Influence of the modifiers on the nature of the stationary phase and the separation of carotenes in sub-critical fluid chromatography

Summary Sixteen solvents were tested as modifiers in the separation of - and -trans/cis carotenes by sub-critical fluid chromatography. The results obtained indicate that the retention behaviour is related to solubility parameters and the dielectric constant of the solvents. The study of the selectivity of thetrans/cis isomers of -carotene leads to a better understanding of the role of the solvents in determining the nature of the stationary phase at the mobile phase interface. The results obtained point out the difference in the conformation of the alkyl chains with three classes of solvents: methanol, acetonitrile, and THF or methylene chloride. The analogy between non aqueous reversed-phase chromatography (NARP) and sub-critical fluid chromatography has also been discussed.     

Enthalpies of transfer of amino acids and sodium chloride from water to aqueous fructose at 25°C

Enthalpies of transfer H ₂ of three amino acids glycine, L-alanine, and L-phenylalanine, and of NaCl, from water to aqueous fructose at low fructose mole fraction were determined at 25°C. Comparison with H ₂ for glycine in the presence and absence of KCl shows that the effect of additional ion-dipole interactions is apparent only in more concentrated fructose solutions. Limiting slopes indicate that glycine resembles NaCl more than it does the other two amino acids. The effect on H ₂ of the polarity of the zwitterion is offset and dominated by the growing non-polar side chain. Values of H ₂ for glycine, L-phenylalanine, and NaCl from water to aqueous urea are compared with the results of the present study. A rough colligative effect similiar to that observed for NaCl in a variety of polar mixed aqueous solvents may also be possible for glycine, since the change of the enthalpies of transfer with mole fraction for glycine are the same for transfer to both aqueous urea and fructose.     

Enhancement of photoinduced intramolecular cross-cycloaddition of α-(9-anthryl)-ω-(1-naphthoyl) end-labeled poly(ethylene glycol) via lipophobic interactions and complexation with metal cations

The fluorescence spectra and photocycloadditions of poly(ethylene glycol) labeled at the chain termini with one 9-anthryl and one 1-naphthoyl group (N-P_n-A) both in non-polar and polar solvents in the presence of alkali-metal cations have been investigated. Lipophobic interactions in non-polar solvents and complexation of the polyether with cations in polar solvents force the two terminal groups of N-P_n-A into proximity, and irradiation of the solutions produces intramolecular photocyclomers to the exclusion of intermolecular products.     

Synthesis ofN,N′-ditosyl-8, 19-dimethoxy-2,5-diaza-[6.1]-naphthyl-cyclophane: crystal and molecular structure of its 1∶1 chloroform solvate

The title compound, C₄₁H₄₀N₂O₆S₂, has been synthesised in good yield and was found to form a 11 inclusion compound with CHCl₃ and other organic solvents. The crystal and molecular structure of the CHCl₃ solvate has been determined by single crystal X-ray analysis and refined to anR-value of 0.034 for 3229 reflections. The compound is monoclinic, space groupP2₁/c, witha=15.316(1),b=14.515(1),c=18.720(3) Å, =101.98(1)^o, andZ=4. One molecule of chloroform is included in the crystal lattice. Supplementary data relevant to this article have been deposited with the British Library as Supplementary Publication No. SUP 82146 (9 pp.).     

Spin-trapping studies on the reaction of NO<Subscript>2</Subscript> with <Emphasis Type="Italic">β</Emphasis>-carotene

Absorption spectrophotometric studies indicated that in chloroform -carotene reacted with NO2 with a stoichiometry of 1:2. To understand the reaction mechanism, free radical intermediates produced during their reaction were trapped by spin traps 2-methyl-2-nitroso propane (MNP) and 4-pyridyl-1-oxide-N-1-butyl nitrone (POBN). The resulting stable spin adducts were detected by electron spin resonance (ESR) spectroscopy. NO₂ alone did not interact with spin traps, but introduction of -carotene-generated ESR signals with different nitrogen and hydrogen hyperfine coupling constants (A _N and A _H) characteristsic of the trapped radical. Two possible pathways exist for the reaction of NO₂ with -carotene, viz., electron transfer and addition to the polyene chain. Our ESR results supported an addition mechanism, where formation of carbon-centered [NO₂ · · · -carotene] free radical adduct was observed.     

