Effect of cyclodextrins on the electrochemical behaviour of ascorbic acid on platinum electrodes in acidic and neutral solutions

The effect of β-CD and α-CD on the electrochemical behaviour of H₂A and HA⁻ on platinum is studied. The adsorption of β-CD on this electrode is demonstrated and proved to be dependent on the base electrolyte composition. The maximum adsorption coverage was reached in phosphate solution at pH 6.95. The homogeneous H₂A---β-CD complex formation produced a decrease in the oxidation current and a positive shift in the oxidation peak potential. These effects are predominant in acid solutions. In neutral solutions the opposite behaviour is observed, i.e. an increase in the oxidation current at lower oxidation potentials. The presence of a parallel oxidation route for the vitamin involving strongly adsorbed CO residues is considered, and evidence for a decrease in CO_ad in the presence of β-CD was given by variations in hydrogen adsorption charges. This fact, more important in neutral solutions, must be responsible for the catalytic effect observed. α-CD was not adsorbed, neither did it modify the electro-oxidation behaviour of H₂A and HA⁻.     

Fluorescent cyclodextrins bearing metal binding sites and their use for chemo- and enantioselective sensing of amino acid derivatives

Ditopic receptors based on cyclodextrins bearing a metal binding site were used as enantioselective fluorescence sensors, which were able to generate different responses in the presence of d- or l-amino acids. The performances of the selectors as a function of their structure were evaluated, and the same analysis was extended to other analytes. In this work, this approach is used for the enantiomers of a series of amino acid derivatives and in particular of 2-aminocaprolactam. The results showed that the ability of these sensors to perform enantiomeric analysis can be extended to other analytes of interest in organic synthesis such as amino acid amides and α-aminolactams.     

Structural studies on supramolecular adducts of cyclodextrins with the complex [Ru([9]aneS3)(bpy)Cl]Cl

The complex [Ru([9]aneS₃)(bpy)Cl]Cl (bpy = 2,2′-bipyridine) was immobilised in plain β-cyclodextrin (β-CD) and permethylated β-CD (TRIMEB) to yield two adducts with a 1:1 host:guest stoichiometry. The adducts were studied by powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), ¹³C{¹H} CP/MAS NMR and vibrational spectroscopy (FT-IR and Raman). Results support the formation of stable supramolecular adducts with a proposed geometry in which the coordinated bypiridine fragment of the guest is partially included in the host cavities, and the bulky [9]aneS₃ fragment protrudes out to the interstitial spaces. A packing mode is proposed for [Ru([9]aneS₃)(bpy)Cl]Cl · TRIMEB, obtained by Monte Carlo optimisation of the XRD data. TRIMEB molecules are stacked in tilted channels, with the voluminous part of the guest molecules in the inter-channel space. The behaviour of [Ru([9]aneS₃])(bpy)Cl]Cl upon CD encapsulation and the chloride ligand hydrolysis process in solution for all compounds were studied in detail by Raman spectroscopy.     

Spectroscopic study and antioxidant properties of the inclusion complexes of rosmarinic acid with natural and derivative cyclodextrins

Measurement of total antioxidant activity/capacity of polyphenols in various solvent media necessitates the use of cyclodextrins to solubilize lipophilic antioxidants of poor aqueous solubility. The inclusion complexes of the slightly water soluble antioxidant, rosmarinic acid (RA), with α-cyclodextrin (α-CD), β-cyclodextrin (β-CD), 2-hydroxypropyl-β-cyclodextrin (HP-β-CD), 2-hydroxyethyl-β-cyclodextrin (HE-β-CD), and methyl-β-cyclodextrin (M-β-CD) were investigated for the first time. The effect of cyclodextrins (CDs) on the spectral features of RA was measured in aqueous medium using UV-vis and steady-state fluorescence techniques by varying the concentrations of CDs. The molar stoichiometry of RA-CD inclusion complexes was verified as 1:1, and the formation constants of the complexes were determined from Benesi-Hildebrand equation using fluorescence spectroscopic data. Among the CDs, maximum inclusion ability was measured in the case of M-β-CD followed by HP-β-CD, HE-β-CD, β-CD and α-CD. Solid inclusion complexes were prepared by freeze drying, and their functional groups were analyzed by IR spectroscopy. Antioxidant capacity of CD-complexed rosmarinic acid was measured to be higher than that of the lone hydroxycinnamic acid by the CUPric Reducing Antioxidant Capacity (CUPRAC) method. The mechanism of the TAC increase was interpreted as the stabilization of the 1-e oxidized o-catechol moiety of RA by enhanced intramolecular H-bonding in a hydrophobic environment provided by CDs, mostly by M-β-CD.     

