Sequence-specific association in aqueous media by integrating hydrogen bonding and dynamic covalent interactions

Oligoamide strands that associate in a sequence-specific fashion into hydrogen-bonded duplexes in nonpolar solvents were converted into disulfide cross-linked duplexes in aqueous media. Thus, by incorporating trityl-protected thiol groups, which allows the reversible formation of disulfide bonds, into the oligoamide strands, only duplexes consisting of complementary hydrogen-bonding sequences were formed in aqueous solution as well as in methanol. The sequence-specific cross-linking of oligoamide strands was confirmed by MALDI-TOF, reverse-phase HPLC, and by isolating a cross-linked duplex. This study demonstrates that the sequence-specificity characteristic of multiply hydrogen-bonded systems can be extended into competitive media through the interplay of H-bonding and reversible covalent interactions, based on which a new class of molecular associating and ligating units that are compatible with both polar and nonpolar environments can be conveniently obtained.     

Metal-free organocatalysis through explicit hydrogen bonding interactions

The metal (ion)-free catalysis of organic reactions is a contemporary challenge that is just being taken up by chemists. Hence, this field is in its infancy and is briefly reviewed here, along with some rough guidelines and concepts for further catalyst development. Catalysis through explicit hydrogen bonding interactions offers attractive alternatives to metal (ion)-catalyzed reactions by combining supramolecular recognition with chemical transformations in an environmentally benign fashion. Although the catalytic rate accelerations relative to uncatalyzed reactions are often considerably less than for the metal (ion)-catalyzed variants, this need not be a disadvantage. Also, owing to weaker enthalpic binding interactions, product inhibition is rarely a problem and hydrogen bond additives are truly catalytic, even in water.     

Fluoride indicator that functions in mixed aqueous media: hydrogen bonding effects

A urea-based fluoride anion indicator, 3, that functions in mixed aqueous media is reported. Under these conditions, indicator 3 and the two control systems 1 and 2 are colorless. Addition of fluoride anions produces an easy-to-visualize colorimetric response (colorless to yellow) in the case of 3, but not 1 or 2 as the result of anion-induced Si–O bond cleavage. Most of the colorimetric response is believed to stem from an alteration in the hydrogen bonding network, rather than changes to the π-conjugation pathway per se.     

Synthesis of 5-fluorouracil-imprinted polymers with multiple hydrogen bonding interactions.

The antitumor active compound 5-fluorouracil (5-FU) was used as a target molecule and 5-FU-imprinted polymers were synthesized using 2,6-bis(acrylamido)pyridine and/or 2-(trifluoromethyl)acrylic acid as functional monomers. The 5-FU-imprinted polymers showed a higher affinity for 5-FU than that for 5-FU derivatives. By using both functional monomers simultaneously, the affinity and separation for 5-FU were improved.     

Assembling a mixed phthalocyanine-porphyrin array in aqueous media through host-guest interactions

A stable 1:1 host-guest complex is formed between a silicon(IV) phthalocyanine conjugated axially with two permethylated beta-cyclodextrin units and a tetrasulfonated porphyrin. The complex exhibits a light-harvesting property and works as an efficient photosensitizing system, killing HT29 human colon adenocarcinoma cells with an IC50 value of 0.09 microM. [structure: see text].     

Polymer/clay nanocomposites through multiple hydrogen‐bonding interactions

An 2‐ureido‐4[1H]pyrimidinone (UPy) motif with self‐association capability (through quadruple hydrogen bonds) was successfully anchored onto montmorillonite clay layers. Polymer/clay nanocomposites were prepared by specific hydrogen bonding interactions between surface functionalized silica nanoclays and UPy‐bonded supramolecular poly(ethylene glycol) or poly(?‐caprolactone). The mixed morphologies including intercalated layers with a non‐uniform separation and exfoliated single layers isolated from any stack were determined by combined X‐ray diffraction and transmission electron microscopic measurements. Thermal analyses showed that all nanocomposites had higher decomposition temperatures and thermal stabilities compared with neat polymer. The differential scanning calorimetric data implied that the crystallinity of polymers did not show essential changes upon introduction of organomodified UPy clays. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 650–658     

The electrochemically tuneable hydrogen bonding interactions between a phenanthrenequinone-functionalized self-assembled monolayer and a phenyl-urea terminated dendrimer

We report the application of electrochemistry to detect and modulate hydrogen bonding interactions between a phenanthrenequinone-functionalized self-assembled monolayer and a phenyl-urea terminated dendrimer.     

