 9) [101]. In the latter case, in addition to the fact that both right- and left-handed helical ribbons are observed, two or three different and well-defined helical pitches coexist (Fig. 9) [101]. 

There are many speculations on the origin of chirality of biosystems. Most interesting for the self assembly of reproducing catalytic systems are theories on the amplification of enantiomeric excess. Frankl proposed a general mechanism for spontaneous asymmetric synthesis. He showed that if the production of living molecules of life is rare and, hence, slow compared with their rate of multiplication, the whole Earth is likely to be extensively populated with the progeny of the first event before another appears. A living entity is defined as one able to reproduce its own kind. Frank showed that a simple and sufficient life model is a chemical substance which is a catalyst for its own production (hence, autocatalytic) and an anticatalyst for the production of its optical enantiomers. 

As discussed previously, different interactions between molecules, solvent, and substrate play an important role in two-dimensional crystal engineering. Another key factor which can strongly influence the outcome of the self-assembly process is chiralityThis structural aspect also assumes immense importance within a broad range of research domains such as catalysis and materials science. Separation of enantiomers is still a main concern in the pharmaceutical industry and the study of chirality on surfaces could be of great potential in this field. STM has been considered as an ideal technique to investigate the expression of chirality at a variety of interfaces with sub-molecular resolution.We focus on ambient conditions since it resembles the environment in which many important chemical and biological processes take place. 

As mentioned above, SAMs are organic assemblies formed by the adsorption of molecules from solution or the gas phase onto the surface of solids. If the adsorbed molecules are chiral, the self assembled monolayer is also rendered chiral. The chirality of the molecule can be distributed within the monolayer interior or located at the terminus of the molecule. However, the chirality is only expressed when the chiral constituent is exposed at the monolayer surface. Chiral SAMs are used in chiral systems. The SAMs can be used to specifically interact with chiral species, such as proteins or amino acids. Chiral SAMs have been used in enantioselective crystallization. In this case, a racemic solution of a chiral molecule is crystallized on a chiral SAM. The chiral SAM serves as a nucleating surface for one of the enantiomers, thereby increasing its crystallization on the SAM. Thus, enantioselective crystallization is achieved. 

To make a knot is a task easily accomplished by a child playing with a rope, or even by a careless adult with shoelaces. For a chemist playing with molecules this is an intrinsically difficult problem and so far only a few of these topologically interesting compounds have been synthesized. A strategy that finally led to success is based on the self-assembly of double-helical copper phenanthroline complexes. The helix represents the core structure from which the trefoil knot 30 in Fig. 15 is obtained in a final cyclization step. It is important to note that a trefoil knot is chiral. The resolution of the two enantiomers was recently accomplished by fractional crystallization of the diastereoisomers obtained with a chiral counterion. 

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