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This paper describes the design, synthesis, and characterization of a hydrogen-bonded molecular duplex (3‚4). Two oligoamide molecular strands, 3 and 4, with the complementary hydrogen-bonding sequences ADAADA and DADDAD, respectively, were found to form an extremely stable (K a ) (1.3 ( 0.7) × 10 9 M -1 ) molecular duplex (3‚4) in chloroform. Evidence from 1D and 2D 1 H NMR spectroscopy, isothermal titration calorimetry, and thin-layer chromatography confirmed the formation and the high stability of the duplex. The exceptional stability is explained by positive cooperativity among the numerous hydrogen-bonding and van der Waals interactions and the preorganization of the individual strands by intramolecular hydrogen bonds. This design has opened a new avenue to supramolecular recognition units with programmable specificities and stabilities.

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The assembly of well-defined protein secondary structures, leads to a bewildering array of tertiary structures. 1 As the first step toward developing artificial oligomers and polymers that fold like biomacromolecules, there is currently an intense interest in designing unnatural building blocks that adopt well-defined secondary structures. 2,3 Here we report a new class of oligoamides with backbones that adopt well-defined, crescent conformations.Our design is based on diaryl amide oligomers, shown as 1. The presence of the three-center hydrogen bonding system consisting of the S(5) and S(6) type 4 intramolecularly hydrogen bonded rings should lead to rigidification of the amide linkage. Oligoamides containing such amide linkages should have a rigid backbone. With the two amide linkages on the same benzene ring being meta to each other, the resulting oligomer should have a crescent conformation. 5 Ab initio molecular orbital calculations (in vacuo) were performed on amide 2. 6 Conformations 2a-b are constrained to be planar. The relative energy of each conformation is shown in parentheses. The computational results indicated significant differences in the relative energies of the four conformations: 2 was overwhelmingly favored over the alternative conformations 2a,b. The desired conformation, 2, was planar and had two strong intramolecular hydrogen bonds with O‚‚‚H ) 1.87 Å (S(6)) and 2.14 Å (S(5)), respectively.

Synthetic water channels were developed with an aim to replace aquaporins for possible uses in water purification, while concurrently retaining aquaporins’ ability to conduct highly selective superfast water transport. Among the currently available synthetic water channel systems, none possesses water transport properties that parallel those of aquaporins. In this report, we present the first synthetic water channel system with intriguing aquaproin-like features. Employing a “sticky end”-mediated molecular strategy for constructing abiotic water channels, we demonstrate that a 20% enlargement in angstrom-scale pore volume could effect a remarkable enhancement in macroscopic water transport profile by 15 folds. This gives rise to a powerful synthetic water channel able to transport water at a speed of ∼3 × 109 H2O s–1 channel–1 with a high rejection of NaCl and KCl. This high water permeability, which is about 50% of aquaporin Z’s capacity, makes channel 1 the fastest among the existing synthetic water channels with high selectivity.

A general strategy for creating nanocavities with tunable sizes based on the folding of unnatural oligomers is presented. The backbones of these oligomers are rigidified by localized, three-center intramolecular hydrogen bonds, which lead to well-defined hollow helical conformations. Changing the curvature of the oligomer backbone leads to the adjustment of the interior cavity size. Helices with interior cavities of 10 Å to >30 Å across, the largest thus far formed by the folding of unnatural foldamers, are generated. Cavities of these sizes are usually seen at the tertiary and quaternary structural levels of proteins. The ability to tune molecular dimensions without altering the underlying topology is seen in few natural and unnatural foldamer systems

We describe here a modularly tunable molecular strategy for construction and combinatorial optimization of highly efficient K-selective channels. In our strategy, a highly robust supramolecular H-bonded 1D ensemble was used to order the appended crown ethers in such a way that they roughly stack on top of each other to form a channel for facilitated ion transport across the membrane. Among 15 channels that all prefer K over Na ions, channel molecule 5F8 shows the most pronounced optimum for K while disfavoring all other biologically important cations (e.g., Na, Ca and Mg). With a K/Na selectivity of 9.8 and an EC value of 6.2 μM for K ion, 5F8 is clearly among the best synthetic potassium channels developed over the past decades.

The three-center hydrogen bond in diaryl amide 1 was examined by IR and 1H NMR spectroscopy, X-ray crystallography, and ab initio calculations. By comparing 1 with its structural isomers 2, 3 and 4, and with its conformational isomers 1a-c, it was found that the two two-center components of the three-center interaction reinforce each other, that is, the enhanced stability of the three-center hydrogen bond is a result of positive cooperativity between the two components. Substituents not involved in hydrogen bonding have little effect on the strength of the two- and three-center hydrogen bonds. To our knowledge, this is the first three-center hydrogen-bonding system that has been shown to exhibit positive cooperativity. Ab initio calculations of the geometries, vibrational modes, and 1H NMR chemical shifts also support the experimental findings. These results have provided a new insight into the three-center intramolecular hydrogen bonding in a partially rigidified structure and have provided a reliable motif for designing stably folded structures.