Metal‐Helix Frameworks from Short Hybrid Peptide Foldamers

The potential of structured peptides has not been explored much in the design of metal‐organic frameworks (MOFs). This is partly due to the difficulties in obtaining stable secondary structures from the short α‐peptide sequences. Here we report the design, crystal conformations, coordination site dependent different silver coordinated frameworks of short α,γ‐hybrid peptide 12‐helices consisting of terminal pyridyl moieties and the utility of metal‐helix frameworks in the adsorption of CO₂. Upon silver ion coordination the 12‐helix terminated by the 3‐pyridyl derivatives adopted a 2:2 macrocyclic structure, while the 12‐helix terminated by the 4‐pyridyl derivatives displayed remarkable porous metal‐helix frameworks. Both head‐to‐tail intermolecular H‐bonds of the 12‐helix and metal ion coordination have played an important role in stabilizing the ordered metal‐helix frameworks. The studies described here open the door to design a new class of metal‐organic‐frameworks from peptide foldamers.     

Halogen Bonding Directed Supramolecular Quadruple and Double Helices from Hydrogen‐Bonded Arylamide Foldamers

Halogen bonding has been used to glue together hydrogen‐bonded short arylamide foldamers to achieve new supramolecular double and quadruple helices in the solid state. Three compounds, which bear a pyridine at one end and either a CF₂I or fluorinated iodobenzene group at the other end, engage in head‐to‐tail N???I halogen bonds to form one‐component supramolecular P and M helices, which stack to afford supramolecular double‐stranded helices. One of the double helices can dimerize to form a G‐quadruplex‐like supramolecular quadruple helix. Another symmetric compound, which bears a pyridine at each end, binds to ICF₂CF₂I through N???I halogen bonds to form two‐component supramolecular P and M helices, with one turn consisting of four (2+2) molecules. Half of the pyridine‐bearing molecules in two P helices and two M helices stack alternatingly to form another supramolecular quadruple helix. Another half of the pyridine‐bearing molecules in such quadruple helices stack alternatingly with counterparts from neighboring quadruple helices, leading to unique quadruple helical arrays in two‐dimensional space.     

Directional Threading and Sliding of a Dissymmetrical Foldamer Helix on Dissymmetrical Axles

We have investigated the self‐assembly of a dissymmetrical aromatic oligoamide helix on linear amido‐carbamate rods. A dissymmetric sequence bearing two differentiated ends is able to wrap around dissymmetric dumbbell guest molecules. Structural and thermodynamic investigations allowed us to decipher the mode of binding of the helix that can bind specifically to the amide and carbamate groups of the rod. In parallel kinetic studies of threading and sliding of the helix along linear axles were also monitored by ¹H NMR. Results show that threading of a dissymmetrical host can be kinetically biased by the nature of the guest terminus allowing a preferential sense of sliding of the helix. The study presented below further demonstrates the valuable potential of foldaxanes to combine designed molecular recognition patterns with fine control of self‐assembly kinetics to conceive complex supramolecular events.     

Metal‐Coordination‐Assisted Folding and Guest Binding in Helical Aromatic Oligoamide Molecular Capsules

The development of foldamer‐based receptors is driven by the design of monomers with specific properties. Herein, we introduce a pyridazine‐pyridine‐pyridazine diacid monomer and its incorporation into helical aromatic oligoamide foldamer containers. This monomer codes for a wide helix diameter and can sequester metal ions on the inner wall of the helix cavity. Crystallographic studies and NMR titrations show that part of the metal coordination sphere remains available and may then promote the binding of a guest within the cavity. In addition to metal coordination, binding of the guest is assisted by cooperative interactions with the helix host, thereby resulting in significant enhancements depending on the foldamer sequence, and in slow guest capture and release on the NMR time scale. In the absence of metal ions, the pyridazine‐pyridine‐pyridazine monomer promotes an extended conformation of the foldamer that results in aggregation, including the formation of an intertwined duplex.