Study on the aggregation and electrochemical properties of Rose Bengal in aqueous solution of cyclodextrins

The interactions of Rose Bengal (RB) with alpha-cyclodextrin (alpha-CD), hydroxypropyl-beta-cyclodextrin (HP-beta-CD), hydroxypropyl-gamma-cyclodextrin (HP-gamma-CD), heptakis(2,3,6-tri-O-methyll)-beta-cyclodextrin (TM-beta-CD) were studied in aqueous solutions of 0.1 M KClO(4) and 0.1 M LiClO(4) by vis absorption, fluorescence spectroscopy as well as electrochemical measurements at 298 K. The spectrophometric results indicate that RB is included in all beta- and gamma-CDs forming complexes with a stoichiometry 1:1 whose stability is slightly higher in KClO(4) than in LiClO(4) solutions. The complex stability constants determined for salt-containing CD solutions are lower than those for water solutions. The complexation of RB with beta- and gamma-CD and the differences between the complexes obtained in the presence of the two salts were confirmed by an electrochemical study.     

Spectroscopic investigation of Rose Bengal/cyclodextrin interactions in aqueous solution: the case of the hydroxypropyl-cyclodextrins

The interaction of Rose Bengal (RB) with hydroxypropyl-α-cyclodextrin (HP-α-CD), hydroxypropyl-β-cyclodextrin (HP-β-CD) and hydroxypropyl-γ-cyclodextrin (HP-γ-CD) has been studied in water and in acetate buffer at pH 4.5 by UV–Vis absorption, fluorescence spectroscopy and Induced Circular Dichroism at 298 K. Evidence of the complex formation between the RB and all HP-CDs have been obtained both in water and in buffer. Binding constants and stoichiometry of RB/HP-CD complexes in water have been determined by applying the modified Benesi-Hildebrand equation to the fluorescence measurements.     

Diffusion of cyclodextrins in aqueous polymer solutions

Diffusion of α and β cyclodextrin (α-CD and β-CD, respectively) has been studied in aqueous solutions of poly(methacrylic acid), sodium poly(styrene sulfonate), having three different degrees of sulfonation (DS), and copoly(styrene-methacrylic acid) containing three different amounts of styrene. N-Acetylglucosamine and raffinose were included as reference diffusants. It was found that a decrease of the diffusion coefficients of the CD's in these polymer solutions is characteristically dependent on the polymer concentration, DS, Styrene content, and the degree of neutralization. The results were interpreted by assuming a 1:1 complex formation between CD and an appropriate residue in the polymer. The complex diffusion behavior of CD in the copolymer solutions suggested that the ability of the polymer residue to form complexes with the CD is lost when the polymer chain dimensions are reduced with decreasing neutralization.     

Complexation of cyclodextrins with flavines in aqueous solutions

Data on the binding mode and thermodynamics of complex formation for various cyclodextrins (CDs) with flavines are summarized. It is shown that the governing factors of complexation are the size, degree of hydration, and hydrophobicity of the guest molecule. It is found that the presence of small hydrophobic substituents in a flavine’s structure increases their affinity toward cyclodextrin cavities, raising the stability of a complex. In contrast, the presence of bulky and polar side groups in a flavine’s structure prevents its inclusion in a macrocyclic cavity and weakens complexation. The size of a CD cavity plays a minor role in the interaction between CDs and flavines, since the inclusion of a guest molecule is only partial.     

Interactions of xanthine and its derivatives with cyclodextrins in aqueous solutions

Using calorimetry, ¹H NMR, UV spectroscopy, and solubility methods, the interactions of natural and hydroxypropylated 6h-, β-, and γ-cyclodextrins with xanthine and its methylated derivatives (theophylline, theobromine, and caffeine) were studied in aqueous solutions at 298.15 K. Cyclodextrins revealed low complexation ability toward xanthine and its methylated derivatives. Hydroxypropyl-γ-cyclodextrin with the largest internal cavity is the most effective solubilizing agent for this type of compounds. The calculated thermodynamic parameters are discussed in terms of structural effects of cyclodextrins and purine alkaloids on the character of their intermolecular interactions in aqueous medium.     

Inclusion complexes of spin-labeled indoles with cyclodextrins in aqueous solutions

Guest-host complexes of β- and γ-cyclodextrins (CDs) with two spin-labeled indole derivatives having the same molecular weights but different structures were studied by EPR spectroscopy in aqueous solutions and semiempirical quantum-chemical calculations of these systems were carried out. In the presence of CD the polarity of the NO group environment decreases and the rotational correlation time (τ) of guest molecules increases. Both indole derivatives form 1 : 1 complexes with γ-CD, the binding constants of the complexes being different more than twice. Simulation of EPR spectra made it possible to determine the indole ring orientation relative to the plane of the host molecule (at angles in the range 30–60°) and the rotational diffusion coefficients of the complexes, which corresponded to the hydrodynamic volume of one γ-CD molecule. In contrast to the complexes with γ-CD the rotational correlation times, τ, of the complexes with β-CD correspond to a hydrodynamic volume which much exceeds the volume of a single β-CD molecule. The complexes with β-CD are also characterized by more hydrophobic environment for guest molecules and absence of spin exchange with Ni²⁺ ions in the aqueous solution. There results are consistent with a dimeric structure of β-CD in the complex and with the orientation of the long axis of the guest molecule along the dimer axis. The energies and geometric parameters were calculated for all complexes by the PM3 method with a conventional set of parameters. The optimized energetically stable structures of the 1 : 1 complexes with γ-CD and of the 1 : 2 complexes with β-CD are consistent with experimental data. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1139–1147, May, 2005.     

