Self-assembled molecules symmetry breaking

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... 

Crystallization and reactivity in two-dimensional (2D) and 3D crystals provide a simple route for mirror-symmetry breaking. Of particular importance are the processes of the self assembly of non-chiral molecules or a racemate that undergo fast racemization prior to crystallization, into a single crystal or small number of enantiomorphous crystals of the same handedness. Such spontaneous asymmetric transformation processes are particularly efficient in systems where the nucleation of the crystals is a slow event in comparison to the sequential step of crystal growth (Havinga, 1954 Penzien and Schmidt, 1969 Kirstein et al, 2000 Ribo et al 2001 Lauceri et al, 2002 De Feyter et al, 2001). The chiral crystals of quartz, which are composed from non-chiral Si02 molecules is an exemplary system that displays such phenomenon. 

The above examples demonstrate that mirror symmetry breaking by self-assembly of non-chiral molecules into chiral architectures is indeed a feasible process. However, in order to preserve the handedness and amplify the stochastically-generated chirality, it is imperative to couple such chance events with efficient sequential autocatalytic processes. We refer now to several experimental systems that illustrate the occurrence of such scenarios. We shall allude in particular to systems undergoing amplification via non-linear asymmetric catalysis processes, via the formation of 2-D and 3-D crystalline systems and amplification of homochiral bio-like polymers in general and oligopeptides in particular. 

Early theoretical models on the feasibility of stochastic mirror symmetry breaking at prebiotic conditions have been successfully realized under laboratory conditions, particularly in studies in crystal and surface science, asymmetric auto catalysis and polymer chemistry. The first step, common in all these scenarios, is the self-assembly of non-chiral or chiral molecules to form diastereoisomeric supramolecular architectures that display different physico-chemical properties. 

Percec et al. [41,42] studied and reviewed various complex self-assembled structures in detail that can be obtained from dendritic molecules. Especially multicompartment capsules obtained by the self-assembly of a library of amphiphilic Janus-type dendrimers are impressive. The chemical linkage of two dissimilar dendritic building blocks results in Janus dendrimers and produces a break in the spherical symmetry characteristic to dendrimers. Consequently, these stmctures spontaneously promote the self-assembly upon injection of its ethanol solution to form stable unilamellar vesicular nanostructures and other complex architectures (Eig. 6.17). Dendriniersom.es... 

The asymmetric polymerization in crystalline architectures provides an excellent environment to conduct the absolute asymmetric synthesis of polymers, and also provides an effident route for the ampHfication of chirality. Mirror-symmetry breaking might occur either through total asymmetric transformations, either in enantiomorphous crystals that have self-assembled from achiral molecules, or within racemic crystalline architectures which are delineated by chiral rims or surfaces when one of the chiral faces is blocked by an interface. The self-assembly of nonracemic mixtures into a mixture comprising eutectic compositions of a racemic compound and an enantiomorphous assembly, followed by asymmetric transformation, provides a series of thermodynamically controlled, alternative routes for the effident ampHfication of homochirality. 

Here, the uneven Roman numerals, 2N— 1, represent a series of maturation steps of a molecular system in growth or development and the even Arabic numerals 2n stand for ingredients of the solution which react only with the relevant preliminary or intermediate product, 2N—I. The species 2/1 can themselves be products of self-assembly processes. The target molecule at the end of the growth process would be formed by some kind of (near equilibrium) symmetry breaking steps. The information it carries could, in principle, be transferred to other systems [13],... 

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