Guest recognition of a tetrapyridinohemicarcerand through hydrogen bonding and constrictive binding interactions

Tetrapyridinohemicarcerand 2 having four hydrogen-bonding acceptors of inward-directing pyridyl units was synthesized and their binding properties for a variety of organic guest molecules have been investigated. Tetrapyridinohemicarcerand 2 formed kinetically stable complexes with various sulfonic acids via intermolecular -SO₃H-pyridyl hydrogen bonding and constrictive binding interactions in C₂D₂Cl₄ at 25 °C. But carboxylic acids or alcohols cannot be a stable guest at the same conditions. Tetrapyridinohemicarcerand 2 also binds various disubstituted benzenes. Especially 1,4-diiodobenzene forms stable hemicarceplex 1,4-diiodobenzene@2, which seems to be stabilized by constrictive binding as well as by -C-H?I interactions between dioxymethylene of 2 and iodo group of guest.     

Heat-resistant polyimides with low CTE and water absorption through hydrogen bonding interactions

A novel diamine, 1H,1′H-(2,2′-bibenzimidazole)-5,5′-diamine (DPABZ), containing bisbenzimidazole unit was successfully synthesized, and used to prepare a series of copolyimides BPDA:(ODAm/DPABZn) by polycondensation with 4,4-diaminodiphenyl ether (ODA) and 4,4-biphthalic anhydride (BPDA). For comparison, a series of copolyimides BPDA:(ODAm/PABZn) based on another benzimidazole diamine 5-amino-2-(4-aminobenzene)-benzimidazole (PABZ) was also prepared. As a result, with the increase of PABZ or DPABZ content, the heat resistance (T_g and T_d) and mechanical properties (σ and E) of the resulting polyimide (PI) films increased, while the coefficient of thermal expansion (CTE) decreased. Overall, the DPABZ-based PIs showed higher T_g values and much lower CTE values than PABZ. As the content of PABZ increased, the water absorption of PABZ-based PIs increased obviously, but no significant change in DPABZ-based PIs. The intramolecular hydrogen bonding in DPABZ-based PIs caused by the diamine DPABZ was believed to be the reason for the aforementioned differences. The BPDA: DPABZ film with low-water adsorption of 2.1%, high-T_g value of 436°C and low-CTE value of 5.4 ppm/°C could be a promising new generation of flexible display substrates.     

Interplay of olefin metathesis and multiple hydrogen bonding interactions: covalently cross-linked zippers

Hydrogen-bonded zippers bearing terminal alkene groups were treated with Grubbs' catalyst, leading to covalently cross-linked zippers without violating H-bonding sequence specificity. The yield of a cross-linked zipper depended on the stability of its H-bonded precursor, with a weakly associating pair giving reasonable yields only at high concentrations while strongly associating pairs showed nearly quantitative yields. The integration of thermodynamic (H-bonding) and kinetic (irreversible C═C bond formation) processes suggests the possibility of developing many different covalent association units for constructing molecular structures based on a self-assembling way.     

The role of hydrogen bonding in water-metal interactions

The hydrogen bond interaction between water molecules adsorbed on a Pd <111> surface, a nucleator of two dimensional ordered water arrays at low temperatures, is studied using density functional theory calculations. The role of the exchange and correlation density functional in the characterization of both the hydrogen bond and the water-metal interaction is analyzed in detail. The effect of non local correlations using the van der Waals density functional proposed by Dion et al. [M. Dion, H. Rydberg, E. Schr?der, D. C. Langreth and B. I. Lundqvist, Phys. Rev. Lett., 2004, 92, 246401] is also studied. We conclude that the choice of this potential is critical in determining the cohesive energy of water-metal complexes. We show that the interaction between water molecules and the metal surface is as sensitive to the density functional choice as hydrogen bonds between water molecules are. The reason for this is that the two interactions are very similar in nature. We make a detailed analogy between the water-water bond in the water dimer and the water-Pd bond at the Pd <111> surface. Our results show a strong similarity between these two interactions and based on this we describe the water-Pd bond as a hydrogen bond type interaction. These results demonstrate the need to obtain an accurate and reliable representation of the hydrogen bond interaction in density functional theory.     

