Stable Host Guest Research Articles

Effect of molecular clips and tweezers on enzymatic reactions by binding coenzymes and basic amino acids

The tetramethylene-bridged molecular tweezers bearing lithium methanephosphonate or dilithium phosphate substituents in the central benzene or naphthalene spacer-unit and the dimethylene-bridged clips containing naphthalene or anthracene sidewalls substituted by lithium methanephosphonate, dilithium phosphate, or sodium sulfate groups in the central benzene spacer-unit are water-soluble. The molecular clips having planar naphthalene sidewalls bind flat aromatic guest molecules preferentially, for example, the nicotinamide ring and/or the adenine-unit in the nucleotides NAD(P)+, NMN, or AMP, whereas the benzene-spaced molecular tweezers with their bent sidewalls form stable host–guest complexes with the aliphatic side chains of basic amino acids such as lysine and argenine. The phosphonate-substituted tweezer and the clips having an extended central naphthalene spacer-unit or extended anthracene and benzo[k]fluoranthene sidewalls, respectively, form highly stable self-assembled dimers in aqueous solution, evidently due to non-classical hydrophobic interactions. The phosphate-substituted molecular clip containing naphthalene sidewalls inhibits the enzymatic, ADH-catalyzed ethanol oxidation by binding the cofactor NAD+ in a competitive reaction. Surprisingly, tweezer-bearing phosphate substituents in the central benzene spacer-unit are more efficient inhibitors for the ethanol oxidation than the correspondingly substituted naphthalene clip, even though the tweezer does not bind the cofactor NAD+ within the limits of detection. The phosphate-substituted naphthalene clip is, however, a highly efficient inhibitor of the enzymatic oxidation of glucose-6-phosphate (G6P) with NADP+ catalyzed by glucose-6-phosphate dehydrogenase (G6PD), whereas the phosphonate-substituted clip only functions as an inhibitor by forming a complex with the cofactor. Detailed kinetic, thermodynamic, and computational modeling studies provide insight into the mechanism of these novel enzyme inhibition reactions.

Development of an electroactive layer-by-layer assembly based on host–guest supramolecular interactions

The development of a novel electrochemically stable host–guest supramolecular complex at a host surface is described. It was constructed by combining a self-assembled monolayer (SAM) of mono-(6-deoxy-6-mercapto)-β-cyclodextrin (βCDSH), iron (III) tetra-(N-methyl-4-pyridyl)-porphyrin (FeTMPyP) as the guest-link-molecule and β-cyclodextrin-functionalized gold nanoparticles (βCDAuNP). The building process of the layer-by-layer assembly was monitored by surface plasmon resonance spectroscopy (SPR). The binding processes between the host-functionalized gold surface and the linker molecule (FeTMPyP) were verified to be monovalent, and for host-functionalized gold nanoparticles with FeTMPyP, the interaction was determined to be bivalent. Finally, the electrochemical properties of the electroactive supramolecular multivalent film were determined.

Inclusion complex of riboflavin with cucurbit[7]uril: study in solution and solid state

The aqueous solution of riboflavin and cucurbit[7]uril complex has been studied based on fluorescence and 1H NMR spectroscopic results. Upon addition of cucurbit[7]uril, the fluorescence intensity of riboflavin was quenched and a slight red shift was observed for the maximum emission peak. These results indicated that the cucurbit[7]uril–riboflavin complex was formed at a 1:1 mole ratio. The temperature-dependent inclusion constants were calculated, from which ΔH and ΔS values were calculated. Meanwhile, rationale of the interaction mechanism was also discussed based on 1H NMR results. The solid inclusion complex was prepared from co-evaporation method and characterised by differential thermal analysis and fluorescence lifetime analysis methods. The experimental results indicated that riboflavin and cucurbit[7]uril formed stable host–guest inclusion complex in both solution and solid states.

Encapsulation of a β-carboline in cucurbit[7]uril

Inclusion of a biological photosensitizer and prototype of β-carbolines, norharmane (NHM), into the cavity of cucurbit[7]uril (CB[7]) has been investigated for the first time, by using 1H NMR and UV–visible spectroscopy, and ab initio calculations. Protonated NHM forms a very stable host–guest complex with CB[7] in aqueous solution, with a binding constant of (9.0 ± 0.5) × 104 M−1. The encapsulation of NHM into CB[7] has driven the prototropic equilibrium of NHM to protonated NHM (NHMH+) at neutral pH. A pH titration for the host–guest complex revealed a moderate shift of the acid–base equilibrium in the ground-state (from 7.2 to 7.9), which may be caused by the low polarity microenvironment of the CB[7] cavity. The CB[7] provides a binding pocket for the hydrophobic molecule, and the polar, carbonyl-lined portals offering an anchoring site for the positive charge of the cationic species NHMH+.

