Nanotubes from Hydrogen Bonded Cyclic Peptides

1 Nanotubes from Hydrogen Bonded Cyclic Peptides 

Cyclic peptides composed of fi, 8, and s-amino acids can also form tubular assemblies. Seebach and co-workers prepared three cychc tetrapeptides that consist of yff -amino acid residues of different chirality (Fig. 8.9a) [44]. These cyclic yS-peptides were revealed to stack to form nanotubes in the same way as cychc 

Dory and co-workers synthesized a tripeptide composed of a, -unsaturated (5-amino acids (Fig. 8.10a) [45]. The trans geometry of the vinyl group was used to induce the peptide to adopt a flat conformation required for the self-assembling process. In a similar way to the nanombes from y5-peptides, the peptide backbone has an even number of atoms between the carbonyl and amino groups of each residue. As a result, all the hydrogen bonds were parallel in the nanotube (Fig. 8.10b). The same group further extended this pattern to di-, tri-, and tetrapeptides of -amino acids with a trans yS,(5-double bond (Fig. 8.10c). AU of these cyclic peptides were found to self-assemble into hydrogen bonded nanotubes [46]. 

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Fig. 8.8 a Nanotube from hydrogen bonding induced self-assembly of cyclic D, L-a-peptides. b Model of the transmembrane channels in the lipid bilayer. Reprinted with permission from Ref. [41]. Copyright 2013, American Chemical Society... 

Fig. 8.9 Nanotubes fonned from hydrogen bonding-induced self-assembly of cyclic -peptides, a Stracture of cyclic peptide containing -amino acids with diifcm nt chirality, b The corresponding stacking model of the three isomers in the solid stale (Me substituents and H atoms on the backbones are omitted for clarity). Re ninted with permission from Ref [44]. Copyright 1997, Wiley-VCH Verlag Gmbh and Co. KGaA, Weinheim...

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. 

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