Ferrocene incorporating host-guest dyads with electrochemically controlled three-pole hydrogen bonding properties

We describe the electrochemically controlled hydrogen bonding interactions between the isobutyl flavin/2,6-diferrocenylamidopyridine (2·5) and 9,10-phenanthrenequinone/1-ferrocenyl-3-hexylurea (4·6) dyads. Cyclic and square wave voltammetry studies have shown that the binding efficiencies between these moieties can be electrochemically actuated in non-polar (CH₂Cl₂ for 2·5) or polar (DMF for 4·6) organic solvents between three distinct states.     

Singlet-Oxygen Quenching by Carotenoids: Steady-state luminescence experiments

The near-infrared luminescence of singlet oxygen (¹O₂) has been measured in order to determine the efficiency of ¹O₂ quenching by two carotenoid compounds, β-carotene and canthaxanthin. 1H-Phenalen-1-one and rose bengal have been used as photosensitizers in those steady-state luminescence experiments. Stern-Volmer analysis of the ¹O₂ luminescence in solutions of CCl₄ and CD₃OD, containing different concentrations of the carotenoids, has shown a very efficient quenching by canthaxanthin. The rate constants are about a factor of 2 below the diffusion limited values for the given solvents, confirming earlier results in benzene. In comparison, the efficiency of ¹O₂ quenching by β-carotene is slightly lower than that by canthaxanthin in non-polar solvents and is reduced by an order of magnitude in CD₃OD, due to the aggregation of this quencher.     

The β-carotene dilemma: ESR study with coordinated peroxyl radicals

The apparently unpredictable behaviour of β-carotene in the supplementation of the diet of smokers is discussed in the light of the reactions of peroxyl radicals with β-carotene in the absence of oxygen. The decay of tert-butylperoxyl radicals in the presence of β-carotene was studied at ambient temperature in non-polar solvents by ESR spectroscopy. The primary reaction in the absence of oxygen is interpreted as a spin-trapping effect of a peroxyl radical by β-carotene producing an intermediate labile free radical, which disappears after recombination with a second tert-butylperoxyl radical. The result is the transformation of β-carotene to a diamagnetic compound with two peroxy bonds. In the presence of chelating transition metals with unpaired d-electrons as electron donors the peroxy group of the oxidized β-carotene can be split to alkoxyl free radicals. The primary attack of tert-butylperoxyl radicals is completely inhibited in the presence of vitamin E followed by production of free aryloxy radicals and the presence of oxygen has no significant effect on this reaction. Spin-trapping of peroxyl radicals by the double bond of vitamin A leads to its oxidation in the absence of vitamin E. Transition metal ions such as Co, Cr, Fe, and Mn, known to be present in the aerosol of cigarette smoke, homolyse the peroxyl bonds of peroxidised β-carotene, which results in cell damage.     

Charge Transfer Salt with Substituted Lindquist Polyoxometalate: Synthesis, Crystal Structure, and Physical Properties of α-(BEDT-TTF)6(VW5O19)⋅DMF⋅2H2O

A new charge transfer salt based on bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and the vanadium substituted Lindquist anion [VW₅O₁₉]^3– was synthesized and characterized by X-ray and spectroscopic analysis. Monoclinic, C2, F _w =3789.2, a=41.5019(5), b=11.2600(2), c=12.7514(2) Å, =101.877(5)°, Z=2, R=0.060, based on 12095 reflections with I>2(I). The structure consists of alternating organic and inorganic layers. The inorganic layer is generated by [VW₅O₁₉]^3– anions, dimethylformamide (DMF) and water molecules. The organic layer, which is generated by three different BEDT-TTF units, belongs to the -type of packing and consists of trimerized chains parallel to the c direction. The compound is a semi-conductor with _{300 K} =1.1 Scm^–1 and E _a =0.1 eV. Polarized reflectance spectra of single crystals were measured over the spectral range from 650 to 25000 cm^–1. FT-NIR Raman spectra of powdered crystals dispersed in KBr pellets were also recorded. Vibrational and electronic features are discussed.     

