Two‐Component Self‐Assembly: Hierarchical Formation of pH‐Switchable Supramolecular Networks by π–π Induced Aggregation of Ion Pairs

Two‐component self‐assembly is a promising approach to construct functional nanomaterials. Interaction of a flexible guanidiniocarbonyl pyrrole tetra‐cation ( 1 ) with naphthalene diimide dicarboxylic acid (NDIDC) in aqueous DMSO leads to the formation of supramolecular networks. First, the carboxylate groups of NDIDC bind to the guanidiniocarbonyl pyrrole cations of 1 in a 1:2 stoichiometry. Further π–π induced aggregation then leads to 3D networks, as established by dynamic light scattering studies (DLS), NMR, fluorescence titration, viscosity measurements, AFM, and TEM microscopy. Due to ion pairing, the resulting aggregates can be switched between the monomers and the aggregates reversibly using external stimuli like protonation or deprotonation. At high concentration, a stable colloidal solution is formed, which shows an extensive Tyndall effect. Increasing the concentrations even further leads to formation of a supramolecular gel.     

Anion-dependent dimerization of a guanidiniocarbonyl pyrrole cation in DMSO

With spherical counteranions such as chloride or hexafluorophosphate, the glycine-derived guanidiniocarbonyl pyrrole cation 1 self-assembles into discrete dimers in DMSO, as can be seen by NMR and ESI mass spectral analysis. According to concentration- and temperature-dependent NMR studies, the dimerization is endothermic and therefore entropy driven. Molecular modeling suggests that the dimers are held together by hydrogen bonding in combination with pi-pi interactions. In the presence of picrate anions, dimerization of cation 1 does not occur, probably due to the formation of pi-stacked ion pairs.     

Functional Disruption of the Cancer‐Relevant Interaction between Survivin and Histone H3 with a Guanidiniocarbonyl Pyrrole Ligand

The protein Survivin is highly upregulated in most cancers and considered to be a key player in carcinogenesis. We explored a supramolecular approach to address Survivin as a drug target by inhibiting the protein–protein interaction of Survivin and its functionally relevant binding partner Histone H3. Ligand L1 is based on the guanidiniocarbonyl pyrrole cation and serves as a highly specific anion binder in order to target the interaction between Survivin and Histone H3. NMR titration confirmed binding of L1 to Survivin's Histone H3 binding site. The inhibition of the Survivin–Histone H3 interaction and consequently a reduction of cancer cell proliferation were demonstrated by microscopic and cellular assays.     

Highly stable self-assembly in water: ion pair driven dimerization of a guanidiniocarbonyl pyrrole carboxylate zwitterion

The synthesis of a novel water-soluble guanidiniocarbonyl pyrrole carboxylate zwitterion 2 is described, and its self-association in aqueous solutions is studied. Zwitterion 2 forms extremely stable 1:1 dimers which are held together by an extensive hydrogen bonding network in combination with two mutual interacting ion pairs as could be shown by ESI MS and X-ray structure determination. NMR dilution studies in different highly polar solvents showed that dimerization is fast on the NMR time scale with association constants ranging from an estimated 10(10) M(-1) in DMSO to a surprisingly high 170 M(-1) in water. Hence, zwitterion 2 belongs to the most efficient self-assembling systems solely on the basis of electrostatic interactions reported so far. Furthermore, an amidopyridine pyrrole carboxylic acid 10 was developed as a neutral analogue of zwitterion 2, which also dimerizes with an essentially identical hydrogen bonding pattern (according to ESI MS and X-ray structure determination) but lacking the ionic interactions. NMR binding studies demonstrated that the solely hydrogen-bonded neutral dimer of 10 is stable only in organic solvents of low polarity (K > 10(4) M(-1) in CDCl3 but <10 M(-1) in 5% DMSO in CDCl3). The comparison of both systems impressively underlines the importance of ion pair interactions for stable self-association of such H-bonded binding motifs in water.     

