F⋅⋅⋅HN Hydrogen Bonding Driven Foldamers: Efficient Receptors for Dialkylammonium Ions

Solvent Dependent Folding of an Amphiphilic Polyaramid

A series of synthetic alternating and amphiphilic aromatic amide polymers were synthesized by a step growth polymerization. Alternating meta- and para-linkages were introduced to force the polymer chain into a helical shape in the highly polar solvent water. The polymers were analyzed by 1H NMR spectroscopy and SEC in polar aprotic solvents such as DMSO and DMF. However, the polymers also showed good solubility in water. 1H NMR spectroscopy, small-angle X-ray scattering, and dynamic light scattering provided clear evidence of polymer folding in water but not DMF. We employed parallel tempering metadynamics in the well-tempered ensemble (PTMetaD-WTE) to simulate the free energy surfaces of an analogous model polymer in DMF and water. The simulations gave a molecular model of an unfolded structure in DMF and a helically folded tubular structure in water.

Hybridization Approach to Identify Salicylanilides as Inhibitors of Tubulin Polymerization and Signal Transducers and Activators of Transcription 3 (STAT3)

The superimposition of the X-ray complexes of cyclohexanediones (i.e., TUB015), described by our research group, and nocodazole, within the colchicine binding site of tubulin provided an almost perfect overlap of both ligands. This structural information led us to propose hybrids of TUB015 and nocodazole using a salicylanilide core structure. Interestingly, salicylanilides, such as niclosamide, are well-established signal transducers and activators of transcription (STAT3) inhibitors with anticancer properties. Thus, different compounds with this new scaffold have been synthesized with the aim to identify compounds inhibiting tubulin polymerization and/or STAT3 signaling. As a result, we have identified new salicylanilides (6 and 16) that showed significant antiproliferative activity against a panel of cancer cells. Both compounds were able to reduce the levels of p-STAT3^Tyr705 without affecting the total expression of STAT3. While compound 6 inhibited tubulin polymerization and arrested the cell cycle of DU145 cells at G2/M, similar to TUB015, compound 16 showed a more potent effect on inhibiting STAT3 phosphorylation and arrested the cell cycle at G1/G0, similar to niclosamide. In both cases, no toxicity towards PBMC cells was detected. Thus, the salicylanilides described here represent a new class of antiproliferative agents affecting tubulin polymerization and/or STAT3 phosphorylation.

Fluorofoldamer-Based Salt- and Proton-Rejecting Artificial Water Channels for Ultrafast Water Transport

Here, we report on a novel class of fluorofoldamer-based artificial water channels (AWCs) that combines excellent water transport rate and selectivity with structural simplicity and robustness. Produced by a facile one-pot copolymerization reaction under mild conditions, the best-performing channel (AWC 1) is an n-C₈H₁₇-decorated foldamer nanotube with an average channel length of 2.8 nm and a pore diameter of 5.2 Å. AWC 1 demonstrates an ultrafast water conduction rate of 1.4 × 10¹⁰ H₂O/s per channel, outperforming the archetypal biological water channel, aquaporin 1, while excluding salts (i.e., NaCl and KCl) and protons. Unique to this class of channels, the inwardly facing C(sp2)-F atoms being the most electronegative in the periodic table are proposed as being critical to enabling the ultrafast and superselective water transport properties by decreasing the channel's cavity and enhancing the channel wall smoothness via reducing intermolecular forces with water molecules or hydrated ions.

Ultrafast water permeation through nanochannels with a densely fluorous interior surface

Ultrafast water permeation in aquaporins is promoted by their hydrophobic interior surface. Polytetrafluoroethylene has a dense fluorine surface, leading to its strong water repellence. We report a series of fluorous oligoamide nanorings with interior diameters ranging from 0.9 to 1.9 nanometers. These nanorings undergo supramolecular polymerization in phospholipid bilayer membranes to form fluorous nanochannels, the interior walls of which are densely covered with fluorine atoms. The nanochannel with the smallest diameter exhibits a water permeation flux that is two orders of magnitude greater than those of aquaporins and carbon nanotubes. The proposed nanochannel exhibits negligible chloride ion (Cl^-) permeability caused by a powerful electrostatic barrier provided by the electrostatically negative fluorous interior surface. Thus, this nanochannel is expected to show nearly perfect salt reflectance for desalination.

