Helical conformation self-assembly

The alicyclic structure must have a small amount of twist. This may be built into a rigid alicyclic skeleton, or it may be achieved by means of conformational flexibility in the ring system. It aids the generation of helicity during self-assembly. 

Figure 5.24 Model of hierarchical self-assembly of chiral rodlike monomers.109 (a) Local arrangements (c-f) and corresponding global equilibrium conformations (c -f) for hierarchical selfassembling structures formed in solutions of chiral molecules (a), which have complementary donor and acceptor groups, shown by arrows, via which they interact and align to form tapes (c). Black and the white surfaces of rod (a) are reflected in sides of helical tape (c), which is chosen to curl toward black side (c ). (b) Phase diagram of solution of twisted ribbons that form fibrils. Scaled variables relative helix pitch of isolated ribbons h hh /a. relative side-by-side attraction energy between fibrils eaur/e. Reprinted with permission from Ref. 109. Copyright 2001 by the National Academy of Sciences, U.S.A.

Sheets also play an important role in the construction of peptide-based functional nanohbrous materials. /3-sheets are preferred over a-helices as molecular building blocks in the fabrication of artificial nanostructured materials perhaps because of the growing interest in understanding the self-assembly of two types of namral /3-sheet products silk protein and amyloid-Uke /3-sheets. Furthermore, extended /3-sheet conformation is relatively easy to achieve. Indeed, preventing their formation, particularly in high concentration or at high temperature, can be difficult in both synthetic and natural constructs. 

Lehn and coworkers also discussed the principles of design of self-assembling helical structures on the basis of conformational control. Depending on the solvent used, they were able to obtain helicate fibers and bundles possessing extended molecular channels characterized by hollow cores of limited diameter ofca. 8 A. This finding could form the basis of functional materials for multichannel ion active transport [28]. 

Figure 20 The last frames (representative) of two 600 ps simulations of a self-assembled monolayer (SAM) on Au(l 11). When a defect was introduced by shifting roughly half of the chains to the next interstitial site on the Au(l 11) surface (this resulted in the loss of four chains in the system below), the chains reoriented to a helical conformation to fill the voids caused by the defect. A 1.5 nm layer of water was placed on top of the SAM, and the computations were carried out in the presence of 2-D periodic boundaries to mimic an infinite SAM. Only the non-water molecules and atoms of the primary system are shown for clarity...

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