Separation of dimethylcyclohexane stereoisomers by selective guest inclusion of host compound of guanidinium o-terphenyl-4,4′-disulfonate

Using the molecular recognizability of guanidinium o-terphenyl-4,4′-disulfonate (G 2 o-TPDS) host compound to guests, the stereoisomers of dimethylcyclohexane (DMCH) were separated. In a binary competition of cis- and trans-isomers of 1,2-, 1,3- and 1,4-DMCH, it was found that the equatorial, equatorial (e,e) conformers ( trans-1,2-, cis-1,3- and trans-1,4-DMCH) were predominantly selected as guests to the host inclusion against the axial, equatorial (a,e) conformers ( cis-1,2-, trans-1,3- and cis-1,4-DMCH) due to the spatial configuration of the nano-cavity in the host frame. Furthermore, among the three e,e-conformers, trans-1,2- and trans-1,4-DMCH were preferentially accommodated to the host frame rather than cis-1,3-DMCH. However, there was little guest selectivity of the host frame between trans-1,2- and trans-1,4-DMCH. From powder X-ray diffraction and differential scanning calorimetry of the host–guest inclusion compounds, it was revealed that trans-1,2- and trans-1,4-DMCH were guest templates for stronger G 2 o-TPDS host-frame structure than other isomers, resulting in the more stable host–guest inclusion compounds of G 2 o-TPDS ( trans-1,2-DMCH) and G 2 o-TPDS ( trans-1,4-DMCH). So, they required higher energy and onset temperature to release guest molecules from the host frames than other inclusion compounds. This thermal analysis of the host–guest inclusion compounds was consistent with the guest selectivity of the host frame in the binary mixture, implying the potential method to predict the selective guest in the inclusion compound in the guest mixture.

An Extremely Stable Host–Guest Complex That Functions as a Fluorescence Probe for Calcium Ions

Herein, we report a crown ether based molecular cage that forms extremely stable supramolecular complexes with dimethyldiazapyrenium (DMDAP) ions in CD(3)CN through the collaboration of multiple weak C-HO hydrogen bonds. The very strong binding affinity in this host-guest system allows the molecular cage to bleach the fluorescence signal of DMDAP substantially in equimolar solutions at concentrations as low as 1 x 10(-5) M. Remarkably, a 1x10(-5) M equimolar solution of the molecular cage and DMDAP is highly selective toward Ca(2+) ions-relative to other biologically important Li(+), Na(+), K(+), and Mg(2+) ions-and causes a substantial increase in the fluorescence intensity of the solution. As a result, this molecular cage/DMDAP complex behaves as a supramolecular fluorescence probe for the detection of Ca(2+) ions in solution.

Highly Stable Host–Guest Photorefractive Polymer Composite with Low Glass Transition Temperature

We studied the phase stability and photorefractive properties of a new poly(N-vinylcarbazole) (PVK)-based photorefractive polymer composite by modifying the structure of a nonlinear optical chromophore (NLO), 4-butoxy-3-propyl-1-(4'-nitrophenyazo)benzene. The shelf life of a sample with a modified NLO, 4-(2-ethylhexyloxy)3-propyl-1-(4'-nitrophenyazo) benzene was extended by more than five months even though it had a low glass transition temperature (Tg) of -19°C. This material also had a short photorefractive response time constant of 33 ms and a high diffraction efficiency of 49% at an electric field of 100 V/µm and a total writing intensity of 942 mW/cm2. These desirable characteristics are believed to be due to the following reasons: first, the new chromophore acts as a plasticizer, thereby enhancing the orientation of the NLO and second, the solubility of the chromophore in a polymer matrix increases, inhibiting crystallization and providing a high optical quality.

Synthesis and Supramolecular Properties of Molecular Clips with Anthracene Sidewalls