Solubilization of lanthanide ions by cyclodextrins in basic aqueous solutions

Cyclodextrins form complexes with lanthanide ions in basic aqueous solutions. This complex formation in basic solution dramatically enhances the solubility of lanthanide ions, which are otherwise insoluble due to the formation of hydroxide gels. Solutions of the -cyclodextrin-Ce³⁺ complex effectively hydrolyze 2-deoxyadenosine-5-monophosphate to 2-deoxyadenosine.     

Calorimetric study of the complex formation of cyclodextrins with nicotinic and niflumic acids in aqueous solutions

Russian Journal of General Chemistry - Thermodynamic parameters of the inclusion complex formation of native and hydroxypropylated cyclodextrins with pharmacologically active niflumic and nicotinic...     

Inclusion complexation of volatile chlorinated hydrocarbons in aqueous solutions of branched cyclodextrins

The inclusion complexation of five volatile chlorinated hydrocarbons, i.e., chloroform, carbon tetrachloride, trichloroethylene, tetrachloroethylene, and monochlorobenzene, with branched cyclodextrins (CDs) such as monoglucosyl--CD and monomaltosyl--CD in aqueous solutions was examined. Their inclusion complexes were found to be very water soluble and the solubilities of the chlorinated hydrocarbons in aqueous solutions increased almost linearly or gradually with increasing concentration of the branched CDs. The amounts of the chlorinated hydrocarbons included in the branched CDs were well related to their molecular size and shape. In addition, the viability of application to pollution prevention is discussed.     

Capture of volatile chlorinated hydrocarbons by aqueous solutions of branched cyclodextrins

The use of aqueous solutions of branched cyclodextrins was examined in order to develop an effective method of capturing toxic volatile chlorinated hydrocarbons such as trichloroethylene and monochlorobenzene. From the experiments in which trichloroethylene diluted with nitrogen gas came into contact with aqueous solutions of branched cyclodextrin mixtures, it was found that absorption could be performed without the formation of inclusion complex solids, which should simplify the whole process of absorption and recovery.     

Comparative electrochemical study of self-assembly of octanethiol from aqueous and aqueous ethanol solutions on a gold electrode

The self-assembly of octanethiol (OT) on the surface of a polycrystalline gold electrode in aqueous and aqueous ethanol thiol-containing (1 × 10^–4 М) 0.1 М NaClO₄ solutions was studied. The blocking properties and electrochemical stability of monolayer OT films were studied by chronopotentiometry during OT adsorption under the open circuit conditions (chronoamperometry at a fixed potential) combined with cyclic voltammetry for modified Au/OT electrodes. It was found from the change in the rate of electrochemical reactions in the range of monolayer stability potentials that in aqueous media, compact insulating OT monolayer films formed at a open circuit potential within ~100 s, and the shift of the adsorption potential toward negative values (to–0.6 V) allowed a considerable decrease in the monolayer self-assembly time. The potential shift toward higher negative values (–0.9 V) leads to a removal of OT from the electrode surface during the reductive desorption, with a multipeak current signal recorded on the voltammograms. A transition from aqueous to aqueous ethanol solutions accelerated the formation of an insulating OT monolayer (≈6 s) and led to a change in the shape of the desorption current peak, whose value was almost independent of the ОТ accumulation time and potential.     

Spectroscopic evidence of monomeric aluminium phthalocyanine tetrasulphonate in aqueous solutions

Aqueous solutions of aluminium phthalocyanine tetrasulphonate (AlPcS(4)) were investigated by means of absorption and fluorescence spectroscopy. The absorption spectrum of AlPcS(4) is independent of concentration in a wide range (from 10(-8) to 10(-4) M). The fluorescence spectrum measured with a standard setup is strongly dependent on AlPcS(4) concentration, and the fluorescence maximum is gradually red-shifted with increasing concentration. Calculations that take into account reabsorption of fluorescence (inner-filter effect) fit the experimental observations at low concentrations (up to 10(-6) M). Disagreement between the calculations and spectra recorded at higher concentrations (above 10(-5) M) shows that the reabsorbed light may be reemitted as fluorescence. The influence of inner-filter effects on the spectral shape was demonstrated by the experiments where a fibre-optic front-face fluorescence setup was applied: Under such conditions the shape of the fluorescence spectra for a high concentration (10(-3) M) coincided with that of a low concentration (10(-8) M). In conclusion, the present spectroscopic results show that AlPcS(4) does not form aggregates and is a very stable compound in aqueous solutions.     

