Enhancing effectiveness of capillary electrophoresis as an analytical tool in the supramolecular acidity modification

A strategic modification of acidity (pK_a values) by the non-covalent host-guest interactions is one of the most promising concepts in current supramolecular chemistry. This work is aimed at enhancing the effectiveness of capillary electrophoresis (CE) in determination of pK_a shifts caused by such interactions and their thermal dependencies crucial in a deep thermodynamic description. We show how to (i) minimize the systematic errors related to Joule heating, (ii) minimize the influence of a voltage ramp time, (iii) speed up pK_a shift identification and estimation, (iv) interpret thermal effects related to two overlapped dynamic equilibria, and (v) determine pK_a shifts by an alternative spectrophotometric method (CE-DAD). The proposed solutions were implemented to examine the supramolecular pK_a shifts of several coumarin derivatives, caused by a variety of structurally different cyclodextrins. It was revealed that a specific host substitution pattern determines the magnitude of apparent pK_a shifts. Accordingly, heptakis(2,6-di-O-methyl)-β-cyclodextrin induces the much stronger shifts than both non-methylated-β-cyclodextrin and heptakis(2,3,6-tri-O-methyl)-β-cyclodextrin applied at the same concentration. We also show that insofar as the complexation of 4-hydroxycoumarin and its derivative (coumatetralyl) are similarly exothermic, the thermal effects accompanying the deprotonation process are remarkably different for both molecules. The pK_a shift induced by complexation with calixarene was also for the first time determined by a CE method. These observations throw a new light on the background of acidity modification and confirm the applicability of CE as an analytical tool.

Revealing Nanostructures through Plasmon Polarimetry

Polarized optical dark-field spectroscopy is shown to be a versatile noninvasive probe of plasmonic structures that trap light to the nanoscale. Clear spectral polarization splittings are found to be directly related to the asymmetric morphology of nanocavities formed between faceted gold nanoparticles and an underlying gold substrate. Both experiment and simulation show the influence of geometry on the coupled system, with spectral shifts Δλ = 3 nm from single atoms. Analytical models allow us to identify the split resonances as transverse cavity modes, tightly confined to the nanogap. The direct correlation of resonance splitting with atomistic morphology allows mapping of subnanometre structures, which is crucial for progress in extreme nano-optics involving chemistry, nanophotonics, and quantum devices.

Modulation in the acidity constant of acridine dye with cucurbiturils: stimuli-responsive pKa tuning and dye relocation into live cells

Cucurbit[7/8]uril–acridine complexes result in striking modulation of the excited state properties of acridine and allowed a tunable upward pK_a shift, which is applied for controlled relocation of the dye from the host to live cells.

Metal ion-induced supramolecular pKa tuning and fluorescence regeneration of a p-sulfonatocalixarene encapsulated neutral red dye

Supramolecular pK_a shift and fluorescence quenching in a neutral red dye in the presence of p-sulfonatocalix[4/6]arenes have been demonstrated, which are relevant for the off–on switch, ion sensitive electrodes and drug delivery vehicles.

Detection, inhibition and disintegration of amyloid fibrils: the role of optical probes and macrocyclic receptors

This article provides a brief account of the recent reports on the early detection of amyloid fibril formation using fluorescent dyes and inhibition and disintegration of fibrils using macrocyclic receptors, which find applications in the treatment of fibril associated neurodegenerative diseases.

Light-induced piston nanoengines: ultrafast shuttling of a styryl dye inside cucurbit[7]uril

A proof of principle for an ultrafast molecular shuttle based on the light-operated movement of a styryl dye inside cucurbit[7]uril was described.

Cyclodextrin-assisted modulation of the photophysical properties and acidity constant of pyrene-armed calix[4]arene

The supramolecular pK_a shift and modulation in the monomer and excimer emission behaviour of pyrene-armed calixarene with cyclodextrins find applications in ratiometric sensing.

Host-Guest Complexes of Carboxylated Pillar[n]arenes With Drugs

Pillar[n]arenes are a new family of nanocapsules that have shown application in a number of areas, but because of their poor water solubility their biomedical applications are limited. Recently, a method of synthesizing water-soluble pillar[n]arenes was developed. In this study, carboxylated pillar[n]arenes (WP[n], n = 6 or 7) have been examined for their ability to form host-guest complexes with compounds relevant to drug delivery and biodiagnostic applications. Both pillar[n]arenes form host-guest complexes with memantine, chlorhexidine hydrochloride, and proflavine by ¹H nuclear magnetic resonance and modeling. Binding is stabilized by hydrophobic effects within the cavities, and hydrogen bonding and electrostatic interactions at the portals. Encapsulation within WP[6] results in the complete and efficient quenching of proflavine fluorescence, giving rise to "on" and "off" states that have potential in biodiagnostics. The toxicity of the pillar[n]arenes was examined using in vitro growth assays with the OVCAR-3 and HEK293 cell lines. The pillar[n]arenes are relatively nontoxic to cells except at high doses and after prolonged continuous exposure. Overall, the results show that there could be a potentially large range of medical applications for carboxylated pillar[n]arene nanocapsules.

Synthesis of novel TEMPO stable free (poly)radical derivatives and their host–guest interaction with cucurbit[6]uril

The line shape analysis of the ESR spectra of TEMPO derivatives in the absence and in the presence of cucurbit[6]uril provides information on host–guest complexes.

Encapsulation of rhodamine-6G within p-sulfonatocalix[n]arenes: NMR, photophysical behaviour and biological activities

Partial inclusion of rhodamine-6G within sulfonatocalixarene macrocycles engenders fluorescence quenching and demonstrates remarkable antiproliferative activities.