Interplay between anion-pi and hydrogen bonding interactions

The interplay between two important noncovalent interactions involving aromatic rings is studied by means of high level ab initio calculations. They demonstrate that synergistic effects are present in complexes where anion-pi and hydrogen bonding interactions coexist. These synergistic effects have been studied using the "atoms-in-molecules" theory and the Molecular Interaction Potential with polarization partition scheme. The present study examines how these two interactions mutually influence each other.     

Matrix polycondensation through hydrogen bonding interaction

Polycondensation of diesters having hydroxyl or pyridine groups was carried out with hexamethylenediamine (HMD) in the presence of (vinyl alcohol/vinyl acetate) copolymers as a matrix polymer. Apparent rates of the polycondensation of dimethyl tartrate (DMT) with HMD increased with increasing contents of PVA units in the copolymers and a strong entanglement between growing polyamide chains and PVA copolymers took place through the adsorption of HMD and DMT on the matrix copolymers. 2,6-Dimethyl pyridine dicarboxylate (2,6-DMP) reacted with HMD in the presence of the PVA copolymers or polysaccharide, while the rate enhancement effect of the matrix polymers was not significantly observed, as in the case of DMT. The effect of the matrix polymers on the polycondensation was discussed in terms of hydrogen bonding interactions.     

Lamellar bridged silsesquioxanes: self-assembly through a combination of hydrogen bonding and hydrophobic interactions

The synthesis of four bis(trialkoxysilylated) organic molecules capable of self-assembly--(EtO)3Si(CH2)3NHCONH-(CH2)n-NHCONH(CH2)3Si(OEt)3 (n = 9-12)--associating urea functional groups and alkylidene chains of variable length is described. These compounds behave as organogelators, forming supramolecular assemblies thanks to the intermolecular hydrogen bonding of urea groups. Whereas fluoride ion-catalysed hydrolysis in ethanol in the presence of a stoichiometric amount of water produced amorphous hybrids, acid-catalysed hydrolysis in an excess of water gave rise to the formation of crystalline lamellar hybrid materials through a self-organisation process. The structural features of these nanostructured organic/inorganic hybrids were analysed by several techniques: attenuated Fourier transformed infrared (ATR-FTIR), solid-state NMR spectroscopy (13C and 29Si), scanning and transmission electron microscopy (SEM and TEM) and powder X-ray diffraction (PXRD). The reaction conditions, the hydrophobic properties of the long alkylidene chains and the hydrogen-bonding properties of the urea groups are determining factors in the formation of these self-assembled nanostructured hybrid silicas.     

Stabilization of copper(II) thiosulfonate coordination complexes through cooperative hydrogen bonding interactions

A series of copper(II) thiosulfonate complexes have been prepared via the reaction of [Cu(Me 3tren)(OH 2)](ClO 4) 2 (Me 3tren = tris(2-methylaminoethyl)amine) with three thiosulfonate ligands (RSO 2S (-), where R = Me, Ph, and MePh) and characterized by microanalysis, FTIR spectroscopy, and X-ray crystallography. In these complexes, the distorted trigonal bipyramidal copper(II) coordination sphere is occupied by four amine nitrogen atoms from the tripodal tetramine ligand and an apically bound sulfur atom from the thiosulfonate ligand. By using the tripodal tetramine ligand the oxidation of the thiosulfonate has been restricted, allowing the isolation of the complexes. The Cu-S distances were found to be similar to those in related thiosulfate complexes, indicating coordinative interactions of similar strength. Two types of intramolecular hydrogen bonding interactions were evident which enhance the binding of the thiosulfonate to the copper(II) center. These interactions, which involve two amine N-H groups and either one or two thiosulfonate oxygens, were found to be weaker than in the corresponding thiosulfate complexes. The complex formation constants for the thiosulfonate complexes (log K f = 0.3-0.7) were found to be two orders of magnitude lower than compared to the thiosulfate analogues. This correlates well with a lower strength of intramolecular hydrogen bonding.     

