Dendrophanes: Water-soluble dendritic receptors as models for buried recognition sites in globular proteins

Water-soluble dendritic cyclophanes (dendrophanes) of first ( 1 , 4 ), second ( 2 5 ), and third generation ( 3 6 ) with poly(ether amide) branching and 12, 36, and 108 terminal carboxylate groups, respectively, were prepared by divergent synthesis, and their molecular recognition properties in aqueous solutions were investigated. Dendrophanes 1 – 3 incorporate as the initiator core a tetraoxa[6.1.6.1]paracyclophane 7 with a suitably sized cavity for inclusion complexation of benzene or naphthalene derivatives. The initiator core in 4 – 6 is the [6.1.6.1]cyclo-phane 8 shaped by two naphthyl(phenyl) methane units with a cavity suitable for steroid incorporation. The syntheses of 1 – 6 involved sequential peptide coupling to monomer 9 , followed by ester hydrolysis (Schemes 1 and 4), Purification by gel-permeation chromatography (GPC; Fig. 3) and full spectral characterization were accomplished at the stage of the intermediate poly(methyl carboxylates) 10 – 12 and 23 – 25 , respectively. The third-generation 108-ester 25 was also independently prepared by a semi-convergent synthetic strategy, starting from 4 (Scheme 5). All dendrophanes with terminal ester groups were obtained in pure form according to the ¹³C-NMR spectral criterion (Figs, 1 and 5). The MALDI-TOF mass spectra of the third-generation derivative 25 (mol. wt. 19328 D) displayed the molecular ion as base peak, accompanied by a series of ions [M – n(1041 ± 7)]⁺, tentatively assigned as characteristic fragment ions of the poly(ether amide) cascade. A similar fragmentation pattern was also observed in the spectra of other higher-generation poly(ether amide) dendrimers. Attempts to prepare monodisperse fourth-generation dendrophanes by divergent synthesis failed. ¹H-NMR and fluorescence binding titrations in basic aqueous buffer solutions showed that dendrophanes 1 – 3 complexed benzene and naphthalene derivatives, whereas 4 – 6 bound the steroid testosterone. Complexation occurred exclusively at the cavity-binding site of the central cyclophane core rather than in fluctuating voids in the dendritic branches, and the association strength was similar to that of the complexes formed by the initiator cores 7 and 8 , respectively (Tables 1 and 3). Fluorescence titrations with 6-(p-toluidino)naphthalene-2-sulfonate as fluorescent probe in aqueous buffer showed that the micropolarity at the cyclophane core in dendrophanes 1 - 3 becomes increasingly reduced with increasing size and density of the dendritic superstructure; the polarity at the core of the third-generation compound 3 is similar to that of EtOH (Table 2). Host-guest exchange kinetics were remarkably fast and, except for receptor 3 , the stabilities of all dendrophane complexes could be evaluated by ¹H-NMR titrations. The rapid complexation-decomplexation kinetics are explained by the specific attachment of the dendritic wedges to large, nanometer-sized cyclophane initiator cores, which generates apertures in the surrounding dendritic superstructure.     

Catalytic Dendrophanes as Enzyme Mimics: Synthesis,Binding Properties,Micropolarity Effect,and Catalytic Activity of Dendritic Thiazolio-cyclophanes