Luminarin 9: A new fluorescent reagent for the precolumn derivatizatioin of main volatile and non-volatile N-nitrosamines in RP-HPLC

Summary A sensitive precolumn fluorescence derivatization method for low level detection of the, volatile (N-nitrosodimethylamine and N-nitrosopyrrolidine) and non-volatile N-nitrosamines (N-nitrosoproline and N-nitrosodiethanolamine) an high-performance liquid chromatography was developed. This method is based on the denitrosation of the compounds of interest by a mixture of hydrobromic acid and acetic acid to produce the corresponding secondary amines. These are, then, able to react with, a quinolizinocoumarin derivative (luminarin 9^®) to form highly fluorescent labelled N-nitrosamine derivatives. The structural elucidation of the luminarin 9^® derivatives of N-nitrosoproline and N-nitrosodimethylamine by way of example, were established by liquid chromatography-mass spectrometry (LC-MS) and by direct chemical ionization-mass spectrometry (CI-MS). The separation, derivatization and detection conditioins were optimized for all the studied compounds. The detection limits (signal to noise ratio=3) were between 0.4 and 1.0 pmol injected depending on the compound. The calibration graphs were linear for derivatized amounts in the range of 0.5–40 nmol for N-nitrosodimethylamine and N-nitrosopyrrolidine, 0.4–2- nmol for N-nitrosoproline and 1.0–40 nmol for N-nitrosodiethanolamine. The repeatability (RSD less than 3.5%, n=6) and reproducibility (RSD less than 4.8%, n-6) were satisfactory.     

The effect of cyclodextrins on the hydrolysis of carboxylic acid esters

Cyclodextrins and their derivatives act as inverse phase-transfer catalysts in the hydrolysis of carboxylic acid esters. The reaction is affected by temperature, choice of cyclodextrin and also by the stirring rate. Lipophilic esters are better hydrolyzed under inverse phase-transfer catalysis conditions than under classic phase-transfer catalysis (PTC) conditions. Some typical features of the reaction are emphasized and compared with classical PTC reactions.     

Application of automated docking to the binding of naphthalenes to βCD in water: correlation with spectrofluorimetric data

Automated semi-rigid docking has been explored as an alternative approach for the theoretical study of the inclusion complexes with cyclodextrins. To this purpose we have chosen as a model for the binding to βCD some naphthalene derivatives (naphthalene, 2-ethylnaphthalene, 2-acetylnaphthalene, 1-naphthyl acetate, 2-naphthyl acetate and 1-naphthol). For comparison purposes, the binding constants in water and the associated thermodynamic parameters have been obtained under the same experimental conditions by steady-state fluorescence spectroscopy. The calculations of the automated docking regarding the topology of the guest inside the cavity produce a cluster of structures that qualitatively agrees with fluorescence results and literature data. However, the predicted values of the free energy of binding are lower than the experimental ones by ca. −10 kJ mol⁻¹, and very close to the experimental enthalpy of binding deduced from the temperature dependence of the association constants. The differences are ascribed mainly to the assumption of rigidity of the CD into the auto-docking scheme.     