Introducing the Chiral Constrained α-Trifluoromethylalanine in Aib foldamers to Control, Quantify and Assign the Helical Screw-Sense

Oligomers of α-aminoisobutyric acid (Aib) are achiral peptides that adopt 3 10 helical structures with equal population of left- and right-handed conformers. Yet, the screw-sense preference of the helical chain may be controlled by a single chiral residue located at one terminus. 1 H and 19 F NMR, X-ray crystallography and circular dichroism studies on new Aib oligomers show that the incorporation of a chiral quaternary α-trifluoromethylalanine at their N -terminus induces a reversal of the screw-sense preference of the 3 10 -helix compared to that of a non-fluorinated analogue having an l-α-methyl valine residue. This work demonstrates that, among the many particular properties of introducing a trifluoromethyl group into foldamers, its stereo-electronic properties are of major interest to control the helical screw sense. Its use as an easy-to-handle 19 F NMR probe to reliably determine both the magnitude of the screw-sense preference and its sign assignment is also of remarkable interest.

The helical supramolecular assembly of oligopyridylamide foldamers in aqueous media can be guided by adenosine diphosphates

A metal-free and achiral tri-pyridylamide foldamer, DM 11, containing a critical naphthalimide side chain self-assembles in a left-handed helical manner in the presence of chiral adenosine phosphates, under physiological conditions. Surprisingly, a very high degree of helicity in the foldamer assemblies was observed with ADP compared to other nucleoside phosphates, including ATP.

A new method for detecting intramolecular H-bonds of aromatic amides based on the de-shielding effect of carbonyl groups on β-protons

Aromatic amide foldamers with highly predictable conformations possess potential for application in the fields of stereoselective recognition, charge transport and catalysis, whose conformations are commonly limited by the intramolecular hydrogen bonding between amide groups and hydrogen-bonding receptors. Herein, on the basis of the de-shielding effect of carbonyl groups on β-protons, we develop a new method for detecting intramolecular hydrogen bonds of aromatic amide compounds. The solvent-related changes in the βH chemical shifts (Δ(δβH)) and NH chemical shifts (Δ(δNH)) of three kinds of amide compounds, which are frequently used as building blocks of aromatic amide foldamers, were recorded in chloroform, nitromethane, acetonitrile and DMSO. The Δ(δβH) method is found to be highly suitable for studying methoxy-benzamides and fluoro-benzamides in chloroform and DMSO. It is worth noting that a reference compound is not required for applying the Δ(δβH) method, which is an advantage over the Δ(δNH) method. In addition, we extend the Δ(δNH) method from methoxy-benzamides to pyridine-carboxamides and fluoro-benzamides in chloroform and DMSO, and propose that nitromethane and acetonitrile will be possible alternatives for the Δ(δNH) method if a test compound is not soluble in chloroform.

Studies of novel trifluoroacetylated diaryl hydrazone molecular photoswitches in solution and in the solid state

Photoisomerization of trifluoroacetylated diaryl hydrazones induces a change in color due to the formation of a new hydrogen bond in solution and in the solid state.

Bi-functional hydrogen and coordination bonding surfactant: A novel and promising collector for improving the separation of calcium minerals