Catalytic dendrophanes 9 and 10 were prepared as functional mimics of the thiamine-diphosphate-dependent enzyme pyruvate oxidase, and studied as catalysts in the oxidation of naphthalene-2-carbaldehyde ( 4 ) to methyl naphthalene-2-carboxylate ( 8 ) (Scheme 1). They are composed of a thiazolio-cyclophane initiator core with four generation 2 (G-2) poly(etheramide) dendrons attached. The two dendrophanes were synthesized by a convergent growth strategy by coupling dendrons 11 and 12 , respectively (Scheme 2) with (chloromethyl)-cyclophane 42 (Scheme 5) and subsequent conversion with 4-methylthiazole (Scheme 7). The X-ray crystal structures of cyclophane precursors 30 (Scheme 3), 37 , and 38 (Scheme 5) on the way to dendrophanes were determined (Fig. 1). The crystal-structure analysis of the benzene clathrate of 37 revealed the formation of channel-like stacks by the cyclophane which incorporate its morpholinomethyl side chain and the enclathrated benzene molecule (Fig. 2). The interactions of the enclathrated benzene molecule with the phenyl rings of the two adjacent cyclophane molecules in the stack closely resemble those between neighboring benzene molecules in crystalline benzene (Fig. 3). The characterization by MALDI-TOF-MS (Fig. 4), and ¹H- and ¹³C-NMR spectroscopy (Fig. 5) proved the monodispersity of the G-2 dendrophanes 9 and 10 with molecular weights up to 11500 Da (for 10 ). ¹H-NMR and fluorescence binding titrations in H₂O and aqueous MeOH showed that 9 and 10 form stable 1 : 1 complexes with naphthalene-2-carbaldehyde ( 4 ) and 6-(p-toluidino)naphthalene-2-sulfonate ( 48 , TNS) (Tables 1 and 2). The evaluation of the fluorescence emission maxima of bound TNS revealed that the dendritic branching creates a microenvironment of distinctly reduced polarity at the cyclophane core by limiting its exposure to bulk solvent. Initial rate studies for the oxidation of naphthalene-2-carbaldehyde to methyl naphthalene-2-carboxylate in basic aqueous MeOH in the presence of flavin derivative 6 revealed only a weak catalytic activity of dendrophanes 9 and 10 (Table 3), despite the favorable micropolarity at the cyclophane active site. The low catalytic activity in the interior of the macromolecules was explained by steric hindrance of reaction transition states by the dendritic branches.     

Dendrophanes: Novel Steroid-Recognizing Dendritic Receptors. Preliminary Communication

The H₂O-soluble dendritic cyclophanes (dendrophanes) 3–5 of first to third generation with molecular weights up to nearly 20 kD were synthesized, purified, and characterized. Cyclophane 2 , which served as the initiator core (generation zero), was prepared from tetrabromocyclophane 10 in a four-step sequence which involved as the first transformation a high-yielding, four-fold Pd(0)-catalyzed Suzuki cross-coupling reaction with 4-(benzyloxy)-phenyl-boronic acid to give 18 . The X-ray crystal-structure analysis of tetrabromocyclophane 10 displayed an open, nearly rectangular box with opposite aromatic walls being 8.3 and 11.4 Å apart and of suitable size for the incorporation of steroidal substrates. ¹H-NMR Binding titrations in borate-buffered D₂O/CD₃OD 1:1 showed that cyclophane-tetracarboxylate 2 forms 1:1 inclusion complexes with steroids (Table 2). Complexation was found to be enthalpically driven with higher binding affinities measured for the more apolar substrates. ¹H-NMR Titrations in the same solvent also provided clear evidence for core-selective complexation of testosterone ( 21 ) by the dendrophanes 3 (1st), 4 (2nd), and 5 (3rd generation) carrying up to 108 carboxylate surface groups. The stability of the corresponding 1:1 complexes was hardly affected by the size of the dendritic shell, although some generation-dependent conformational changes in the receptor cavity seemed to take place. Remarkably, host-guest exchange kinetics in all recognition processes were fast on the ¹H-NMR time scale.     

New Cyclophanes as Initiator Cores for the Construction of Dendritic Receptors: Host-guest complexation in aqueous solutions and structures of solid-state inclusion compounds

