Nanotubes from Hydrogen Bonded Cyclic Molecules

6 Nanotubes from Hydrogen Bonded Cyclic Molecules 

Another common method which has been used to build nanotubes is based on macrocyclic molecules that are capable of stacking with each other to form the tubular structure. The main advantage of this method is the precise control of the nanotube diameter, which depends only on the size of the macrocycle used in the self-assembly. 

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The first dipeptide nanotube system was L-Val-L-Ala (VA), which forms crystals with narrow hydrophobic channels (diameter about 5 A) lined by peptide side chains.This structure is conceptually different from those of the cyclic peptides in that the pores are generated from self-assembly of small molecules, which are hydrogen bonded, head-to-tail, into helices (Fig. 3a). The extremely robust three-dimensional hydrogen-bonding scaffold was since observed for a series of other hydro-phobic dipeptides.Pore size can be regulated from 3.3 A (L-Ile-L-Val) to 5.2 A (L-Ala-L-Val) by adjusting the bulk of the hydrophobic side chains. Furthermore, L-Ala-L-Val has pores that can adapt their shapes and sizes to absorb large solvent molecules like 2-butanol. 

Cyclic ureas have also been used for nanotube formation in which the ureas are responsible for establishing the necessary hydrogen bonds to allow the association and orientation of cycloureas. One example of this process concerns the use of diphenyl ether linkers between ureas (Figure 24g). " The system forms a ring that leaves the C=0 and NH from the ureas in the appropriate position to form hydrogen bonds between the different subunits (position perpendicular to the plane of the backbone of the molecule). In the crystal structure, the nanotubes derived from these ureas contain acetic acid molecules in their cavity. 

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