Zsombor Miskolczy

Photochromism in cucurbit[8]uril cavity: inhibition of hydrolysis and modification of the rate of merocyanine-spiropyran transformations.

The effect of inclusion complex formation on the photochromic behavior of a spirobenzopyran dye and its merocyanine isomer was studied in aqueous solution using cucurbit[8]uril (CB8) as a host. The merocyanine (MC) and protonated merocyanine (MCH(+)) were the most stable forms both in water and inside the cavity of CB8. The equilibrium constant of 1:1 complexation with CB8 was found to be 1.7 × 10(5) and 2.0 × 10(6) M(-1) for the former and latter species, respectively. The encapsulation led to significant change in the rate of the photoinduced and thermal photochromic transformations and hindered the hydrolysis of MC. The effect of CB8 on the reaction kinetics strongly altered with pH. The transition from the spiropyran form to trans-MC in a thermal reaction had 33 kJ mol(-1) lower activation energy and more than 5 orders of magnitude smaller Arrhenius pre-exponential factor in CB8 than in water.

Kinetics and Thermodynamics of Berberine Inclusion in Cucurbit[7]uril

The kinetics and thermodynamics of berberine inclusion in cucurbit[7]uril was studied by stopped-flow method, fluorescence titrations, and isothermal calorimetry in neat water. The ~500-fold fluorescence intensity enhancement upon encapsulation was exploited to monitor the complex formation in real time at various temperatures. The increase in the fluorescence intensity could be fitted well by assuming a simple 1:1 binding equilibrium without any intermediate formation. For the rate constants of association and dissociation, (1.9 ± 0.1) × 10(7) M(-1) s(-1) and (0.81 ± 0.08) s(-1) were found at 298 K, respectively. The ingression into the cavity of CB7 had 32 ± 2 kJ mol(-1) activation enthalpy, implying a constrictive binding, whereas 69 ± 2 kJ mol(-1) was obtained for the activation enthalpy of the egression. Substantial structural change had to occur when berberine passed through the tight carbonyl-rimmed portal of the macrocycle to reach the transition state. An enthalpy-driven complexation took place with a slight entropy gain.

4-Sulfonatocalix[6]arene-induced aggregation of ionic liquids

The interaction of 4-sulfonatocalix[6]arene (SCX6) macrocycle with 1-alkyl-3-methylimidazolium type of ionic liquids possessing dodecyl, tetradecyl, or hexadecyl substituent was studied in aqueous solution at 298 K. Host-guest complexation promoted the spontaneous self-assembly into nanoparticles of 7:1 ionic liquid:SCX6 stoichiometry. Positively charged and stable nanoparticles were produced in solutions of 7-200-fold excess of ionic liquid as compared to the amount of SCX6. The negatively charged nanoparticles formed in solutions having 2-7 ionic liquid:SCX6 molar ratios evolved into larger species. The stability of the nanoparticles increased with the lengthening of aliphatic chain of the ionic liquid. Cryo-TEM experiments showed dense particles generally with spherical shape and multilayered structure, which has been confirmed by small-angle neutron scattering.

Thermodynamics of inclusion complex formation between 1-alkyl-3-methylimidazolium ionic liquids and cucurbit[7]uril

The thermodynamics of 1:1 inclusion complex formation between 1-alkyl-3-methylimidazolium type ionic liquids and cucurbit[7]uril was studied by isothermal titration calorimetry in aqueous solution at 298 K. The encapsulation proved to be enthalpy driven for all cations used. The enthalpy change upon binding (ΔH) became more negative when the 1-alkyl moiety of the imidazolium ring was gradually lengthened reaching the most exothermic association with the hexyl derivative. Further increase of the number of carbon atoms in the aliphatic chain led to less negative ΔH values. The much smaller entropy change followed the trend of ΔH. The slope of the linear enthalpy–entropy correlation found in the present work is significantly smaller than that reported previously for cyclodextrin complexes, because the more rigid CB7 macrocycle cannot undergo significant conformational change upon complexation.

Photochromism of a merocyanine dye bound to sulfonatocalixarenes: effect of pH and the size of macrocycle on the kinetics.

The effect of 1:1 complex formation on the photochromic behavior of the merocyanine isomer of a nitro-substituted spirobenzopyran dye was studied in aqueous solution using 4-sulfonatocalixarene (SCXn) cavitands possessing four or eight phenol units. The binding constants were independent of the size of the macrocycle, and about 7-fold more stable associates were produced at pH 2.3 than in slightly alkaline solution. The complexation with SCXn diminished the acidity of protonated merocyanine (trans-MCH(+)) and precluded its photoinitiated transition to spirobenzopyran form, but did not affect the reactions in basic media. Upon exposure to light, the complexed trans-MCH(+) was converted to cis isomer. The association with 4-sulfonatocalix[4]arene slowed down the thermal back reaction in the dark to a larger extent than the confinement to 4-sulfonatocalix[8]arene. Both the activation energy and the Arrhenius A factor were significantly larger when the smaller, more rigid macrocycle served as a host.

