Self-assembling cyclic peptide nanotubes (SPNs) have been extensively studied due to their potential applications in biology and material sciences. Cyclic γ-peptides, which have a larger conformational space, have received less attention than the cyclic α- and β-peptides. The self-assembly of cyclic homo-γ-tetrapeptide based on cis-3-aminocyclohexanecarboxylic acid (γ-Ach) residues, which can be easily synthesized by a one-pot process is investigated. Fourier transform infrared (FTIR) and NMR analysis along with density functional theory (DFT) calculations indicate that the cyclic homo-γ-tetrapeptide, with a non-planar conformation, can self-assemble into nanotubes through hydrogen-bond-mediated parallel stacking. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) experiments reveal the formation of bundles of nanotubes in CH₂Cl₂/hexane, but individual nanotubes and bundles of only two nanotubes are obtained in water. The integration of TEG (triethylene glycol) monomethyl ether chains and cyclopeptide backbones may allow the control of width of single nanotubes.

Density functional theory calculations (B3LYP) have been carried out to investigate the 4π-electron systems of 2,4-disila-1,3-diphosphacyclobutadiene (compound 1) and the tetrasilacyclobutadiene dication (compound 2). The calculated nucleus-independent chemical shift (NICS) values for these two compounds are negative, which indicates that the core rings of compounds 1 and 2 have a certain amount of aromaticity. However, deep electronic analysis reveals that neither of these two formal 4π-electron four-membered ring systems is aromatic. Compound 1 has very weak, almost negligible antiaromaticity, and the amidinate ligands attached to the Si atoms play an important role in stabilizing this conjugated 4π-electron system. The monoanionic bidentate ligand interacts with the conjugated π system to cause π-orbital splitting. This ligand-induced π-orbital splitting effect provides an opportunity to manipulate the gap between occupied and unoccupied π orbitals in conjugated systems. Conversely, compound 2 is nonaromatic because its core ring does not have a conjugated π ring system and does not fulfill the requirements of a Hückel system.

The structures and conformational energies of twelve MeN-, O-, S-, MeP-, CO-bridged homocalix[4]arenes and two kinds of O-bridged alternate hybrid-calix[4]arenes have been calculated at the B3LYP/6-31G* level of theory. The 1,3-alternate or twisted-1,3-alternate conformations are found to be the lowest energy structures in all cases except for MeP-bridged calix[4]pyridine and calix[4]benzene, for which the twist-pinched cone and partial cone are the most stable conformations, respectively. The conformational energy differences calculated between the lowest energy and the next conformation are on the order of 2.0–3.0 kcal/mol, but smaller for the S- and MeP-bridged compounds. Analysis of the structures and relative energies shows that the phenyl hydrogens have electrostatic attractions with the lone pairs of the heteroatoms and steric repulsions with the methyl groups on bridgehead groups. Conversely, the lone electron pair in the pyridyl compounds engage in a repulsive electrostatic interaction. The interactions of 1,3-aromatic rings play a minor but important role in the relative stability sequence. This detailed understanding of the factors governing the conformational space of hetero calixarences can be used to design conformationally biased analogs of these interesting compounds. Copyright © 2011 John Wiley & Sons, Ltd.