Handedness self-assembled molecules, chirality

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. 

The fourth section of the chapter focuses on the various approaches to experimentally address chirality in self-assembled fibers. The unique chi-roptical properties of chiral molecules can be studied by circular dichro-ism and give access to valuable information about the conformations and relative positions of the molecules in the fibers. This section also gives an overview of the techniques that have been used to observe and assign fiber handedness. 

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]. 

---