Although cis-trans lactam amide rotation is fundamentally important, it has been little studied, except for a report on peptide-based lactams. Here, we find a consistent relationship between the lactam amide cis/trans ratios and the rotation rates between the trans and cis lactam amides upon the lactam chain length of the stapling side-chain of two 7-azabicyclo[2.2.1]heptane bicyclic units, linked through a non-planar amide bond. That is, as the chain length increased, the rotational rate of trans to cis lactam amide was decreased, and consequently the trans ratio was increased. This chain length-dependency of the lactam amide isomerization and our simulation studies support the idea that the present lactam amides can spin through 360 degrees as in open-chain amides, due to the occurrence of nitrogen pyramidalization. The tilting direction of the pyramidal amide nitrogen atom of the bicyclic systems is synchronized with the direction of the semicircle-rotation of the amide.

Both (R- and S-) enantiomers of the bridgehead-substituted β-proline analogue (i.e., 7-azabicyclo[2.2.1]heptane-2-carboxylic acid or Abh amino acid (Abh-AA)) can work as a new scaffold for single β-strand enforcement and propagation.

β-Strands are formed by extended linear peptide chains that are usually paired to form β-sheet structure through interstrand hydrogen bonding. Linking a structured organic molecule with α-amino acid(s) can enforce or stabilize β-strand-like extended structures of the jointed amino acids. Spectroscopic and simulation studies indicated that the presence of a C-terminal 7-azabicyclo[2.2.1]­heptane amine (Abh) favors a β-strand-like extended conformation of the adjacent α-amino acid on the N side. The bridgehead substitution of the Abh unit biases the amide cis–trans equilibrium of the adjacent α-amino acid residue to cis conformation. The proximity, specified by the presence of bond paths (such as H–H bond path) between the bridgehead proton of Abh and the α-proton of the α-amino acid provides a driving force favoring the extended conformation, which is independent of solvents. These results provide a basis for de novo design of β-strand-mimicking extended peptides by using β-strand enforcer/stabilizer even in the absence of the interstrand hydrogen bonding.

Although multiple hydrophobic, aromatic π–π, and electrostatic interactions are proposed to be involved in amyloid fibril formation, the precise interactions within amyloid structures remain poorly understood. Here, we carried out detailed quantum theory of atoms-in-molecules (QTAIM) analysis to examine the hydrophobic core of amyloid parallel and antiparallel β-sheet structures, and found the presence of multiple inter-strand and intra-strand topological neighborhoods, represented by networks of through-space bond paths. Similar bond paths from side chain to side chain and from side chain to main chain were found in a single β-strand and in di- and tripeptides. Some of these bond-path networks were enhanced upon β-sheet formation. Overall, our results indicate that the cumulative network of weak interactions, including various types of hydrogen bonding (X-H—Y; X, Y = H, C, O, N, S), as well as non -H- non -H bond paths, is characteristic of amyloid β-sheet structure. The present study postulated that the presence of multiple through-space bond-paths, which are local and directional, can coincide with the attractive proximity effect in forming peptide assemblies. This is consistent with a new view of the van der Waals (vdW) interactions, one of the origins of hydrophobic interaction, which is updating to be a directional intermolecular force.