Oxoanion binding by flexible guanidiniocarbonyl pyrrole-ammonium bis-cations in water

The syntheses of several bis-cations 1-5 with a simple primary ammonium cation attached via flexible linkers of varying length to a guanidiniocarbonyl pyrrole oxo anion binding site are reported. In UV-binding studies in aqueous buffer solution these bis-cations showed efficient binding of various N-acetyl amino acid carboxylates. However, complex affinity is significantly depending on both the anion and the length of the linker in the bis-cation. With increasing linker length, complex stability first increases until an optimum is reached for bis-cation 3 with a C4-linker. Then the complex stability decreases again. The best binding substrate in this series is N-acetyl phenyl alanine, most likely due to additional cation-pi-interactions between the aromatic ring and the guanidiniocarbonyl pyrrole cation. The formation of the complex between bis-cation 3 and N-acetyl phenyl alanine carboxylate was investigated further by fluorescence titrations and NOE studies, as well as molecular mechanics calculations.     

Stereoselective self-sorting in the self-assembly of a Phe-Phe extended guanidiniocarbonyl pyrrole carboxylate zwitterion: formation of two diastereomeric dimers with significantly different stabilities

The 'dipeptide extended' guanidiniocarbonyl pyrrole carboxylate zwitterion GCP-Phe-Phe 1 forms stable dimers in DMSO. However, dimerization is highly stereoselective. Only homochiral dimers are formed and the (L,L)·(L,L) dimer (K(dim) > 10(5) M(-1)) is significantly more stable by a factor of 10(3) than the diastereomeric (D,L)·(D,L) dimer (K(dim) = 120 M(-1)).     

Carboxylate binding by 2-(guanidiniocarbonyl)pyrrole receptors in aqueous solvents: improving the binding properties of guanidinium cations through additional hydrogen bonds

A series of guanidiniocarbonyl pyrrole receptors has been synthesized which bind carboxylates by ion pairing in combination with multiple hydrogen bonds. Their binding properties with various carboxylates have been investigated using NMR titration studies in 40% water/DMSO (v/v). The best receptor has association constants which are in the order of K approximately/= 10(3) mol(-1) and hence some 30 times larger than with the simple acetyl guanidinium cation. Through a systematic variation of the receptor structure, semiquantitative estimates for the energetic contributions of the individual binding interactions could be derived. These data show that the various hydrogen bonds are not equally important for the binding but differ significantly in their energetic contribution to the overall complexation process. Furthermore, the receptor can be made chiral and shows selectivity upon binding of enantiomeric amino acid carboxylates. Molecular modeling was used to obtain structural information for the various receptor carboxylate complexes and served as a basis to explain the observed differences in binding constants.     

Amino acid binding by 2-(guanidiniocarbonyl)pyridines in aqueous solvents: a comparative binding study correlating complex stability with stereoelectronic factors

A series of guanidiniocarbonylpyridine receptors has been synthesized, and these compounds bind amino acids (carboxylate forms) in aqueous DMSO with association constants ranging from K = 30 to 460 M(-1) as determined by NMR titration experiments. The differences in the complex stabilities can be correlated with steric and electrostatic effects with the aid of calculated complex structures. For example, the electrostatic repulsion between the pyridine nitrogen lone pair and the bound carboxylate makes anion binding less efficient than with the analogous pyrrole receptors previously introduced by us for carboxylate binding in water. Furthermore, steric interactions between the receptor side chain as in 2 b and the bound substrate also disfavor complexation.     

Reversible Self-Assembly of Gold Nanoparticles Based on Co-Functionalization with Zwitterionic and Cationic Binding Motifs**

We report a pH- and temperature-controlled reversible self-assembly of Au-nanoparticles (AuNPs) in water, based on their surface modification with cationic guanidiniocarbonyl pyrrole (GCP) and zwitterionic guanidiniocarbonyl pyrrole carboxylate (GCPZ) binding motifs. When both binding motifs are installed in a carefully balanced ratio, the resulting functionalized AuNPs self-assemble at pH 1, pH 7 and pH 13, whereas they disassemble at pH 3 and pH 11. Further disassembly can be achieved at elevated temperatures at pH 1 and pH 13. Thus, we were able to prepare functionalized nanoparticles that can be assembled/disassembled in seven alternating regimes, simply controlled by pH and temperature.     

New guanidinium-based carboxylate receptors derived from 5-amino-pyrrole-2-carboxylate: synthesis and first binding studies

The syntheses of two new guanidinium-based carboxylate receptors 2a,b derived from 5-amino pyrrole-2-carboxylate 4 are described. These receptors bind N-acetyl alanine carboxylate and O-acetyl lactate efficiently in aqueous DMSO as could be shown by NMR studies. However, compared to previously reported guanidiniocarbonyl pyrrole receptors 1, the reversal in the direction of the amide group in 2a,b changes both the substrate selectivity (amides are now preferred over esters) and their relative binding affinities. Both effects can be explained based on the calculated complex structure.     