Influence of solvent polarity on the separation of leucine enantiomers by β-cyclodextrin: a molecular mechanics and dynamics simulation

The interaction between leucine and β-cyclodextrin with different solvents was studied by molecular mechanics and dynamics simulations. In order to analyse the influence of the solvent polarity on the inclusion complex formation and separation process of leucine enantiomers by β-cyclodextrin, the organic modifiers were characterised by the same value of dielectric constant in the electrostatic contribution to the interaction energy, and a different molecular configuration of amino acids (neutral or zwitterion). The complexes formed in polar solvents were more stable than those in non-polar solvents with the same dielectric constant, because the electrostatic contribution is negative for the former and positive for the latter. The optimized structures obtained for leucine enantiomers and β-cyclodextrin in vacuo are non-inclusion complexes. The solvent polarity contributes to increasing the probability of the presence in an inner position for the guest, whereas the results for non-polar configurations were smaller and distributed in larger areas. The regions where the enantiomers spend more time in the simulation correspond to locations with greater chiral discrimination. d-Leu was the first eluted enantiomer in every case, except for a polar solvent with $\epsilon =26$.     

HPLC determination of carotenoids in human serum by gradient elution technique

Summary Polarities of the carotenoids in human serum are very different; many nonpolar carotenoid hydrocarbons (e.g. -carotene, lycopene) and highly polar hydroxycarotenoids (e. g. -cryptoxanthin, zeaxanthin, lutein) can be found among them.Gradient elution chromatography was used for the separation of -carotene, lycopene, -cryptoxanthin, lutein and zeaxanthin in serum samples applying amino and cyano packings (Chromsil-NH₂, Chromsil-CN). The effects of different stationary phases on the selectivity were compared. The method is particularly suitable for the direct determination of serum levels of carotenoid in serum extracts.     

On the interpretation of solubilization results obtained from semi-equilibrium dialysis experiments

The interpretation of intramicellar solubilization data obtained from semi-equilibrium dialysis (SED) experiments is described, and methods are presented for determining equilibrium constants for the solubilization of organic species by aqueous surfactant solutions as well as activity coefficients of both the organic solute and the surfactant within the micelle. The solubilization equilibrium constant of an organic solute in an aqueous micellar solution (K) is defined as the ratio of the mole fraction of organic solute in the micellar pseudophase (X) to the concentration of the unsolubilized monomeric organic solute in the aqueous phase (c ₀). Expressions compatible with the Gibbs-Duhem equation are used to represent the concentration dependence of activity coefficients of both the solubilizate and surfactant in the micellar pseudophase; the analysis leads to calculated values of the concentrations of free and intramicellar surfactant and organic solute in both compartments of the equilibrium dialysis cell. Solubilization equilibrium constants for many amphiphiles are well correlated by the simple expressionK=K ₀(1-BX)², whereB is an empirical constant andK ₀ is the limiting value ofK asX approaches 0.     