Mono-functional chelating collectors exhibit limited selectivity in the flotation of minerals. In particular, the selective separation of calcium minerals presents a significant challenge because mono-functional chelating collectors, such as fatty acid, indistinguishably adsorb onto mineral surfaces by coordinating with the same metal cation (Ca²⁺). Thus, there is an urgent need to develop new-mode-functional collectors to separate calcium minerals and a need to understand the underlying chemoselectivity. Given the difference of the hydrogen bonding ability of anions with fluorite, calcite and scheelite surfaces, the introduction of additional hydrogen bonding functional groups into collector molecules is a novel strategy to improve selectivity. In this study, a hydrogen and coordination bonding (bi-functional) collector, 2-cyano-N-ethylcarbamoyl acetamide (CEA) was developed, which could form coordination bonds with the Ca²⁺ ions (by carbonyl groups) and hydrogen bonds with the anions (by amino groups) on calcium mineral surfaces. The results of flotation tests showed that CEA can selectively separate fluorite and calcite from scheelite at pH 7. The promising selectivity of CEA lies in both the electrical properties and the anions' hydrogen bonding ability with the three calcium minerals. The negatively charged scheelite surfaces are not conducive to coordination bonding with CEA while the positively charged fluorite and calcite surfaces are. Besides, the hydrogen bonding ability of fluorite (F^-) and calcite (CO₃^2-) with carbamido in CEA is higher than that of scheelite (WO₄^2-), and this also plays an essential role. This coordination and hydrogen bonding based surfactant design protocol has a great potential in the development of tail-made collectors/depressants for the separation of other oxidized minerals.

The Diverse World of Foldamers: Endless Possibilities of Self-Assembly

Different classes of foldamers, which are synthetic oligomers that adopt well-defined conformations in solution, have been the subject of extensive studies devoted to the elucidation of the forces driving their secondary structures and their potential as bioactive molecules. Regardless of the backbone type (peptidic or abiotic), the most important features of foldamers are the high stability, easy predictability and tunability of their folding, as well as the possibility to endow them with enhanced biological functions, with respect to their natural counterparts, by the correct choice of monomers. Foldamers have also recently started playing a starring role in the self-assembly of higher-order structures. In this review, selected articles will be analyzed to show the striking number of self-assemblies obtained for foldamers with different backbones, which will be analyzed in order of increasing complexity. Starting from the simplest self-associations in solution (e.g., dimers of β-strands or helices, bundles, interpenetrating double and multiple helices), the formation of monolayers, vesicles, fibers, and eventually nanostructured solid tridimensional morphologies will be subsequently described. The experimental techniques used in the structural investigation, and in the determination of the driving forces and mechanisms underlying the self-assemblies, will be systematically reported. Where applicable, examples of biomimetic self-assembled foldamers and their interactions with biological components will be described.

Close Amide NH···F Hydrogen Bonding Interactions in 1,8-Disubstituted Naphthalenes

In this note, we present a series of N-(8-fluoronaphthalen-1-yl)benzamide derivatives designed to maximize amide-NH···F hydrogen bond interactions therein. A combination of IR and NMR spectroscopy indicates a linear correlation between the high energy shift in NH stretching frequency and the electron withdrawing nature of the substituent, consistent with the trend predicted by DFT calculations. Additionally, a limiting case of hydrogen bonding is observed when the benzamide derivatives are replaced with trifluoroacetamide, causing an additional red shift of 44 cm^-1 in the NH stretching frequency. Most importantly, ¹H-¹⁹F coupling constants in this series are among the largest measured for amide-NH···F interactions. X-ray crystallography reveals face-to-face alignment of naphthalene rings in these derivatives resulting in part from the NH···F hydrogen bonds. This motif also dictates the formation of sheets composed of stacked naphthalene rings in the crystal structure as opposed to unfluorinated analogues wherein NH···OC hydrogen-bonding interactions force benzamide and naphthalene rings to engage in T-shaped π-π interactions instead. Additionally, the NH proton in the trifluoroacetamide derivative engages in extended H-bond interactions in its crystal structure.

Intramolecular hydrogen bond directed distribution of conformational populations in the derivatives of N′-benzylidenebenzohydrazide

The extensive NMR investigations reveal the presence of E-isomers in the derivative of N′-benzylidenebenzohydrazide. The different conformer populations are controlled by the strength of intramolecular hydrogen bonds.

Combined experimental and theoretical study of long-range H–F interactions in α-fluoro amides

A combined experimental and computational approach provided insight into the nature and conformational dependence of long-range ⁴J_HF couplings in α-fluoro amides.

Halogen and hydrogen bonding-driven self-assembly of supramolecular macrocycles and double helices from hydrogen-bonded arylamide foldamers

Halogen bonding has been used to hold hydrogen bonded short aromatic amide foldamers to form 2 + 2 or 1 + 1 macrocycles.