Cyclophanes 3 and 4 were prepared as initiator cores for the construction of dendrophanes (dendritic cydophanes) 1 and 2 , respectively, which mimic recognition sites buried in globular proteins. The tetra-oxy[6.1.6.1]paracyclophane 3 was prepared by a short three-step route (Scheme 1) and possesses a cavity binding site shaped by two diphenylmethane units suitable for the inclusion of flat aromatic substrates such as benzene and naphthalene derivatives as was shown by ¹H-NMR binding titrations in basic D₂O phosphate buffer (Table 1). The larger cyclophane 4 , shaped by two wider naphthyl(phenyl)methane spacers, was prepared in a longer, ten-step synthesis (Scheme 2) which included as a key intermediate the tetrabromocyclophane 5 . ¹H-NMR Binding studies in basic borate buffer in D₂O/CD₃OD demonstrated that 4 is an efficient steroid receptor. In a series of steroids (Table 1), complexation strength decreased with increasing substrate polarity and increasing number of polar substituents; in addition, electrostatic repulsion between carboxylate residues of host and guest also affected the binding affinity strongly. The conformationally flexible tetrabromocyclophane 5 displayed a pronounced tendency to form solid-state inclusion compounds of defined stoichiometry, which were analyzed by X-ray crystallography (Fig. 2). 1,2-Dichloroethane formed a cavity inclusion complex 5a with 1:1 stoichiometry, while in the 1:3 inclusion compound 5b with benzene, one guest is fully buried in the macrocyclic cavity and two others are positioned in channels between the Cyclophanes in the crystal lattice. In the 1:2 inclusion compound 5c , two toluene molecules penetrate with their aromatic rings the macrocyclic cavity from opposite sides in an antiparallel fashion. On the other hand, p-xylene (= 1,4-dimethylbenzene) in the 1:1 compound 5d is sandwiched between the cyclophane molecules with its two Me groups penetrating the cavities of the two macrocycles. In the 1:2 inclusion compound 5e with tetralin (= 1,2,3,4-tetrahydronaphthalene), both host and guest are statically disordered. The shape of the macrocycle in 5a – e depends strongly on the nature of the guest (Fig. 4). Characteristic for these compounds is the pronounced tendency of 5 to undergo regular stacking and to form channels for guest inclusion; these channels can infinitely extend across the macrocyclic cavities (Fig. 6) or in the crystal lattice between neighboring cyclophane stacks (Fig. 5). Also, the crystal lattice of 5c displays a remarkable zig-zag pattern of short Br…?O contacts between neighboring macrocycles (Fig. 7).     

Structure-Function Relationships in the Complexation of Steroids by a Synthetic Receptor

The complexation between the double-decker cyclophane (±)- 1 and a series of 30 steroids was investigated in CD₃OD by ¹H-NMR titrations. The geometries of the complexes, in which the substrates are axially included in the 13-Å deep and 9 Å×12 Å wide receptor cavity, were estimated based on the complexation-induced changes in chemical shift (CIS) of the steroidal Me group resonances. Computer modeling provided additional support for the geometries deduced from the experimental data. The log P (octanol/H₂O) values of the steroids were determined experimentally by HPLC or calculated using the program CLOGP. Although steroids with a high log P form some of the most stable complexes with (±)- 1 , a general correlation between the thermodynamic driving force for association −ΔG⁰ and the partition coefficient was not observed. It can, therefore, be concluded that inclusion complexation is not only driven by the preference of the steroid to transfer from the polar solvent into the lipophilic binding cavity but also by specific host-guest interactions. A series of structure-function relationships was revealed. i) Steroids with an isoprenoidal side chain at C(17) form some of the most stable complexes (−ΔG⁰ up to 4.8 kcal mol⁻¹), with side-chain encapsulation contributing as much as 1.2 kcal mol⁻¹ to the association strength. In these complexes, the receptor is slipping in a dynamic process over both the tetracyclic core and the lipophilic side chain. ii) Pregnane derivatives, which lack the isoprenoidal side chain, are tightly encapsulated with their tetracyclic core. Upon introduction of double bonds, the core flattens, and binding affinity drops substantially. iii) The presentation of steroidal OH groups to the receptor cavity is accompanied by energetically unfavorable functional-group desolvation, which strongly reduces the host-guest binding affinity. In contrast, inclusion of steroidal carboxylate or keto groups into the cavity does not substantially change complexation strength as compared to the unsubstituted derivatives. iv) Addition of extra Me groups to the steroidal A ring does not have a large effect on the association strength; however, complex geometries may change significantly. v) Receptor (±)- 1 shows a remarkably high affinity towards progesterone (−ΔG⁰=4.7 kcal mol⁻¹) despite the low log P value (3.87) of this steroid. Small changes in the progesterone structure lead to large reductions in complex stability, which clearly demonstrates that the double-decker cyclophane is a selective molecular receptor.     