High-efficiency cucurbit[7]uril capillary column for gas chromatographic separations of structural and positional isomers

The CB7–OV capillary column by incorporation of CB7 into OV-210 achieved extremely high column efficiency and outstanding resolving performance for various types of structural and positional isomers.

Recognition-mediated cucurbit[7]uril-heptamolybdate hybrid material: a facile supramolecular strategy for99mTc separation

Employing a novel cucurbit[7]uril (CB7)-heptamolybdate hybrid material developed as a solid generator bed, we demonstrate an efficient and facile supramolecular strategy to separate the^99mTc nuclide from⁹⁹Mo solution for its use in theranostic applications.

Mixed-metal metallocavitands: a new approach to tune their electrostatic potentials for controllable selectivity towards substituted benzene derivatives

We have successfully developed a new synthetic approach to modulate the electrostatic potentials of metallocavitands and thus their selective recognition towards substituted benzene derivatives via integrating two metal cations of different electronegativity into a self-assembled system.

Metal-ion-mediated assemblies of thiazole orange with cucurbit[7]uril: a photophysical study

The formation of molecular superstructures by metal-ion-mediated noncovalent self-assembly has been demonstrated using the macrocycle, cucurbit[7]uril (CB7), and the dye, thiazole orange (TO), as building blocks. Interestingly, the association of these molecular building blocks can be tuned by the chemical environment, leading to self-assembled structures of different stoichiometries, which is supported by absorption, fluorescence, (1)H NMR, and AFM measurements. Most importantly, the self-assembly process of the CB7/TO/metal ion system is observed to be remarkably different for alkali (Na(+)) and alkaline earth (Ca(2+)) metal ions. Fluorescence enhancement is observed in the presence of Ca(2+) ions, which is attributed to the formation of short dimeric structures composed of two 1:1 CB7-TO complexes. Solution turbidity is detected in the presence of Na(+) ions, which is proposed to be due to the formation of extended structures by the assembly of many 1:1 CB7-TO complexes.

Molecular binding behavior of bipyridium derivatives by water-soluble carboxylato-biphen[3]arene

Complexation of paraquat derivatives and bis(N-mono-substituted bipyridine) cations by a negatively charged carboxylato-biphen[3]arene in water is described.

Modulation of pKa by cyclodextrins; subtle structural changes induce spectacularly different behaviors

Supramolecular acidity tuning: it is shown that minor structural changes of guests and hosts molecules may be of great importance for the complexation-induced pK_a shifts and their temperature dependency.

Cucurbit[8]uril-templated H and J dimers of bichromophoric coumarin dyes: origin of contrasting emission

Supramolecular control of the novel dimer emission from bichromophoric coumarin dyes caught in the cucurbit[8]uril cavity; a facile approach to developing photo-responsive molecular assemblies.

Selective stepwise oxidation of 1,4-decamethoxypillar[5]arene

Pillar[n]arene[m]quinone was synthesized in high yields, and a rule for determining its structure based on the NMR spectra was deduced.

Enthalpy–entropy relations in the acid–base equilibrium of warfarin and 10-hydroxywarfarin; joint experimental and theoretical studies

Warfarin and 10-hydroxywarfarin are structurally similar molecules, however, they exhibit considerably different thermodynamics of acid dissociation. Intramolecular H-bonds and solvent composition are the factors of great importance.

Triple emission from p-dimethylaminobenzonitrile-cucurbit[8]uril triggers the elusive excimer emission

The intriguing dual-emission behavior of p- dimethylaminobenzonitrile (DMABN) and the identity of the associated excited states is, arguably, the most extensively investigated and also controversially discussed molecule- specific phenomenon of modern photochemistry. We have now found a new, third fluorescence band when DMABN is encapsulated within the water-soluble molecular container cucurbit[8]uril (CB8). It is centered between the previously observed emissions and assigned to the elusive excimer emission from DMABN through 1:2 CB8:DMABN complex formation. Heating of the CB8⋅(DMABN)2 complex from 0 to 100 °C results in the dissociation of the ternary complex and restoration of the dual-emission properties of the monomer. Alternatively, monomer emission can be obtained by selecting cucurbit[7]uril (CB7), a host homologue that is too small to accommodate two DMABN molecules, or by introducing ethyl instead of methyl groups at the amino terminus of the aminobenzonitrile guest.

Stimuli-responsive supramolecular micellar assemblies of cetylpyridinium chloride with cucurbit[5/7]urils

This article demonstrates, for the first time, construction of novel cucurbituril (CB)-adorned supramolecular micellar assemblies of a cationic surfactant, cetylpyridinium chloride (CPC), through noncovalent host-guest interactions. The distinct cation receptor features and cavity dimensions of the CB5 and CB7 homologues assert that the macrocyclic hosts remain complexed with the CPC monomers and take part in the micelle formation, a unique observation in contrast to that of the classical host, β-cyclodextrin. The cooperative contributions of the CB macrocycles in the micelle formation have been documented by the photochemical, surface tension, conductivity, DOSY NMR, and SANS measurements. The contrasting downward and upward shifts in the cmc of the CPC surfactant, respectively, with CB5 and CB7 hosts provide a unique opportunity for the controlled tuning of the micellization region for CPC from 0.57 to 1.6 mM, by using a combination of the macrocyclic hosts. The article also establishes the reversible response of these soft supramolecular micellar structures to thermal-stimuli, which projects their utility for on-demand smart drug-delivery vehicles.