Making Molecular and Macromolecular Helical Tubes: Covalent and Noncovalent Approaches

Aromatic foldamers possess well-defined cavity that can be stabilized by discrete intramolecular interactions including hydrogen bonding, solvophobicity, electrostatic repulsion, or coordination. Long foldamers can form dynamic deep helical tubular architectures that are not only structurally attractive but also useful hosts for guest encapsulation, chirality induction, delivery, and catalysis. This kind of helical tubular structures can be formed by single molecules or macromolecules or by connecting short-folded or helical segments through noncovalent or covalent forces. This perspective summarizes the recent advances on the construction of helical tubes and their properties and functions.

Intramolecular hydrogen bond directed stable conformations of benzoyl phenyl oxalamides: unambiguous evidence from extensive NMR studies and DFT-based computations

A number of benzoyl phenyl oxalamide derivatives have been synthesized and characterized by the extensive utility of one- and two-dimensional NMR experimental techniques. The manifestation of intramolecular hydrogen bonds in all of the synthesized molecules, convincingly established using NMR studies, governs the stable conformations of the molecules. In the fluorine substituted molecules, the coupling between two NMR active nuclei mediated through hydrogen bonds has been detected. The measured chemical shift difference of an NH proton has been employed to calculate the energy of the HBs. NMR analysis revealed the electrostatic nature of the hydrogen bonds in all of the molecules. The NMR experimental findings have been validated using Density Functional Theory (DFT)-based Non Covalent Interactions (NCIs) and Quantum Theory of Atoms In Molecules (QTAIM) computations.

Intramolecular Hydrogen Bonding Involving Organic Fluorine: NMR Investigations Corroborated by DFT-Based Theoretical Calculations

The combined utility of many one and two dimensional NMR methodologies and DFT-based theoretical calculations have been exploited to detect the intramolecular hydrogen bond (HB) in number of different organic fluorine-containing derivatives of molecules, viz. benzanilides, hydrazides, imides, benzamides, and diphenyloxamides. The existence of two and three centered hydrogen bonds has been convincingly established in the investigated molecules. The NMR spectral parameters, viz., coupling mediated through hydrogen bond, one-bond NH scalar couplings, physical parameter dependent variation of chemical shifts of NH protons have paved the way for understanding the presence of hydrogen bond involving organic fluorine in all the investigated molecules. The experimental NMR findings are further corroborated by DFT-based theoretical calculations including NCI, QTAIM, MD simulations and NBO analysis. The monitoring of H/D exchange with NMR spectroscopy established the effect of intramolecular HB and the influence of electronegativity of various substituents on the chemical kinetics in the number of organic building blocks. The utility of DQ-SQ technique in determining the information about HB in various fluorine substituted molecules has been convincingly established.

Surprisingly High Selectivity and High Affinity in Mercury Recognition by H-Bonded Cavity-Containing Aromatic Foldarands

In the absence of macrocyclic ring constraints, few synthetic systems, possessing a mostly solvent-independent well-folded conformation that is predisposed for highly selective and high affinity recognition of metal ions, have been demonstrated. We report here such a unique class of conformationally robust modularly tunable folding molecules termed foldarands that can recognize Hg²⁺ ions surprisingly well over 22 other metal ions. Despite the lack of sulfur atoms and having only oxygen-donor atoms in its structure, the best foldarand molecule, i.e., tetramer 4, exhibits a selectivity factor of at least 19 in differentiating the most tightly bound Hg²⁺ ion from all other metal ions, and a binding capacity that is ≥18 times that of thio-crown ethers. These two noteworthy binding characters make possible low level removal of Hg²⁺ ions. With a [4]:[Hg²⁺] molar ratio of 5:1 and a single biphasic solvent extraction, the concentration of Hg²⁺ ions could be reduced drastically by 98% (from 200 to 4 ppb) in pure water. 4 could also effect a highly efficient reduction in mercury content by 98% (from 500 to 10 ppb) in artificial groundwater via multiple successive extractions with an overall consumption of 4 being 9:1 in terms of [4]:[Hg²⁺] molar ratio.

