Fine Tuning of ß-Peptide Foldamers: a Single Atom Replacement Holds Back the Switch from an 8-Helix to a 12-Helix.

Cyclic homologated amino acids are important building blocks for the construction of helical foldamers. N-aminoazetidine-2-carboxylic acid (AAzC), an aza analogue of trans-2-aminocyclobutanecarboxylic acid (tACBC), displays a strong hydrazino turn conformational feature, which is proposed to act as an 8-helix primer. tACBC oligomers bearing a single N-terminal AAzC residue were studied to evaluate the ability of AAzC to induce and support an 8-helix along the oligopeptide length. While tACBC homooligomers assume a dominant 12-helix conformation, the aza-primed oligomers preferentially adopt a stabilized 8-helix conformation for an oligomer length up to 6 residues. The (formal) single-atom exchange at the N terminus of a tACBC oligomer thus contributes to the sustainability of the 8-helix, which resists the switch to a 12-helix. This effect illustrates atomic-level programmable design for fine tuning of peptide foldamer architectures.

Solution state conformational preferences of dipeptides derived from N-aminoazetidinecarboxylic acid: an assessment of the hydrazino turn.

Four model compounds and four dipeptides containing N-aminoazetidinecarboxylic acid (AAzC) and a particular stereoisomer of 2-aminocyclobutanecarboxylic acid (ACBC) were studied to establish their solution state conformational preferences, particularly regarding the ability of AAzC to induce a three-center hydrogen-bonded folding feature known as a "hydrazino turn". On the basis of IR and NMR experiments, supported by molecular modeling, the AAzC residue adopted a trans configuration amenable to the formation of a cyclic eight-membered hydrogen bond conformation in solution, in all cases studied. The implication of the heterocyclic nitrogen atom of AAzC in the trans-like structure was demonstrated via a refined (1)H-(15)N HMBC experiment giving exploitable data at natural (15)N isotopic abundance, providing unprecedented evidence for the solution state hydrazino turn conformation. The predominance of this secondary structural feature depended on the configuration of the neighboring ACBC residue in the dipeptides: while the trans-ACBC derivatives prefer the hydrazino turn, the cis-ACBC derivatives may also populate low-energy 10-membered hydrogen-bonded ring structures. X-ray diffraction analysis of three compounds confirmed the presence of a solid state hydrazino turn in two cases, with geometries similar to those deduced from the solution state studies, but in the third compound, no intramolecular hydrogen-bonding feature was in evidence.

Fast-pulsing NMR techniques for the detection of weak interactions: successful natural abundance probe of hydrogen bonds in peptides.

Structural investigations of peptides using NMR spectroscopy rarely include the detection of N-H···O=C and N-H···N hydrogen bonds, because the relevant heteronuclei have a low natural abundance while the small trans hydrogen bond scalar couplings reduce the sensitivity. Fast repetition NMR techniques combined with state of the art spectrometer specifications allowed the enhancement of the sensitivity for detection of hydrogen bonds at natural isotopic abundance.