Combined electrochemical and radiotracer study of anion sorption from aqueous solutions into polypyrrole films

The sorption of SO²⁻₄ and Cl⁻ ions into polypyrrole films has been studied by the radiotracer method under potential cycling and steady state conditions using labelled H₂SO₄ and HCl. Although a potential dependent migration and penetration of anions in the film can be detected, no strong correlation was found between the amount of charge consumed in the oxidation and reduction processes and the number of sorbed anions. The number of positively charged sites attracting anions into the film seems to be significantly lower than that expected from the amount of charge involved in the electrochemical transformations.     

Spectroscopic investigations of the structure of liquid water and aqueous solutions

IR spectroscopy is a powerful tool for investigating the structure of aqueous systems. Changes in vibrational frequencies and intensities of the absorption band provide information about the structure of the associated water molecules. The water molecule has C_2v symmetry when the intermolecular interaction is symmetrical to both OH bonds as in 2:1 complexes. In this case the frequency difference of the two stretching vibrations ν₃ and ν₁ is nearly constant. If the intermolecular interaction is unsymmetric to the OH bonds as in 1:1 complexes the band separation of ν₃ and ν₁ increases markedly in relation to the increase of the unsymmetry. The IR overtone region is more suitable for the study of the structure of liquid water or aqueous solutions than the IR fundamental region. The reason is the higher intensity of the absorption bands of the “free” OH vibration compared to the H-bonded OH groups. The ratio of the intensities is inverse in the fundamental region. Furthermore it is possible to measure quantitatively in the overtone region and there are no experimental difficulties. The results are estimations of the H-bonded and the free OH groups in different aqueous systems.     

Effect of molecular interactions on the photophysics of Rose Bengal in polyelectrolyte solutions and self-assembled thin films

Solutions and layer-by-layer self-assembled thin films containing Rose Bengal and poly(diallyldimethylammonium chloride) are studied with the aim of understanding the interactions controlling their structures and the photophysics of the dye in both media. A detailed spectroscopic and theoretical analysis shows that hydrophobic interactions among dye molecules contribute to the coiling of the polyelectrolyte chain in solution at low polyelectrolyte/dye ( P/ D) ratios, whereas extensive aggregation of the dye takes place even at ratios as high as 10(4) (expressed in monomeric units). A polyelectrolyte elongated form prevails in self-assembled thin films, providing an environment that reduces hydrophobic interactions and lowers the aggregation tendency. Self-assembled films with a roughly estimated overall dye concentration around 1 M at a P/ D ratio in the order of seven are fluorescent and photogenerate singlet molecular oxygen. This contrasts with the behavior of polyelectrolyte solutions, which are almost nonfluorescent and do not evidence triplet state generation at the same P/ D ratio.     

Conductivity and electrochemical stability of concentrated aqueous choline chloride solutions

Russian Journal of Physical Chemistry A - The conductivity and electrochemical stability of choline chloride (ChCl) solutions with water contents ranging from 20 to 39 wt % are studied. Exposing...     

Photophysical behaviours of some 2-styrylindolium dyes in aqueous solutions and in the presence of cyclodextrins

Host–guest inclusion type association between native β-cyclodextrin and randomly substituted methyl-β-CD and two 2-styrylindolium cationic dyes, e.g. 1,3,3-trimethyl-2-(4-diethylaminostyryl)-3H-indolium iodide (D1) and 1,3,3-trimethyl-2-[4-(N-2-cyanoethyl,N-methyl)-aminostyryl]-3H-indolium iodide (D2), are reported. The described indolium derivatives belong to the rarely investigated class of unsymmetrical polymethines. The complex formation was studied in aqueous buffer solutions with two pH values (7.2 and 3) by means of absorption and steady-state fluorescence spectroscopy. The association equilibrium constant (K), the molar absorptivity and the stoichiometry of the complexes were evaluated using the modified Benesi-Hildebrand approach. The complex stability was affected by the pH of the solution and by the type of CD. The results obtained indicate that D1 forms 1:1 complexes with both β-CD and Me-O-β-CD, whereas D2 does not form stable complexes with Me-O-β-CD and in acidic medium. The fluorescent intensity of D1 in the presence of CDs increases over four times relative to the intensity of the pure dye solutions, presumably via inclusion of the dye into the cyclodextrin cavity due to rigidity of the structure.     

Formation of poly(ethylene glycol) inclusion complexes in aqueous solutions of mixed cyclodextrins

Even though the addition of modified cyclodextrins (modified CDs) accelerates the precipitation in aqueous solutions of poly(ethylene glycol) (PEG) and α-cyclodextrin (α-CD) the final amount of formed solid complex remains unchanged, with no significant presence of modified CDs detected by MALDI-TOF mass spectrometry. Thus unsuitability of kinetic turbidity measurements for determination of binding parameters was confirmed. On the other hand, theoretical calculations based on a model of a chain of freely accessible binding sites demonstrated that the results do not necessarily contradict the finding that individual modified CD molecules can thread onto PEG chains with the efficiency comparable to that of natural (unmodified) α-CD.