Intramolecular hydrogen bonds involving organic fluorine in the derivatives of hydrazides: an NMR investigation substantiated by DFT based theoretical calculations

The rare examples of intramolecular hydrogen bonds (HB) of the type the N-H∙∙∙F-C, detected in a low polarity solvent in the derivatives of hydrazides, by utilizing one and two-dimensional solution state multinuclear NMR techniques, are reported. The observation of through-space couplings, such as, (1h)JFH, and (1h)JFN, provides direct evidence for the existence of intra-molecular HB. Solvent induced perturbations and the variable temperature NMR experiments unambiguously establish the presence of intramolecular HB. The existence of multiple conformers in some of the investigated molecules is also revealed by two dimensional HOESY and (15)N-(1)H HSQC experiments. The (1)H DOSY experimental results discard any possibility of self or cross dimerization of the molecules. The derived NMR experimental results are further substantiated by Density Function Theory (DFT) based Non Covalent Interaction (NCI), and Quantum Theory of Atom in Molecule (QTAIM) calculations. The NCI calculations served as a very sensitive tool for detection of non-covalent interactions and also confirm the presence of bifurcated HBs.

Detection of weak hydrogen bonding to fluoro and nitro groups in solution using H/D exchange

Hydrogen/deuterium (H/D) exchange can be a sensitive technique for measuring the strength of hydrogen bonding to neutral organic nitro and fluoro groups. The slower rates of reaction in comparison to suitable controls suggest that hydrogen bonding is present, albeit rather weak.

Supramolecular chemistry: from aromatic foldamers to solution-phase supramolecular organic frameworks

This mini-review covers the growth, education, career, and research activities of the author. In particular, the developments of various folded, helical and extended secondary structures from aromatic backbones driven by different noncovalent forces (including hydrogen bonding, donor-acceptor, solvophobicity, and dimerization of conjugated radical cations) and solution-phase supramolecular organic frameworks driven by hydrophobically initiated aromatic stacking in the cavity of cucurbit[8]uril (CB[8]) are highlighted.

Folding Patterns in a Family of Oligoamide Foldamers

A series of small, unsymmetrical pyridine-2,6-dicarboxylamide oligoamide foldamers with varying lengths and substituents at the end groups were synthetized to study their conformational properties and folding patterns. The @-type folding pattern resembled the oxyanion-hole motifs of enzymes, but several alternative folding patterns could also be characterized. Computational studies revealed several alternative conformers of nearly equal stability. These folding patterns differed from each other in their intramolecular hydrogen-bonding patterns and aryl-aryl interactions. In the solid state, the foldamers adopted either the globular @-type fold or the more extended S-type conformers, which were very similar to those foldamers obtained computationally. In some cases, the same foldamer molecule could even crystallize into two different folding patterns, thus confirming that the different folding patterns are very close in energy in spite of their completely different shapes. Finally, the best match for the observed NOE interactions in the liquid state was a conformation that matched the computationally characterized helix-type fold.

Organic fluorine involved intramolecular hydrogen bonds in the derivatives of imides: NMR evidence corroborated by DFT based theoretical calculations

The rare occurrence of intramolecular hydrogen bonds (HBs) of the type N–H⋯F–C is detected in the derivatives of imides in a low polarity solvent by using multi-dimensional and multinuclear NMR experiments.