Reversible Nanoparticle–Micelle Transformation of Ionic Liquid–Sulfonatocalix[6]arene Aggregates

The effect of temperature and NaCl concentration variations on the self-assembly of 1-methyl-3-tetradecylimidazolium (C14mim(+)) and 4-sulfonatocalix[6]arene (SCX6) was studied by dynamic light scattering and isothermal calorimetric methods at pH 7. Inclusion complex formation promoted the self-assembly to spherical nanoparticles (NP), which transformed to supramolecular micelles (SM) in the presence of NaCl. Highly reversible, temperature-responsive behavior was observed, and the conditions of the NP-SM transition could be tuned by the alteration of C14mim(+):SCX6 mixing ratio and NaCl concentration. The association to SM was always exothermic with enthalpy independent of the amount of NaCl. In contrast, NPs were produced in endothermic process at low temperature, and the enthalpy change became less favorable upon increase in NaCl concentration. The NP formation was accompanied by negative molar heat capacity change, which further diminished when NaCl concentration was raised.

Photooxidation of Alkaloids: Considerable Quantum Yield Enhancement by Rose Bengal-sensitized Singlet Molecular Oxygen Generation

The photooxidation of sanguinarine, coralyne and berberine was studied in oxygenated alkaline methanol solutions. Rose bengal as photosensitizer significantly accelerates the process, indicating the importance of singlet molecular oxygen in the reaction mechanism. The quantum yield of sensitized oxidation was found to increase significantly with pH and reaches 0.4 for berberine at pH 13.8. The direct oxidation of alkaloids is less efficient, the quantum yield does not exceed 0.01 even in oxygen‐saturated solutions. The photoinduced electron ejection does not play a role in the oxidation. The uncharged pseudobase forms, which are present in alkaline medium, are oxidized much more easily than the alkaloid cations.

Effect of Macrocycle Size on the Self-Assembly of Methylimidazolium Surfactant with Sulfonatocalix[n]arenes

The effect of macrocycle size on the association of supramolecular amphiphiles composed of 4-sulfonatocalix[n]arene and 1-methyl-3-tetradecylimidazolium (C14mim+) was studied in aqueous solutions at pH 7. When the cavitand contained four sulfonatophenol units (SCX4), formation of spherical nanoparticles (NPs) was observed. By contrast, both supramolecular micelle (SM) and NP formation could be attained in the presence of NaCl when the larger, more flexible 4-sulfonatocalix[8]arene (SCX8) served as the host compound. The SCX8-promoted self-assembly into the SM was enthalpically more favorable than the NP production, but the molar heat capacity changes in the two processes barely differed. An addition of 50 mM NaCl significantly increased the enthalpy of C14mim+-SCX8 NP formation, thereby making the self-organization into the SM more favorable. The transformation of SM into NP at high temperatures was due to the substantial entropic contribution to the driving force behind the NP formation. The critical micelle concentration (cmc) and the local polarity in the headgroup domain were considerably lower for the SM compared with those of the conventional C14mim+Br- micelle.

Effect of electrolytes, nucleotides and DNA on the fluorescence of flavopereirine natural alkaloid

The absorption and fluorescence characteristics of flavopereirine, a pharmaceutically important natural alkaloid, were studied to reveal how the complex formation and the change of the microenvironment affect the deactivation kinetics from the singlet-excited state. The fluorescence lifetime was not influenced by the ionic strength, but a significant deuterium effect was observed showing that hydrogen bonding in the singlet-excited state promoted energy dissipation. Nucleotides caused both static and dynamic quenching. The rate constant of the latter process increased when the nucleobase was capable of donating electron to the excited flavopereirine. The spectrophotometric measurements provided evidence for non-cooperative binding to double-stranded DNA with an equilibrium constant of 4.6 × 10(5) M(-1). Time-resolved fluorescence signals showed that three kinds of complexes are formed with distinct fluorescence lifetimes. Flavopereirine binding to chondroitin sulfate was also found, which led to different fluorescence characteristics at pH 2 and 6.

Effect of Headgroup Variation on the Self-assembly of Cationic Surfactants with Sulfonatocalix[6]arene

The effect of headgroup variation on the association of supramolecular amphiphiles composed of 4-sulfonatocalix[6]arene (SCX6) and cationic surfactant possessing tetradecyl substituent was studied in aqueous solutions at pH 7. When the surfactant contained hydrophilic trimethylammonium, pyridinium, or 1-methylimidazolium headgroup, highly reversible temperature-responsive nanoparticle-supramolecular micelle transformation could be attained at appropriately chosen component mixing ratios and NaCl concentrations. In these cases, the substantial negative molar heat capacity change (ΔCp) rendered nanoparticle formation strongly endothermic at low temperature, whereas the assembly to supramolecular micelle was always accompanied by enthalpy gain. The ΔCp values became less negative when the charge density and the hydrophilic character of the surfactant headgroup diminished. The association of the more hydrophobic 6-methoxyquinolinium and quinolinium surfactants with SCX6 did not lead to supramolecular micelle formation because the self-assembly into nanoparticles was highly exothermic.