Thermodynamic Parameters of the Dissolution of 4-Hydroxy-L-Proline and L-Phenylalanine in Mixed Aqueous Solvents at 298 K

The enthalpies of solution of 4-hydroxy-L-proline and L-phenylalanine in binary mixed aqueous solvents containing acetonitrile (AN), 1,4-dioxane (1,4-DO), or acetone (AC) at mole fractions of 0 to 0.25 are determined at T = 298.15 K via isothermal calorimetry. The standard enthalpies of solution (Δ_solH°) and transfer (Δ_trH°) of 4-hydroxy-L-proline and L-phenylalanine from water to mixed aqueous solvents are calculated using the experimental calorimetric data, as are the enthalpy coefficients of paired interactions (h _xy ) between the molecules of the investigated amino acids and the organic solvents. The effects the mixed aqueous solvent composition and the structure of the organic solvent molecules have on the enthalpies of solution and transfer for the investigated amino acids are considered. The correlation between the enthalpy of solution of the amino acids and the electron-donating properties of the organic solvents in the mixed aqueous solvent systems is established.     

Crystal structure of the organic metal (ET)2[Hg(SCN)2Cl] withT M-D=50 K

The composition and structure of the radical-ion salt (ET) ₂ [Hg(SCN) ₂ Cl], a new organic metal withT _M-D=50 K, were established in an x-ray structure investigation. Principal crystallographic data:a=36.69(1),b=8.302(8),c=11.732(8) Å, =90.02(6)°, space groupCc, Z=4,d _calc=2.08 g/cm ³ ,R=0.067. The crystal structure of the salt consists of an alternation of organic (cationic) and inorganic (anionic) layers along the axisa. The anionic layer consists of the polymeric chains in which the [Hg(SCN) ₂ Cl]^– ions are linked together along the axisc through secondary Hg···N bonds with lengths of 2.75(3) and 2.98(8) Å. On account of these interactions the trigonal configuration of the bonds of the Hg atom is built up to trigonal-bipyramidal. In the organic layer with a structure of the type the ET cations form dimers with an interplanar distance of 3.53 Å. ET cations belonging to neighboring dimers are linked together through shortened intermolecular S...S contacts with lengths of 3.40–3.50(2) Å.Institute of Chemical Physics, Russian Academy of Sciences, Chernogolovka 142432. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 4, pp. 903–909, April, 1992.     

One-step regioselective green synthesis of 2,3-mannoepoxy-β-cyclodextrin under aqueous conditions

Abstract  A simple method for the straightforward regioselective synthesis of 2,3-mannoepoxy-β-cyclodextrin, which is a valuable precursor for further functionalization of β-cyclodextrin, is achieved under aqueous conditions without any organic solvents at a moderate yield. Graphical abstract        

Flexibly regulated electrochemiluminescence of all-inorganic perovskite CsPbBr3 quantum dots through electron bridge to across interfaces between polar and non-polar solvents

All-inorganic perovskite quantum dots (QDs) have attracted great interests due to its outstanding properties. But their poor stability in polar solvents seriously hampered wide applications in analytical chemistry. In this work, strong, stable and flexibly regulated the electrochemiluminescence (ECL) emission form CsPbBr₃ QDs was successfully obtained and applied in the analysis of polar solvents through the unique structure of closed bipolar electrode (BPE). To demonstrate the feasibility, it was successfully used in the detection of tetracycline (Tc) aqueous solution. CsPbBr₃ QDs was immersed into organic solution in anode microcell of closed BPE while Tc aqueous solution was added into cathode microcell. The two microcells were physically separated and would not interfere with each other. But the bio-recognition event between aptamer and Tc in cathode microcell would induce the ECL signal change in anode microcell through the electrons conducted by BPE as the bridge. The ECL emission can be flexibly regulated by environmental factors of both polar and non-polar solvents and the interface status of the BPE. Compared with traditional methods to overcome the intrinsic instability in polar medium, the reported method does not need any further surface modifications, has no limitations on the targets and can provide wide development space for further deep research, which may open a new direction for the ECL sensing of CsPbBr₃ QDs.     