Synthesis and self-association properties of flexible guanidiniocarbonylpyrrole-carboxylate zwitterions in DMSO: intra- versus intermolecular ion pairing

We have synthesized a new class of flexible zwitterions 6a-e, in which a carboxylate is linked via an alkyl chain with variable length (one to five methylene groups) to a guanidiniocarbonylpyrrole cation. The self-association properties of these zwitterions were determined by NMR dilution studies in DMSO and by ESI-MS experiments. The stability and hence also the size of the aggregates formed via self-assembly is critically dependent on the length and therefore flexibility of the spacer. Whereas the smallest zwitterion 6a forms large aggregates already at low concentrations, the more flexible zwitterions only form small oligomers (6b) or dimers (6c-e) at much larger concentrations. The differences between the five zwitterions can be explained based on the different extent of intramolecular ion pairing within the monomers. Any intramolecular ion pairing, which becomes possible with increasing linker length, stabilizes the monomer and therefore destabilizes any oligomer.     

How to achieve self-assembly in polar solvents based on specific interactions? Some general guidelines

In general, self-assembly in polar solutions requires a combination of several non-covalent interactions within one binding motif. Besides the combination of H-bonds and hydrophobic or aromatic stacking interactions, in the last few years H-bonded ion pairs have been proven useful in this context. Also the molecular rigidity and the extent of intra- versus intermolecular interactions within the monomer play an important role in determining the self-assembling properties of a given monomer. We present some general guidelines and illustrative examples of various approaches that have been pursued in the literature before finally concentrating on a case study from our own work, the dimerization of a guanidiniocarbonyl pyrrole carboxylate zwitterion. This zwitterion forms stable dimers with K > 10(9) M(-1) in DMSO and >10(2) M(-1) even in water and can not only be used to study the importance of various non-covalent interactions for self-assembly in polar solvents but also to construct large nanostructures.     

Switchable supramolecular polymers from the self-assembly of a small monomer with two orthogonal binding interactions

The low molecular weight heteroditopic monomer 1 forms supramolecular polymers in polar solution as shown, for example, by infrared laser-based dynamic light scattering (DLS), small-angle neutron scattering (SANS), electron microscopy (TEM, cryo-TEM), and viscosity measurements. Self-assembly of 1 is based on two orthogonal binding interactions, the formation of a Fe(II)-terpyridine 1:2 metal-ligand complex and the dimerization of a self-complementary guanidiniocarbonyl pyrrole carboxylate zwitterion. Both binding interactions have a sufficient stability in polar (DMSO) and even aqueous solutions to ensure formation of linear polymers of considerable length (up to 100 nm). The supramolecular polymerization follows a ring-chain mechanism causing a significant increase in the viscosity of the solutions at millimolar concentrations and above. The linear polymers then further aggregate in solution into larger globular aggregates with a densely packed core and a loose shell. Both binding interactions can be furthermore switched on and off either by adding a competing ligand to remove the metal ion and subsequent readdition of Fe(II) or by reversible protonation and deprotonation of the zwitterion upon addition of acid or base. The self-assembly of 1 can therefore be switched back and forth between four different states, the monomer, a metal-complexed dimer or an ion paired dimer, and finally the polymer.     

C-H-pi interactions in 1-n-butyl-3-methylimidazolium tetraphenylborate molten salt: solid and solution structures