Aromatic amide and hydrazide foldamer-based responsive host-guest systems

CONSPECTUS: In host-guest chemistry, a larger host molecule selectively and noncovalently binds to a smaller guest molecule or ion. Early studies of host-guest chemistry focused on the recognition of spherical metal or ammonium ions by macrocyclic hosts, such as cyclic crown ethers. In these systems, preorganization enables their binding sites to cooperatively contact and attract a guest. Although some open-chain crown ether analogues possess similar, but generally lower, binding affinities, the design of acyclic molecular recognition hosts has remained challenging. One of the most successful examples was rigid molecular tweezers, acyclic covalently bonded preorganized host molecules with open cavities that bind tightly as they stiffen. Depending on the length of the atomic backbones, hydrogen bonding-driven aromatic amide foldamers can form open or closed cavities. Through rational design of the backbones and the introduction of added functional groups, researchers can regulate the shape and size of the cavity. The directionality of hydrogen bonding and the inherent rigidity of aromatic amide units allow researchers to predict both the shape and size of the cavity of an aromatic amide foldamer. Therefore, researchers can then design guest molecules with structure that matches the cavity shape, size, and binding sites of the foldamer host. In addition, because hydrogen bonds are dynamic, researchers can design structures that can adapt to outside stimuli to produce responsive supramolecular architectures. In this Account, we discuss how aromatic amide and hydrazide foldamers induced by hydrogen bonding can produce responsive host-guest systems, based on research by our group and others. First we highlight the helical chirality induced as binding occurs in solution, which includes the induction of helicity by chiral guests in oligomeric and polymeric foldamers, the formation of diastereomeric complexes between chiral foldamer hosts and guests, and the induction of helical chirality by chiral guests into inherently flexible backbones. In addition, molecular or ion-pair guests can produce supramolecular helical chirality in the organogel state. Such structures exhibit remarkable time-dependence and a "Sergeants and Soldiers" effect that are not observed for other two-component organogels that have been reported. We further illustrate that the reversible folding behavior of an aromatic amide foldamer segment can modulate the switching behavior of donor-acceptor interaction-based [2]rotaxanes. Finally we show that a folded oligomer can induce folding in one or two attached intrinsically flexible oligomers, an example of a solvent-responsive intramolecular host-guest system.

Conformational preferences of furan- and thiophene-based arylamides: a combined computational and experimental study

We examine the conformational preferences of the furan- and thiophene-based arylamides, N-methylfuran-2-carboxamide (3) and N-methylthiophene-2-carboxamide (4), using a combination of computational methods and NMR experiments. The compound choice stems from their use as foldamer building blocks. We quantify the differences in the conformational rigidity of the two compounds, which governs corresponding foldamer conformations. Specifically, we demonstrate the effects of intramolecular hydrogen bonding (H-bonding), geometrical patterns and solvent polarity on arylamide conformations by comparing 3, 4 and previously studied ortho-methoxy N-methylbenzamide (1) and ortho-methylthio N-methylbenzamide (2). The study reveals that compound 3, despite its non-optimal S(5)-type H-bond geometry, retains a large portion of the H-bonded (eclipsed) conformation even in polar protic solvents. This behaviour is consistent with the quantum mechanical (QM) torsional energy profile. The percentages of H-bonded conformers that 3 retains are just slightly smaller than those of 1, which has a stronger S(6)-type H-bond. As for 2 and 4, the replacement of the O atom in 1 by an S atom in 2 results in a 70–90% loss of the H-bonded conformer in solution. However, the equivalent O to S replacement in 3 (leading to 4) causes only 15–30% loss of the eclipsed conformers in 4. Therefore, conformational preferences of 4 are very different from 2, in contrast to the similarity between 3 and 1. This study shows how the interplay of several forces modulates the conformational flexibility of arylamides. It also attests the strategy we are developing, which leads to accurate prediction of foldamer structure. The vital component of this strategy is the re-parameterization of critical force field parameters based on QM potential energy profiles, as well as validation of these parameters using experimental data in solution.

A conformationally switchable fluorescent oligophenol foldamer for selective sensing of copper(ii) ions

A stimuli-responsive hexameric oligophenol host undergoes amine-induced co-operative folding and fluorescence quenching, enabling easy classification of the bound metal ion guests as well as selective sensing of Cu²⁺ ions.

Intramolecular C–H⋯F hydrogen bonding-induced 1,2,3-triazole-based foldamers

Intramolecular C–H⋯F hydrogen bonding has been utilized to induce 1,2,3-triazole oligomers to fold into stable artificial secondary structures.

Nonaggregational shape-persistent cyclo[6]aramide and its macrocyclic effect toward binding secondary ammonium salts in moderately polar media

Simply by introducing steric side chains, the shape-persistent cyclo[6]aramides were found to exhibit nonaggregational behavior and strong association (3 × 10(4) M(-1)) ability in acetone for binding secondary ammonium salt. The complexation can be switched in an on-and-off fashion using AgPF6 and TBACl, contrasting sharply with their corresponding acyclic pentamer and demonstrating the macrocyclic effect.