Solubilities of the cyclodextrins in the presence of transition metal salts

The solubilities of -, -, and -cyclodextrin have been measured in the presence of the first row transition metals: Cr³⁺, Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺; chlorides, nitrates and sulphates (in this case Fe²⁺), and, for companson, with CaCl₂, the corresponding Group IIa salt. Where possible the measurements are reported as a function of the activity of the salts. In general, for the transition metals the sulphates all show a linear decrease in solubility with increasing salt activity: for the nitrates the solubility increases and then reaches a limiting value; and for the chlorides a small decrease in solubility is observed at low activity followed by an increase in solubility at higher salt activity. Circular dichroism measurements confirm that there is no direct complexation at non-basic pH.Presented at the Sixth International Seminar on Inclusion Compounds, Istanbul, Turkey, 27–31 August 1995.     

Palladium-catalyzed ring opening of azabicyclic olefins with organoindium reagents: a simple, clean, and efficient synthesis of functionalized cyclopentenes

Azabicyclic olefins undergo facile palladium-catalyzed ring opening with organoindium reagents affording trans-3,4-disubstituted hydrazinocyclopentenes in good to excellent yields.     

A new fluorescent derivatization reagent and its application to free fatty acid analysis in pomegranate samples using HPLC with fluorescence detection

A new fluorescent labeling reagent has been developed for the determination of fatty acids (FAs) by HPLC with fluorescence detection. The derivatization conditions including the amount of derivatization reagent, temperature, and type of catalyst were investigated, the results indicated that the reaction proceeded within 30 min at 90°C in the presence of K₂CO₃ catalyst. The maximal yield was obtained with a four‐ to fivefold molar reagent excess. The derivatives exhibited strong fluorescence with an excitation maximum at λ_ex = 245 nm and an emission maximum at λ_em = 410 nm. Twenty‐five FA derivatives were well separated by RP‐HPLC on a Hypersil BDS C₈ column in combination with gradient elution. All FAs were found to give excellent linear responses with correlation coefficients >0.9992. The method gave a low LOQ of 0.85–5.5 ng/mL (S/N of 10). The developed method was employed to analyze free FAs (FFAs) composition in pomegranate samples without any purification. FFAs in samples were doubly identified by HPLC retention time and protonated molecular ion corresponding to m/z [M+H]⁺. This newly developed method allows a highly sensitive determination of trace FFAs from pomegranate and other foodstuffs.     

Exploring the Research Progress about the Applications of Cyclodextrins and Nanomaterials in Electroanalysis

Cyclodextrins and nanomaterials are widely used in the achievement of powerful platforms in supramolecular chemistry and nanotechnology. The relatively hydrophobic internal cavity of the CDs selectively retains molecules having the proper geometry, while the hydrophilic exterior allows CDs to improve the dispersibility and molecular recognition. The nanomaterials provide higher surface area, good conductivity, and electrocatalytic effect. The use of nanomaterials and CDs in electrochemical sensors’ design allows the development of a large variety of devices, explaining the increasing number of papers in the last years that are discussed in this review.     

ESR-spin trapping in the presence of cyclodextrins. Detection of PBN-superoxide spin adduct

In phosphate buffer solution the half-life of the PBN-superoxide spin adduct is very short. However, as a result of the formation of inclusion complexes, its ESR signal was easily detected for up to 25 min when the trapping of superoxide anion with PBN was carried out in the presence of different cyclodextrins. Furthermore, the formation of these inclusion complexes results in a significant protection of the PBN-superoxide adduct against l-ascorbate monoanion reduction.     