Cyclophane-lectin Conjugates as a New Class of Water-soluble Host

A conjugate composed of tetraaza[6.1.6.1]paracyclophane bearing carboxylic acids and lectin, a carbohydrate binding protein, was prepared. The specific saccharide-binding abilities as well as the secondary structural features of the lectin were not disturbed, when the cyclophane were covalently bound to the lectin. The conjugate was found to act as a water-soluble host for inclusion of anionic guest molecules such as 6-p-toluidino-naphthalene-2-sulfonate (TNS) and 8-anilinonaphthalene-1-sulfonate (ANS) in aqueous acetate buffer (pH 4.0) with binding constants of 4.2 × 10⁴ and 1.5 × 10⁴ dm³ mol⁻¹, respectively. The obtained binding constants were much larger than those by untethered water-soluble cyclophane. A highly desolvated microenvironment was provided by the cyclophane cavity on the protein surfaces so that the tight host–guest interaction, which brought about the marked motional repression of the entrapped guests, became effective. The conjugate also showed molecular discrimination capabilities toward the anionic guests through electrostatic repulsion mechanism originating from acid-dissociation equilibrium of carboxylic acids of the cyclophane branches.     

Large Water-Soluble Cyclophanes with Convergent Intracavity Functionality

New tricyclic spacers, readily available through fourfold Mannich reaction of substituted dibenzyl ketones, were introduced into a series of ten H₂O-soluble cyclophanes with spacious preorganized cavity binding sites. These spacers provide H₂O-solubility with amine or crown-ether functionality remote from the cyclophane cavity while directing functional groups such as keto or OH groups in a precise geometrical array inside the cavity. The cyclophanes were designed to include organic substrates via a combination of apolar and specific polar functional group interactions. The X-ray crystal-structure analysis of the tritopic receptor 18 with one potential neutral-molecule and two cation-binding sites showed a large rectangular open cavity with dimensions of roughly 9 × 14 Å and a spacing of 9.7 Å between the O-atoms of two convergent C?O groups. Despite the binding-site preorganization, cyclophanes incorporating two of the new spacers did not show any substrate binding in aqueous solutions. The failure of these systems to function as receptors is mainly due to steric hindrance to important cyclophane aromatic ring-guest interactions. Also, the favorable solvation of the intracavity functionality may prevent the formation of complexes. Hybrid receptors constructed from the novel spacers and diphenylmethane units were found to bind flat aromatic substrates as well as bulky [4.2]paracyclophanes. The observed large differences in stability (ΔΔG°> 2 kcal mol^?1) of the complexes formed by three structurally closely related hybrid receptors with convergent C?O, OH or CH₂ groups and 6-hydroxynaphthalene-2-carbonitrile as guest can be explained by a strong solvation effect of the convergent functional groups on apolar inclusion complexation.     

Dendroclefts: Optically Active Dendritic Receptors for the Selective Recognition and Chiroptical Sensing of Monosaccharide Guests

The enantiomerically pure dendritic receptors with cleft-type recognition sites (dendroclefts) of generation zero ((−)- G0 ), one ((−)- G1 ), and two ((−)- G2 ) (Fig. 1) were prepared for the complexation of monosaccharides via H-bonding. They incorporate a rigid, optically active 9,9′-spirobi[9H-fluorene] core bearing 2,6-bis(carbonylamino)pyridine moieties as H-bonding sites in the 2,2′-positions. The dendritic shells in (−)- G1 and (−)- G2 are made out of a novel type of dendritic wedges of the first ( 8 ; Scheme 2) and second ( 13 ; Scheme 3) generations, which contain only donor O-atoms and are attached to the H-bonding edges of the core via glycine spacers (Scheme 4). The formation of stable 1 : 1 complexes (association constants K_a between 100 and 600 M ⁻¹, T=298 K; Table 2) between the three receptors and pyranosides in CHCl₃ was confirmed by ¹H-NMR and CD binding titrations as well as by Job plot analyses. The degree of dendritic branching was found to exert a profound effect on the stereoselectivity of the recognition processes. The binding enantioselectivity decreases with increasing degree of branching, whereas the diastereoselectivity increases. The ¹H-NMR analysis showed that the N−H⋅⋅⋅O H-bonds between the amide NH groups around the core and the sugar O-atoms become weakened with increasing dendritic generation, presumably due to steric factors and competition from intramolecular H-bonding between these amide groups and the O-atoms of the dendritic shell. The chiroptical properties of the dendroclefts respond to guest binding in a stereoselective manner. Whereas large differential changes are seen in the circular dichroism (CD) spectra of (−)- G0 and (−)- G1 upon complexation of the enantiomeric monosaccharides (Figs. 3 and 4), the CD spectra of the higher-generation derivative (−)- G2 respond to a lesser extent to guest complexation (Fig. 5). This is indicative of a different binding geometry, more remote from the core chromophore. With their higher masses, the dendroclefts (−)- G1 and (−)- G2 are readily recycled from host-guest solutions by gel-permeation chromatography. The strong CD sensory response and the easy recyclability suggest applications of chiral dendroclefts as sensors for biologically important molecules.     