Novel molecular clips with anthracene sidewalls (1 a-c) were synthesized; they form stable host-guest complexes with a variety of electron-deficient aromatic and quinoid molecules. According to single-crystal structure analyses of clip 1 c and 1,2,4,5-tetracyanobenzene (TCNB) complex 14@1 b, the clips' anthracene sidewalls have to be compressed substantially during the complex formation to provide attractive pi-pi interactions between the aromatic guest molecule and the two anthracene sidewalls in the complex. The compression and expansion of aromatic sidewalls are calculated by molecular mechanics to be low-energy processes, so the energy required for compression of the anthracene sidewalls during complex formation is apparently overcompensated by the gain in energy resulting from the attractive pi-pi interactions. The finding that complexes of the clips 1 a-c are more stable than those of the corresponding clips 2 a-c can be explained in terms of the larger van der Waals contact surfaces of the anthracene sidewalls in 1 a-c (relative to the naphthalene sidewalls in 2 a-c). Color changes resulting from charge-transfer (CT) bands are observed in complex formation by 1 a-c: from colorless to red or purple with TCNB (14), and from yellow to green with 2,4,7-trinitro-9-fluorenone TNF (17). Independently, the host 1 b and guest 14 fluoresce from their respective excited singlet states, whilst in the complex 14@1 b the charge-transfer state quenches the higher-energy singlet states of the two components, and as a result luminescence is only observed from this new CT state. To the best of our knowledge, complex 14@1 b is the first example of CT luminescence from a host-guest complex. The binding constant determined for the formation of the TCNB complex 14@1 b from a UV/Vis titration experiment (Ka = 12 400 m(-1)) agrees well with the value (K(a) = 12 800 m(-1)) obtained by 1H NMR titration.

Facile design of poly(3,4-ethylenedioxythiophene)-tris(2,2′-bipyridine)ruthenium (II) composite film suitable for a three-dimensional light-harvesting system

It has been confirmed that octasulfonatocalix[8]arene (Calx-S8) and tris(2,2′-bipyridine)ruthenium (II) (Ru(bpy) 3 2+) can form a stable host–guest complex in aqueous solution. The binding constant for 1:1 [Calx-S8 8−·Ru(bpy) 3 2+] 6− complex formation was estimated to be (2.4±0.8)×10 4 dm 3 mol −1 by fluorescence titration, which indicates that the [Calx-S8 8−·Ru(bpy) 3 2+] 6− complex is the main species in 1:1 molar ratio aqueous solution of Calx-S8 and Ru(bpy) 3 2+. In situ UV–Vis spectroscopic measurements indicated that Ru(bpy) 3 2+ complexes can be readily deposited onto ITO electrode through electrochemical polymerization of 3,4-ethylenedioxythiophene (EDOT) using [Calx-S8 8−·Ru(bpy) 3 2+] 6− host–guest complex as a dopant anion owing to the electrostatic interaction between the cationic conductive polymer and the anionic host–guest complex. The loading degree of the composite film with Ru(bpy) 3 2+ can be determined by Lambert-Beer law modified for the two-dimensional concentration. The obtained composite film showed good photoelectric conversion properties in response to visible light irradiation. This is a novel photocurrent generation system in which the photoexcited state energy is efficiently collected by the conductive polymeric layer.

A stable host–guest electro-optic polymer system with polyisoimide as a host

A stable host–guest electro-optic (EO) polymer system has been prepared for improving not only the EO effect but also the thermal stability of the effect using a polyisoimide as a host. Disperse Red 1 (DR1) and a polyisoimide (PII) synthesized from 2,2-bis(4-aminophenyl)hexafluoropropane and oxydiphthalic anhydride were used as the guest and the host, respectively. Analyses of a model compound reaction and the host–guest film after annealing at various temperatures show that the reaction between the isoimide groups in the PII and the hydroxyl groups in DR1 occurs at around 140 °C. The EO coefficient obtained at a wavelength of 1.55 μm was 13.7 pm/V from an EO polymer film with 30 wt% of DR1 as added guest. The EO signal was thermostable at high temperature due to the chemical reaction between the host and guest during poling.

The Role of Guest Molecules in the Self‐Assembly of Metal—Ligand Clusters

AbstractFor Abstract see ChemInform Abstract in Full Text.

Stabilization of a K + -( bis -Cage-Annulated 20-Crown-6) Complex by Bidentate Picrate

Bis-cage-annulated 18-crown-6 and 20-crown-6 macrocyclic ethers (i.e., 1 and 2, respectively) have been synthesized, and their alkali metal picrate extraction profiles have been determined. Host system 1 proved to be a significantly more avid alkali metal cation complexant than 2 and somewhat more avid than 18-crown-6. Both 1 and 18-crown-6 display modest selectivity toward K+ and Rb+. A stable host–guest complex was prepared by slow evaporation of a CH2Cl2–hexane solution of an equimolar mixture of 2 and potassium picrate. The X-ray crystal structure of this complex reveals that picrate anion functions as a bidentate ligand therein. The gas-phase interaction energy between the 2 ⋅ K+ complex and picrate anion was calculated to be ca. −64.9 kcal mol−1, thereby indicating that participation of picrate anion as an additional bidentate ligand results in significant stabilization of complex 10.