Thermodynamics of hydrogen bonding in hydrophilic and hydrophobic media

The thermodynamics of hydrogen bond breaking and formation was studied in solutions of alcohol (methanol, ethanol, 1-propanol) molecules. An extensive series of over 400 molecular dynamics simulations with an aggregate length of over 900 ns was analyzed using an analysis technique in which hydrogen bond (HB) breaking is interpreted as an Eyring process, for which the Gibbs energy of activation DeltaG can be determined from the HB lifetime. By performing simulations at different temperatures, we were able to determine the enthalpy of activation DeltaH and the entropy of activation TDeltaS for this process from the Van't Hoff relation. The equilibrium thermodynamics was determined separately, based on the number of donor hydrogens that are involved in hydrogen bonds. Results (DeltaH) are compared to experimental data from Raman spectroscopy and found to be in good agreement for pure water and methanol. The DeltaG as well as the DeltaG are smooth functions of the composition of the mixtures. The main result of the calculations is that DeltaG is essentially independent of the environment (around 5 kJ/mol), suggesting that buried hydrogen bonds (e.g., in proteins) do not contribute significantly to protein stability. Enthalpically HB formation is a downhill process in all substances; however, for the alcohols there is an entropic barrier of 6-7 kJ/mol, at 298.15 K, which cannot be detected in pure water.     

Insights into hydrogen bonding and stacking interactions in cellulose

In this quantum chemical study, we explore hydrogen bonding (H-bonding) and stacking interactions in different crystalline cellulose allomorphs; namely, cellulose I(β) and cellulose III(I). We consider a model system representing a cellulose crystalline core made from six cellobiose units arranged in three layers with two chains per layer. We calculate the contributions of intrasheet and intersheet interactions to the structure and stability in both cellulose I(β) and cellulose III(I) crystalline cores. Reference structures for this study were generated from molecular dynamics simulations of water-solvated cellulose I(β) and III(I) fibrils. A systematic analysis of various conformations describing different mutual orientations of cellobiose units is performed using the hybrid density functional theory with the M06-2X with 6-31+G(d,p) basis sets. We dissect the nature of the forces that stabilize the cellulose I(β) and cellulose III(I) crystalline cores and quantify the relative strength of H-bonding and stacking interactions. Our calculations demonstrate that individual H-bonding interactions are stronger in cellulose I(β) than in cellulose III(I); however, the total H-bonding contribution to stabilization is larger in cellulose III(I) because of the highly cooperative nature of the H-bonding network. In addition, we observe a significant contribution from cooperative stacking interactions to the stabilization of cellulose I(β). The theory of atoms-in-molecules (AIM) has been employed to characterize and quantify these intermolecular interactions. AIM analyses highlight the role of nonconventional CH···O H-bonding in the cellulose assemblies. Finally, we calculate molecular electrostatic potential maps for the cellulose allomorphs that capture the differences in chemical reactivity of the systems considered in our study.     

Supramolecular polymers fabricated by orthogonal self-assembly based on multiple hydrogen bonding and macrocyclic host–guest interactions

Supramolecular polymers constructed by orthogonal self-assembly based on multiple hydrogen bonding and macrocyclic host-guest interactions have received increasing attention due to their elegant structures,outstanding properties,and potential applications.Hydrogen bonding endows these supramolecular polymers with good adaptability and reversibility,while macrocyclic host-guest interactions give them good selectivity and versatile stimuli-responsiveness.Therefore,functional supramolecular polymers fabricated by these two highly specific,noninterfering interactions in an orthogonal way have shown wide applications in the fields of molecular machines,electronics,soft materials,etc.In this review,we discuss the recent advances of functional supramolecular polymers fabricated by orthogonal self-assembly based on multiple hydroge n bonding and host-guest interactions.In particular,we focus on crown ether-and pillar[n]arene-based supramolecular polymers due to their compatibility with multiple hydrogen bonds in organic solution.The fabrication strategies,interesting properties,and potential applications of these advanced supramolecular materials are mainly concerned.     

Host-guest interactions with hydrophobic cyclophanes in aqueous media

Hydrophobic cyclophanes containing amide nitrogens in a rigid macrocyclic skeleton and flexible hydrocarbon chains, branched out at such nitrogen atoms, were prepared and their substrate-binding behavior was studied in aqueous media. the fluorescence spectroscopy was primarily adopted for the investigation of host-guest interactions by the aid of various hydrophobic probes. These host molecules provide cavities that are deep and hydrophobic enough to incorporate hydrophobic substrates of various bulkiness through an induced-fit mechanism originated from the flexible character of the alkyl branches. In addition to the hydrophobic interaction, the roles of electrostatic and charge-transfer interactions in molecular recognitions were clarified. Much hydrophobic, less polar, and highly viscous binding sites for hydrophobic guest molecules were provided by the octopus azaparacyclophane bearing eight hydrocarbon chains and the tetraazacyclotetradecane-capped azaparacyclophane having four flexible hydrocarbon chains connecting both macrocycles.