A Perylene Bisimide Cyclophane as a “Turn‐On” and “Turn‐Off” Fluorescence Probe

A rigid, covalently linked perylene‐3,4:9,10‐tetracarboxylic acid bisimide (PBI) cyclophane was synthesized by imidization of a bay‐substituted perylene bisanhydride with p‐xylylenediamine. The interchromophoric distance of approximately 6.5 Å establishes an ideal rigid cavity for the encapsulation of large aromatic compounds such as perylene and anthracene with binding constants up to 4.6×10⁴ M ^?1 (in CHCl₃). For electron‐poor guest molecules, the complexation process is accompanied by a significantly increased fluorescence, whereas the emission intensity is dramatically quenched by more electron‐rich guests because of the formation of charge‐transfer complexes. Furthermore, the influence of the PBI core twist on the binding constant results in a remarkable selectivity towards more flexible aromatic guest molecules.     

Binding behaviour and solubilisation of p-sulfonatocalixarenes to cinchona alkaloids

In this study, we investigated the binding behaviours of three water-soluble p-sulfonatocalixarenes with four cinchona alkaloids in aqueous and phosphate buffer solutions (pH 7.2 and 2.0). The complexation stability constants obtained by fluorescence titrations were comparatively discussed from several aspects: host cavity, pH effect and ionic strength. Among three hosts, p-sulfonatocalix[4]arene (SC4A) forms the most stable complexes with cinchona alkaloids, especially in acidic aqueous conditions. Furthermore, SC4A was elected as model drug carrier for cinchona alkaloids, where solubilisation by the complexation of SC4A and mimic release from the calixarene cavity in the presence of negatively charged micelles were initially studied.     

Complexation of Neutral Molecules by Cyclophane Hosts

Since the discovery of the crown ethers by Pedersen twenty years ago, the chemistry of synthetic hosts for the selective complexation of organic and inorganic guests has seen an extraordinarily rapid development. This article discusses in particular the contributions provided by synthetic cyclophanes as hosts to the understanding of molecular complexation of neutral organic guest molecules in aqueous and organic solvents. In aqueous solution, cyclophanes form stoichiometric complexes with neutral aromatic guests which can approach enzyme-substrate complexes in their stability. Efficient molecular complexation is also observed in organic environments. Here, as a result of large solvation effects, the strength of complexation is strongly dependent on the nature of the organic solvent. Electron donor-acceptor interactions can contribute significantly to the stability of complexes formed between cyclophane hosts and aromatic guests. Force-field calculations together with computer graphics are powerful tools in the design of water-soluble, optically active hosts for chiral recognition of complexed racemic guests. Simple and selective functionalization of the cyclophane framework leads to stable, bioorganic catalysts. Like enzymes, these catalysts bind their substrates in a rapid equilibrium prior to the reaction steps. As a perspective, some fascinating research objectives in the field of molecular recognition and catalysis which can be targeted with designed cyclophane hosts are shown.     

Macrotricyclic Steroid Receptors by Pd°-Catalyzed Cross-Coupling Reactions: Dissolution of cholesterol in aqueous solution and investigations of the principles governing selective molecular recognition of steroidal substrates