Supramolecular recognition: on the kinetic lability of thermodynamically stable host-guest association complexes.

A molecular receptor consisting of a spacer bearing two cofacially disposed terpyridyl-palladium-ligand (terpy-Pd-L) units rigidly separated by about 7 A has been investigated for molecular recognition of planar aromatic molecules. It is found that although the receptor forms stable 1:2 host-guest association complexes with 9-methylanthracene (9-MA), the guest undergoes very rapid site exchange within the receptor and with external free 9-MA. A crystal structure of the 2:1 adduct shows one 9-MA in the molecular cleft defined by the two terpy-Pd-L units and the other resides on an outside face of one terpy-Pd-L unit. To establish the site residency time of the guests, a number of tethered molecules were prepared. These involve an anthracene molecule tethered to a pyridine ligand bound to the palladium atoms to form intramolecular host-guest adducts. Rotating-frame Overhauser effects were used to infer the site residency of the anthracene guests in the receptor. Variable-temperature (1)H NMR spectroscopy of the intramolecular host-guest complexes has revealed that the site residency time of the anthracene guests is 1.6 x 10(-5) sec at 20 degrees C and 1.3 sec at -90 degrees C in acetone solution. Whereas the guests are thermodynamically stable, they are kinetically very labile. A crystal structure of one of the tethered host-guest adducts reveals the expected structure which is the same as that determined in solution by (1)H rotating-frame Overhauser enhancement spectroscopy experiments.

The Role of Guest Molecules in the Self-assembly of Metal—ligand Clusters

Understanding the self-assembly of nanoscale metal—ligand clusters is an important research area in supramolecular chemistry, especially, if one wishes to develop a truly predictive design strategy for synthesizing these nanoscale clusters. As the building blocks for forming these clusters have become larger and more complex, spacious clusters have been synthesized which often contain large cavities. These assemblies can house guest molecules which play a previously uncharacterized role in the self-assembly processes. We seek to analyze this role: do these guest molecules act as templates? Are the guest molecules necessary for cluster formation? Does the guest drive cluster assemble by forming a stable host—guest complex with the cluster? Must a truly rational design strategy for forming metal—ligand clusters incorporate the use of templates? The role of guest molecules in the self-assembly of nanoscale coordination clusters is reviewed in this article.

Host–guest interaction of calixarene molecules with neutral benzotrifluorides: Comparison of luminescence spectral data with results of model calculations relating to complex formation

The ‘host’ properties of calix[4]arene and 4- tert-butyl-calix[6]arene molecules under the effect of different neutral pesticide related trifluoromethyl-benzene molecules as ‘guests’ have been investigated by spectrofluorometric method. Significant dependence of the photoluminescence behaviour of calixarene derivatives on the structure of interactive molecules was detected in chloroform and dimethylformamide solvent. The formation of stable host–guest complexes was checked by quantum-chemical method. The results show that the photoluminescence signal reflects the supramolecular interaction. This makes calixarenes promising candidates as host molecules to be used in chemical sensors measuring neutral organic molecules.

A highly selective host–guest system formed and stabilized due to concerted halogen‥oxygen and C-H‥O non-bonded interactions: X-ray structures of racemic 1,2,3,4,5-penta-O-benzoyl-6-O-tosyl myo-inositol–dihalomethane (CH2X2, X = Cl and Br) inclusion complexes

myo-Inositol derivative as a host assembles around crystallographic 2-fold axis selectively accommodating dihalomethanes as guests having a C2 symmetry; formation of highly stable host–guest complexes is attributed to halogen⋯oxygen and C–H⋯O interactions.

Multilayered supermolecular structures self-assembled from polyelectrolytes and cyclodextrin host–guest complexes

Supermolecular multilayers with high levels of compositional and structural complexities have been fabricated with an example that involves formation of stable host–guest complexes from bolaamphiphiles and cyclodextrins, followed by sequential build-up of multilayered films by these complexes in combination with anionic polystyrene sulfonate polymer chains.

Chiral Recognition of (R)‐(−)‐1,7‐Dioxaspiro‐[5.5]undecane by Hexakis(2,3,6‐tri‐O‐methyl)‐α‐cyclodextrin

Even on a large scale an aqueous solution of 1,7-dioxaspiro[5.5]undecane and a tailored cyclodextrin selectively yields the inclusion complex with the (R) guest (shown on the right), the structure of which was studied both in the solid state and in solution. The yields are higher than would be expected for the racemate because the (S) enantiomer of the guest is transformed into the (R) enantiomer, which forms the more stable host–guest complex.

Inclusion behaviour of azacyclophanes holding various hydrophobic cavities

Abstract