Hierarchical Structures with Supramolecular Chirality

Figure 26.12 Hierarchical structures with supramolecular chirality for a folded polypeptide chain (top) and a cholic acid crystal (bottom), (a) A single molecule, (b) a bimolecular assembly, (c) a helical assembly, (d) a bundle of the helices, and (e) a host-guest system.

As mentioned above, steroidal assemblies consist of hierarchical structures with supramolecular chirality, leading us to find an analogy on the basis of the concept molecular information and their expression of biopolymers. The information originates from sequential arrangements of a-amino acids. Since it is considered that the steroidal molecules consist of chains of methylene units with various substituents, the concept may be applied to steroidal molecules. Such sequential chains may be considered to hold for other related organic molecules, leading to the idea that chiral methylene chains with various substituents function as universal molecular storage. The methylene chains can be chemically modified to various sequential chains, such as polypetides, polynucleotides, polysaccharides, and ster-... 

Since the steroidal molecules are highly asymmetric, the resulting hierarchical assemblies may have the corresponding three-dimensional structures with supramolecular chirality. Starting from molecular chirality, each assembly must be chiral. In this context, we encountered a new problem, how we describe such molecular and supramolecular chirality. The first idea is that the steroidal molecules are analogous to a vertebrate animal which has three-axial chirality based on three directions such as head-leg, right-left, and belly-back. The three-axial chirality enables us to determine the three-axes of the hierarchical assemblies, as in the case of the helices of proteins and DNA. 

However, the particular synthetic requirements in the preparation of conjugated polymers have thus far severely limited the number of similarly hierarchically structured examples. Pu et al. reported different types of conjugated polymers with fixed main-chain chirality containing binaphthyl units in their backbone which exhibited, for example, nonlinear optical activity or were used as enantioselective fluorescent sensors [42—46]. Some chirally substituted poly(thiophene)s were observed to form helical superstructures in solution [47-51], Okamoto and coworkers reported excess helicity in nonchiral, functional poly(phenyl acetylenejs upon supramolecular interactions with chiral additives, and they were able to induce a switch between unordered forms as well as helical forms with opposite helical senses [37, 52, 53]. 

---