Three double-decker cyclophane receptors, (±)- 2 , (±)- 3 , and (±)- 4 with 11–13-Å deep hydrophobic cavities were prepared and their steroid-binding properties investigated in aqueous and methanolic solutions. Pd°-Catalyzed cross-coupling reactions were key steps in the construction of these novel macrotricyclic structures. In the synthesis of D₂-symmetrical (±)- 2 , the double-decker precursor (±)- 7 was obtained in 14% yield by fourfold Stille coupling of equimolar amounts of bis(tributylstannyl)acetylene with dibromocyclophane 5 (Scheme 1). For the preparation of the macrotricyclic precursor (±)- 15 of D₂-symmetrical (±)- 3 , diiodocylophane 12 was dialkynylated with Me₃SiC?CH to give 13 using the Sonogashira cross-coupling reaction; subsequent alkyne deprotection yielded the diethynylated cyclophane 14 , which was transformed in 42% yield into (±)- 15 by Glaser-Hay macrocyclization (Scheme 2). The synthesis of the C₂-symmetrical conical receptor (±)- 4 was achieved via the macrotricyclic precursor (±)- 25 , which was prepared in 20% yield by the Hiyama cross-coupling reaction between the diiodo[6.1.6.1]paracyclophane 19 and the larger, dialkynylated cyclophane 17 (Scheme 4). Solid cholesterol was efficiently dissolved in water through complexation by (±)- 2 and (±)- 3 , and the association constants of the formed 1:1 inclusion complexes were determined by solid-liquid extraction as K_a = 1.1 × 10⁶ and 1.5 × 10⁵ l mol^?1, respectively (295 K) (Table 1). The steroid-binding properties of the three receptors were analyzed in detail by ¹H-NMR binding titrations in CD₃OD. Observed steroid-binding selectivities between the two structurally closely related cylindrical receptors (±)- 2 and (±)- 3 (Table 2) were explained by differences in cavity width and depth, which were revealed by computer modeling (Fig. 4). Receptor (±)- 2 , with two ethynediyl tethers linking the two cyclophanes, possesses a shallower cavity and, therefore, is specific for flatter steroids with a C(5)?C(6) bond, such as cholesterol. In contrast, receptor (±)- 3 , constructed with longer buta-1,3-diynediyl linkers, has a deeper and wider hydrophobic cavity and prefers fully saturated steroids with an aliphatic side chain, such as 5α-cholestane (Fig. 7). In the 1:1 inclusion complexes formed by the conical receptor (±)- 4 (Table 3), testosterone or progesterone penetrate the binding site from the wider cavity side, and their flat A ring becomes incorporated into the narrower [6.1.6.1]paracyclophane moiety. In contrast, cholesterol penetrates (±)- 4 with its hydrophobic side chain from the wider rim of the binding side. This way, the side chain is included into the narrower cavity section shaped by the smaller [6.1.6.1]paracyclophane, While the A ring protrudes with the OH group at C(3) into the solvent on the wider cavity side (Fig. 8). The molecular-recognition studies with the synthetic receptors (±)- 2 , (±)- 3 , and (±)- 4 complement the X-ray investigations on biological steroid complexes in enhancing the understanding of the principles governing selective molecular recognition of steroids.     

Inclusion Complexation of Diphenylamine-4-diazonium Chloride and p-Sulfonatocalix[4]arene

Inclusion complexation of p-sulfonatocalix[4]arene (SC4A) and diphenylamine-4-diazonium chloride (DDC) in aqueous solution was investigated in this study. The inclusion of DDC in the cavity of SC4A leads to ¹H NMR chemical shifts of DDC moving towards higher magnetic field. The complexation of SC4A also results in a bathochromic shift and a decrease in optic intensity of the absorption spectrum of DDC. In the presence of SC4A, the thermal stability of DDC in aqueous solution increases significantly while its photosensitivity still remains high.     

The complexation of ferrocene derivatives by a water-soluble calix[6]arene

The complexation of several ferrocene derivatives by the water-soluble hostp-sulfonato-calix[6]arene was investigated using electrochemical and¹H-NMR spectroscopic techniques. The electrochemical results indicate that both oxidation states of the guests are bound to the calixarene host, although the oxidized (ferrocenium) forms are complexed more strongly than the reduced (ferrocene) species.¹H-NMR spectroscopic data indicate that the complexation phenomena involves the inclusion of the guest's ferrocene moiety into the flexible calixarene cavity.This paper is dedicated to the commemorative issue on the 50th anniversary of calixarenes.     