Protonating power of aqueous and aqueous-alcoholic solutions of HC1 in the presence of inorganic salts

The indicator method has been used to obtain a scale of protonating power of the medium H_o ^s for solutions of HCl in the solvents of constant composition H₂O-EtOH-KCl and H₂O-NH₄Cl at 25C. It is shown that in the solvents investigated the value of H_O ^s in HCl is the same as H₀ in aqueous solution for the same ratios of the concentrations of HCl and of the basic solvents H₂O and EtOH. A method is worked out for estimating the value of of an organic base in aqueous and aqueous-alcoholic solutions of HCl in the presence of inorganic salts.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 996–1000, May, 1991.     

Solubilities of the Silver Halides in Aqueous Mixtures of Methanol,Acetonitrile, and Dimethylsulfoxide

Summary.  The solubilities of the silver halides in three non-aqueous solvents: methanol, acetonitrile, and dimethylsulfoxide, and in their aqueous mixtures, are reviewed. Values for the solubility product, K _SO , the enthalpies of solution, Δ _sol H ^o , and the equilibrium products for AgX ⁽ⁱ⁻¹⁾⁻ _i silver halide complexes, β _i , are listed and, where possible, compared. The solvent systems provide examples for three types of mixed aqueous solvent system: aqueous alcohol mixtures and aqueous mixtures with dipolar aprotic solvents that are weakly or strongly basic. The experimental data are discussed in terms of the solvation of the silver and halide ions in the mixed solvents. E-mail: Earle.Waghorne@ucd.ie Received September 30, 2002; accepted October 29, 2002 Published online April 7, 2003 RID="a" ID="a" Dedicated to Prof. Heinz Gamsj?ger on the occasion of his seventieth birthday     

Rate constants for reaction of 1,2‐dimethylimidazole with benzyl bromide in ionic liquids and organic solvents

The rate constant for the Menschutkin reaction of 1,2‐dimethylimidazole with benzyl bromide to produce 3‐benzyl‐1,2‐dimethylimidazolium bromide was determined in a number of ionic liquids and molecular organic solvents. The rate constants in 12 ionic liquids are in the range of (1.0–3.2) × 10^?3 L mol^?1 s^?1 and vary with the solvent anion in the order (CF₃SO₂)₂ N^? < PF₆^? < BF₄^?. Variations with the solvent cation (butylmethylimidazolium, octylmethylimidazolium, butyldimethylimidazolium, octyldimethylimidazolium, butylmethylpyrrolidinium, and hexyltributylammonium) are minimal. The rate constants in the ionic liquids are comparable to those in polar aprotic molecular solvents (acetonitrile, propylene carbonate) but much higher than those in weakly polar organic solvents and in alcohols. Correlation of the rate constants with the solvatochromic parameter E _T(30) is reasonable within each group of similar solvents but very poor when all the solvents are correlated together. Better correlation is obtained for the organic solvents by using a combination of two parameters, π* (dipolarity/polarizibility) and α (hydrogen bond acidity), while additional parameters such as δ (cohesive energy density) do not provide any further improvement. © 2004 Wiley Periodicals, Inc. *

1 This article is a US Government work and, as such, is in the public domain of the United States of America.

Int J Chem Kinet 36: 253–258, 2004     

Solubility and molecular modeling of triflumizole-β-cyclodextrin inclusion complexes

Solubility enhancement of the fungicide triflumicole by-cyclodextrin is explained using a thermodynamic approach. The influence of organic cosolvents on the overall equilibrium constants of triflumizole complexation with-cyclodextrin in aqueous solutions has been investigated. Their variance in mixed solvents is only partly explained by a competitive inclusion of substrate and cosolvent molecules in-cyclodextrin. The geometries of host-guest complexes have been estimated by molecular mechanics calculations. Their broad structural variety caused by the flexibility of host and guest molecules and different association possibilities of triflumizole have been analysed by a dynamic Monte Carlo docking method. The hydrophobic effect has been simulated by cominimization of the hydrophobic contributions to the solvation energy, calculated from the solvent accessible surface area of the complex and the conformational (potential) energy.