The crystal structure of 1-n-butyl-3-methylimidazolium tetraphenylborate molten salt (1) shows C-H-pi interactions between the hydrogens of the imidazolium cation and the phenyl rings of the tetraphenylborate anion. The imidazolium ring is surrounded by three tetraphenylborate anions that are connected with the same cation by C-H-pi (phenyl rings) interactions. The nearest inter-ion interaction is found between the N-CH-N proton of the cation and the B-phenyl centroid (2.349 A) with a nearly T-shaped geometry. The inter-ionic solution structure of 1 has been investigated by the detection of inter-ionic contacts in 1H NOESY NMR spectra between the protons of the cation and the anion. The 1H-NMR spectra of molten salt 1 is almost independent of its concentration in [D6]DMSO solution, the imidazolium proton chemical shifts are in the expected region and there are no observable NOE effects between the protons of the cation with those of the anion, indicating that 1 behaves in [D6]DMSO as a solvent-separated ion pair. In CDCl3 the 1H-NMR spectra of 1 are concentration dependent and all the imidazolium protons are shielded as compared with those observed in [D6]DMSO. Moreover, the 1H NOESY NMR spectra show all the peaks affected by the interaction between the protons of the imidazolium cation and those of the anion, indicating that in CDCl3 1 possesses a contact ion pair structure. The NCHN proton of the cation exhibits the greatest shielding (up to -4.5 ppm). an indication of the existence of C-H-pi interactions, even in solution. The calculated distance of this proton to the phenyl centroid is 2.3 A for a C-H -pi angle of 180 degrees. The apparent volumes for the cation and anion, calculated from the measured 13C-NMR relaxation times, increase from 38 and 140 A3 in [D6]DMSO to 360 and 600 A3 in CDCl3, respectively; this indicates the formation of floating aggregates of the type (1)(n) in CDCl3 via weak hydrogen bonds, with increasing concentration.     

Sulfur-containing hetero-calix[4]pyrroles as mercury(II) cation-selective receptors: thermodynamic aspects

Two sulfur-containing hybrid calix[4]pyrrole derivatives (III and IV) have been synthesized and fully characterized. Several analytical techniques (1H NMR, conductance measurements, UV-vis spectrophotometry, titration potentiometry, and titration calorimetry) have been used to assess the interaction between these hybrid calixpyrrole receptors and metal cations in acetonitrile and dimethylsulfoxide. The partition constants of calix[4]pyrrole, I, II, and IV in the acetonitrile-hexane solvent system and the solubilities of the ligands in various solvents at 298.15 K were determined. 1H NMR measurements reveal the sites of interaction of calixpyrrole ligands with metal cations in CD3CN. Conductance and UV-vis spectrophotometric measurements were performed to establish the composition of mercury(II) calixpyrrole complexes in acetonitrile at 298.15 K. Titration calorimetry was used to quantitatively assess Hg(II)-calixpyrrole interactions. Thus the thermodynamics of complexation of calixpyrrole ligands with the mercury(II) cation in acetonitrile at 298.15 K are reported. Potentiometric titrations were also used to establish the stepwise stability constants for the complexation of calix[3]thieno[1]pyrrole with the Hg(II) cation in acetonitrile at 298.15 K. The results show that replacement of one or more pyrrole units by thiophene rings in calix[4]pyrrole has tuned significantly the discrimination ability of these ligands between anions and enables the produced hybrid calixpyrroles to bind selectively with Hg(II) in acetonitrile. No interaction was observed between these ligands and other metal cations in acetonitrile.     

Amino acid binding in water by a new guanidiniocarbonyl pyrrole dication: the effect of the experimental conditions on complex stability and stoichiometry

The synthesis and binding properties of a new guanidiniocarbonyl pyrrole dication 2 are reported, which efficiently binds alanine carboxylate with log K_ass = 3.9 in buffered water. Due to the increased charge density in this dication, the binding constant is five times larger than for the parent guanidiniocarbonyl pyrrole monocation 1 (log K = 3.2). However, the experimental conditions for determining the binding constant significantly influence both complex stability and stoichiometry. With increasing amount of substrate added during the titration, the overall complex stability decreases due to the increasing ionic strength of the solution. Furthermore, the formation of 1:2 complexes between 2 and 7 becomes increasingly important. Therefore, for the comparison of binding data it has to be assured that exactly the same experimental conditions are used for their determination.     

Structure of a 4-nitroso-5-aminopyrazole and its salts: tautomerism, protonation, and E/Z isomerism

The structures of 1-benzyl-4-nitroso-5-aminopyrazole (1) and its hydrochloride (1H+) have been determined in the solid state and in solution in DMSO, methanol, and ethanol. The free base exists in solution as a mixture of amino/nitroso tautomers 2a and 2b rather than in the imino/oxime tautomers 3. The conjugated cation 1H+ results from the protonation of the nitroso group. X-ray crystallography showed that both amino hydrogen atoms of 2a form NH...O=N hydrogen bonds: one is intramolecular, the other links adjacent molecules in an infinite chain.     