Inclusion complexes of dihydroartemisinin with cyclodextrin and its derivatives: characterization,solubilization and inclusion mode

The characterization, inclusion complexation behavior and binding ability of the inclusion complexes of dihydroartemisinin with β-cyclodextrin and its derivatives, sulfobutyl ether β-cyclodextrin (SBE-β-CD), mono[6-(2-aminoethylamino)-6-deoxy]-β-cyclodextrin (en-β-CD) and mono{6-[2-(2-aminoethylamino)ethylamino]-6-deoxy}-β-cyclodextrin (dien-β-CD), were studied using phenolphthalein as a spectral probe. Spectral titration was performed in aqueous buffer solution (pH ca. 10.5) at 25 °C to determine the binding constants. The inclusion complexation behaviors were investigated in both solution and solid state by means of NMR, TG, XRD. The results showed that the water solubility and thermal stability of dihydroartemisinin were significantly increased in the inclusion complex with cyclodextrins (CDs). According to ¹H NMR and 2D NMR spectroscopy (ROESY), the A, B rings of dihydroartemisinin can be included into the cavity of CDs. The enhanced binding ability of CDs towards dihydroartemisinin was discussed from the viewpoint of the size/shape-fit concept and multiple recognition mechanism between host and guest.     

Conformations and complexation abilities of chemically modified cyclodextrins in aqueous solution

Circular dichroism spectral and fluorescence decay methods have been employed to determine the conformations of mono[6-(p-tolylseleno)-6-deoxy]-β-CD(1), mono(6-anilino-6-deoxy) −β -CD (2) and mono[6-(L-tryptophan)-6-deoxy]−β -CD (3) in phosphate buffer solution (pH 7.2, 0.1 mol dm⁻³) at 298.15 K. The results indicate that compounds 2 and 3 formed self-inclusion complexes in aqueous buffer solution, while the substituent of compound 1 was not included into cyclodextrin cavity at all. Furthermore, the complex stability constant (logK _s) and Gibbs free en-ergy change (−ΔAG °) of these three cylcodextrin derivatives with several cycloalkanols have been determined by circular dichroism spectral titration in phosphate buffer solution at 298.15 K. It is found that the location of the substituent affects the stability of host-guest complex in aqueous solution.     

Mimicking the Vancomycin Carboxylate Binding Site: Synthetic receptors for sulfonates,carboxylates, and N-protected α-amino acids in water

The novel H₂O-soluble cyclophanes 1 and 2 incorporating different anion-recognition sites were prepared in short synthetic routes (Schemes 1 and 2) as first-generation mimics of the natural, D -Ala-D -Ala binding antibiotic vancomycin. The X-ray crystal structure of 1 , a tris(hydrochloride)salt, revealed an open, preorganized cavity of sufficient size for the incorporation of small aliphatic residues (Fig. 3). In the crystal, molecules of 1 are arranged in parallel stacks, generating two types of channels, an ‘intra-stack’ channel passing through the cyclophane cavities and an ‘inter-stack’ channel located between cyclophane stacks (Fig. 4). The strongest intermolecular interactions between macrocycles in the crystal are C?O…?H? N H-bonds between the carboxamide residues of adjacent cyclophanes in neighboring stacks (Fig. 5). The ‘intra-’ and ‘inter-stack’ channels incorporate the three ordered Cl^? counterions and several, partially ordered solvent molecules (4 MeOH, 1 H₂O) (Fig. 6). Counterion Cl(2) is located within the ‘intra-stack’ channel and interacts with a protonated piperazinium N-atom and both ‘intra-stack’ MeOH molecules. The two other counterions, Cl(1) and Cl(3), are located within the ‘inter-stack’ channel. They are connected to two MeOH and one H₂O molecules and also interact both with the NH group of the protonated spiropiperidinium ring in 1 , forming an infinite, chain-like H-bonding network …?Cl(1)…?HOH…?MeOH…?Cl(3)…?HNH…?Cl(1′)…?. Both ‘intra-’ and ‘inter-stack’ MeOH molecules undergo weak CH…?π interactions with neighboring aromatic rings. Cyclophane 1 complexed aromatic sulfonates in 0.5M KCl/DCl buffer in D₂O, whereas the tetrakis(quaternary ammonium) receptor 2 bound the sodium salts of aliphatic and aromatic carboxylates and sulfonates, of N-acylated α-amino acids as well as of N-acetyl-D -alanyl-D -alanine (Ac-D -Ala-D -Ala), a substrate of vancomycin, in pure H₂O. In all of these complexes, ion pairing between the cationic recognition site in the periphery of the cyclophane receptor and the anionic substrates represents the major driving force for host-guest association. The ¹H-NMR analysis of complexation-induced changes in chemical shift clearly demonstrated that, in solution, this ion pairing exclusively takes place outside the cavity. Nevertheless, the macrocyclic bridges are essential for the efficiency of the anion-recognition sites in the two cyclophane receptors 1 and 2 . Control compounds 3 and 4 possess nearly the same anion-recognition sites than 1 and 2 , but lack their macrocyclic preorganization; as a consequence, they do not form stable ion-pairing complexes with mono-anionic substrates in the considered concentration ranges ( < 50 mM ) in D₂O.     

