Theoretical Study on Tertiary Structural Elements of β-peptides: Nanotubes Formed from Parallel-Sheet-Derived Assemblies of β-Peptides

Parallel or polar strands of β-peptides spontaneously form nanotubes of different sizes in a vacuum as determined by ab initio calculations. Stability and conformational features of [CH3CO−(β-Ala)k−NHCH3]l (1 ≤ k ≤ 4, 2 ≤ l ≤ 4) models were computed at different levels of theory (e.g., B3LYP/6-311++G(d,p)//B3LYP/6-31G(d), with consideration of BSSE). For the first time, calculations demonstrate that sheets of β-peptides display nanotubular characteristics rather than two-dimensional extended β-layers, as is the case of α-peptides. Of the configurations studied, k = l = 4 gave the most stable nanotubular structure, but larger assemblies are expected to produce even more stable nanotubes. As with other nanosystems such as cyclodextrane, these nanotubes can also incorporate small molecules, creating a diverse range of applications for these flexible, biocompatible, and highly stable molecules. The various side chains of β-peptides can make these nanosystems rather versatile. Energetic and structural features...