活性艳红K-2G与环糊精相互作用的极谱伏安法研究

研究了单偶氮染料活性艳红K-2G的极谱伏安行为。实验表明：在底液为0．1mol／L的NaGl溶液中活性艳红K-2G有一稳定的、灵敏的还原峰,峰电位约为-0．78mV（vs．SEE）。用线性扫描二阶导数极谱法研究了活性艳红K-2G与不同环糊精的相互作用。采用“电流法”测定了包结常数,比较了不同类型环糊精的包结能力,初探了包结点的可能位置,并对其进行了理论分析。     

Thermodynamic Study of the Inclusion Interaction between Gemini Surfactants and Cyclodextrins by Isothermal Titration Microcalorimetry

The inclusion complexes of a series of bis-quarternary ammonium surfactants, (C _n N)₂Cl₂ (where n = 12, 14, 16) and sodium bis(2-ethylhexyl) sulfosuccinae (AOT), with α-cyclodextrin (α-CD), β-cyclodextrin (β-CD) and γ-cyclodextrin (γ-CD) in aqueous solutions were investigated by using isothermal titration calorimetry (ITC) at 298.15 K. The stability constants, stoichiometry, and formation enthalpies, entropies and Gibbs energies for the complexes in aqueous solutions have been derived from the calorimetric data. The values of the binding constant, K ^∘ _i , are very large, which indicates that these complexes are quite stable in their aqueous solutions. The enthalpy changes (ΔH ^∘) for all of the inclusion processes are negative, showing that the complex process is enthalpy driven. The entropy effect (TΔS ^∘) is negative, so the inclusion process is entropically unfavorable. The large negative Gibbs energy changes indicate that formation of host-guest inclusion complexes is generally a spontaneous process. The thermodynamic parameters are discussed in the light of the different structures of the host and guest molecules.     

Relationships between the stability,circular dichroism spectrum and type of substitution in inclusion complexes formed between phenophthalein and different cyclodextrins

The perturbation of the three-site interaction of phenolphthalein on complexation with different cyclodextrins (CDs) has been investigated as a function of the type and degree of substitution of the CD. The UV visible spectra are practically identical, while the circular dichroism spectra change dramatically both in intensities and signs. Parallel to this, the stability constants are influenced not only by the size of the cavity but also by the substitution of its rims, particularly by that of the primary hydroxyls. The two phenomena can be related to allow conclusions concerning the mode of inclusion.     

Improvement of detection sensitivity of T-2 and HT-2 toxins using different fluorescent labeling reagents by high-performance liquid chromatography

T-2 and HT-2 toxins are Fusarium mycotoxins that can occur in cereals and cereal-based products. Three fluorescent labeling reagents, i.e. 1-naphthoyl chloride (1-NC), 2-naphthoyl chloride (2-NC) and pyrene-1-carbonyl cyanide (PCC), were used for the determination of T-2 and HT-2 toxins by high-performance liquid chromatography (HPLC) with fluorescence detection (FD). Pre-column derivatization of T-2 and HT-2 toxins was carried out under mild conditions (50 °C, 10 min) in toluene with 4-dimethylaminopyridine (DMAP) as catalyst. All fluorescent derivatives were identified and characterized by HPLC-tandem mass spectrometry (HPLC-MS/MS). Optimal stoichiometric ratios (toxin:derivatizing reagent:catalyst), linear range and repeatability of the reaction, stability and sensitivity of the derivatives were determined. A wide linear range (10–1000 ng of either derivatized T-2 or HT-2 toxin), good stability (up to 2 weeks at −20 °C or 5 days at room temperature) of the fluorescent derivatives and good repeatability of the reaction (RSD ≤ 8%) were observed. Detection limits (based on a signal-to-noise ratio of 3:1) were 10.0, 6.3 and 2.0 ng for derivatized T-2 toxin and 6.3, 2.3 and 2.8 ng for derivatized HT-2 toxin with 1-NC, 2-NC and PCC, respectively. In terms of sensitivity and repeatability, PCC and 2-NC reagents showed better performance than 1-anthroylnitrile (1-AN), a previously reported labeling reagent for T-2- and HT-2 toxins. Preliminary studies also showed the applicability of PCC and 2-NC as fluorescent labeling reagents for the simultaneous determination of T-2 and HT-2 toxins in cereal grains by HPLC/FD following immunoaffinity column clean-up.