Surface plasmon resonance study on binding interactions of multivalent cyclophane hosts with immobilized guests

Guest‐binding affinities of water‐soluble cyclophane heptadecamer (1) and pentamer (2) with immobilized guests such as 1‐pyrenylmethylamine (PMA) and 2‐(1‐ naphthyl)ethylamine (NEA) were investigated by surface plasmon resonance (SPR) measurements. As a typical example, the binding constants (K) for 1 and 2 with the immobilized PMA as a guest were evaluated to be 2.5 × 10⁷ and 2.7 × 10⁶ M^?1, respectively, and were much larger than that of a monocyclic reference cyclophane (K, 2.5 × 10⁴ M^?1). Interestingly, in the complexation of 1 and 2 with the immobilized guests, more favorable association and dissociation rate constant values (k_a and k_d, respectively) were observed in comparison with those for the monocyclic cyclophane, reflecting multivalent effects in macrocycles. The multivalent effects in macrocycles as well as molecular recognition abilities of the cyclophane oligomers were confirmed even when the guest molecules were immobilized on SPR sensor chip surfaces. Copyright © 2009 John Wiley & Sons, Ltd.     

Syntheses of chiral bishomodiazacalix[4]arenes incorporating amino acid residues: Molecular recognition for racemic ammonium ions by the macrocycles possessing tyrosine residues

Chiral bishomodiazacalix[4]arenes containing amino acid residues were prepared. The ¹H and ¹³C nmr spectra indicated that the macrocycles preferably adopted a cone conformation, which suggested that the cyclophane moiety was in a chiral twisted form. Circular dichroism spectra supported the existence of the chirality of the cyclophane unit, and showed that intramolecular hydrogen bonding plays an important role in the transmission of the chirality from the amino acid residues to the cyclophane moiety. Macrocycles bearing a tyrosine residue have a π‐base cavity large enough to include the ammonium ion, and can serve as a shift reagent for the racemic ammonium ions upon complexation during a ¹H nmr analysis.     

Complexation of neurotransmitters ‒ dopamine,serotonin and melatonin ‒ with a DTPA-based cyclophane of high rigidity: 1H NMR shift and line-broadening

The ¹H NMR signals of the titled neurotransmitters undergo up-field shift accompanied by line-broadening in NMR titration with the DTPA-based cyclophane at pD 7.3; the cyclophane consists of a 4,4′-bis(1,1′-biphenyl-4,4′-dihydroxy)dianiline unit cyclised by a DTPA (diethylenetriaminepentaacetate) group through two amide linkages. Changes in chemical shifts of dopamine indicate the formation of a 1:1 complex with the formation constant K₁ 400 M^?1; the complex of serotonin is likely to form a 2:1 host?guest complex with β₂ ≈ 10⁵ M^?2; melatonin does not form a complex with definite stoichiometry. The primary binding forces in the dopamine and serotonin complexes are electrostatic interaction between cationic neurotransmitter and anionic cyclophane molecules, and the resulting ionic pairs are stabilised by encapsulation. The electrostatic interaction is weakened by electrolytes; in 0.1 M Trizma buffer, dopamine does not yield a definite complex, and serotonin forms a 1:1 complex with K₁ 80 M^?1. Extreme line-broadening of neurotransmitter signals suggests that the molecular motion of the guest molecule is slowed in the complex by interactions with the receptor molecule whose internal molecular motion is quenched partially. The high rigidity of the cyclophane enhances intermolecular interaction in the hydrophobic regions to prolong the lifetime of the complex.