A molecular flytrap for the selective binding of citrate and other tricarboxylates in water

The synthesis and binding properties of a new tricationic guanidiniocarbonyl pyrrole receptor 7 are described. Receptor 7 binds citrate 9 and other tricarboxylates such as trimesic acid tricarboxylate 8 with unprecedented high association constants of K(assoc) > 10(5) M(-1) in water as determined by UV and fluorescence tritration studies. According to NOESY experiments and molecular modeling calculations, the tricarboxylates are bound within the inner cavity of receptor 7 by ion pairing between the carboxylate groups and the guanidiniocarbonyl pyrrole moieties, favored by the nonpolar microenvironment of the cavity. Hence, receptor 7 can be regarded as a molecular flytrap. In the case of the aromatic tricarboxylate 8, additional aromatic interactions further strengthen the complex. The complexes with the tricarboxylates are so strong that even the presence of a large excess of competing anions or buffer salts does not significantly affect the association constant. For example, the association constant for citrate changes only from K(assoc) = 1.6 x 10(5) M(-1) in pure water to K(assoc) = 8.6 x 10(4) M(-1) in the presence of a 170-fold excess of bis-tris buffer and a 1000-fold excess of chloride. This makes 7 one of the most efficient receptors for the binding of citrate in aqueous solvents reported thus far.     

Anion binding studies of fluorinated expanded calixpyrroles

The anion binding properties of fluorinated calix[n]pyrroles (n = 4-6) in aprotic solvents (acetonitrile and DMSO) and modified reaction conditions allowing for the synthesis and isolation of the hitherto missing dodecafluorocalix[6]pyrrole from the condensation of 3,4-difluoro-1H-pyrrole and acetone are described. In acetonitrile solution containing 2% water, the association constants for the 1:1 binding interaction between octafluorocalix[4]pyrrole and chloride anion obtained with isothermal titration calorimetry (ITC) and (1)H NMR titration methods were found to match reasonably well. As compared to its nonfluorinated congener, octafluorocalix[4]pyrrole was found to display enhanced binding affinities for several representative anions in pure acetonitrile as judged from ITC analyses. Similar analyses of the fluorinated calix[n]pyrroles revealed an increase in the relative affinity for bromide over chloride with increasing macrocycle size, as manifest in a decrease in the binding ratio K(a(Cl))/K(a(Br)). Anion binding studies in the solid state, involving single-crystal X-ray diffraction analyses of the chloride and acetate anion complexes of octafluorocalix[4]pyrrole and decafluorocalix[5]pyrrole, respectively, confirmed the expected hydrogen bond interactions between the pyrrolic NH protons and the bound anions.     

Investigation on the nature of the adsorption sites of pyrrole in alkali-exchanged zeolite y by nuclear magnetic resonance in combination with infrared spectroscopy

Multinuclear solid-state NMR and infrared spectroscopy have been applied to investigate the host-guest interactions and the nature of the adsorption sites of pyrrole on alkali-exchanged zeolites Y (LiNaY, NaY, KNaY, and CsNaY). The presence of pyrrole provokes changes in the MAS NMR spectra of (23)Na, (7)Li, and (133)Cs to a degree dependent upon the amount adsorbed. The decrease in the quadrupolar coupling constant for (23)Na as well as the shift for (7)Li and (133)Cs signals are attributed to the interaction of the cation with the pyrrole ring system. The adsorption of pyrrole induces the displacement of cations located at SI' and SII sites toward the supercage to bind the guest molecules. In this way, the distribution of the cations at nonframework sites depends on the amount of adsorbate in the zeolite. At low loadings, pyrrole molecules bind preferentially to more electropositive cation in partially exchanged zeolites Y. Quantitative analysis by (1)H NMR shows that the cation-pyrrole complexes formed possess a stoichiometry of 1:1. The origin of the basic site heterogeneity, evidenced by the presence of several components in the -NH infrared stretching band, is investigated assuming that the heterocycle of pyrrole interacts with cations at SII sites in the supercage and the -NH group forms a hydrogen bond with a basic oxygen atom placed in the framework six-member ring. Making use of the information derived from NMR, it is concluded that the main source of basic site heterogeneity comes from the number of aluminum atoms in the six-member rings of the SII site where the alkaline cation is located.