Enantioselective separation of thalidomide on an immobilized α1-acid glycoprotein chiral stationary phaseglycoprotein chiral stationary phase

Summary An easy and rapid enantioselective separation for assay of racemic thalidomide on an immobilized α₁-acid glycoprotein chiral stationary phase (GPA CSP) is described. The effects of tetrahydrofuran (THF) as organic modifier, buffer concentration to control the ionic strenth, and mobile phase pH were studied. These variations have consequences in terms of chromatographic retention (k), resolution (R _s), selectivity (α), and peak asymmetry (USP tailing factor). The main condition affecting chromatographic retention was mobile phase pH. At pH 4.5, no separation of thalidomide enantiomers was achieved whereas at pH 7.9 chiral separation was optimum. Peak tailing was directly related to changes in pH and to addition of THF as mobile phase modifier. Results also indicated that the resolution factor is THF concentration-dependent, and that the separation factor (α) is the best parameter for evaluating enantioselectivity. The best mobile phase was pH 7.0, 30 mM ammonium acetate containing 0.3% THF. Under these conditions validation including linearity, recovery, and precision was performed. The suitability of this method has been successfully proved in a limited in-vivo study after intravenous administration of thalidomide to a New Zealand male rabbit.     

HPLC Enantioseparation of 1-(α-Aminobenzyl)-2-naphthol and 2-(α-Aminobenzyl)-1-naphthol Analogs on a β-Cyclodextrin-Based Chiral Stationary Phase

The enantiomers of 1-(α-aminobenzyl)-2-naphthol and 2-(α-aminobenzyl)-1-naphthol analogs were separated isothermally on a 3,5-dimethylphenylcarbamoylated β-cyclodextrin-based chiral stationary phase (Cyclobond DMP), with an n-hexane/alcohol modifier as mobile phase. Optimization of the separation was achieved by variation of combinations of the polar mobile phase additives ethanol and methanol. The nature and position of the α-aminobenzyl substituent of the 1- and 2-naphthol analogs influenced the retention and the selectivity.     

Synthesis and characterization of molecularly imprinted polymers for recognition of ciprofloxacin

A molecularly imprinted polymer (MIP), with special molecule recognition properties of ciprofloxacin (CIP), was prepared by thermal polymerization in which ciprofloxacin acted as template molecule, α-methacrylic acid (MAA) acted as functional monomer and trimethylolpropane trimethylacrylate (TRIM) acted as crosslinker. The optimized ratio was determined to be n(CIP): n (MMA):n(TRIM)51:6:16 by investigation of the effects of different concentrations of functional monomer and the crosslinker on the MIP’s recognition properties. Equilibrium binding experiment was used to investigate the adsorption dynamics, the binding ability to template molecule and the substrate selectivity. Scatchard analysis was used to study the MIP’s binding characteristic to template molecule. The results indicated that MIP has higher adsorption ability and selectivity. The equilibrium distribution coefficient K _D was 41.64 and the separation factor α was 1.62. Scatchard analysis showed that two different kinds of binding sites were produced in the polymer matrix and their dissociation constants were calculated to be K _d1 = 5.249 × 10⁻⁵ mol·L⁻¹, K _d2 = 2.237 × 10⁻³ mol·L⁻¹. __________ Translated from Chemistry, 2008, 71(2): 132–137     

A simple model for RPLC retention and selectivity of imidazole enantiomers using β-cyclodextrin as chiral selector

Summary Using reversed phase liquid chromatography (RPLC), this paper investigates the enantioselectivity variations, in a series of weak polar R, S-imidazole derivatives, with β-cyclodextrin concentration in the mobile phase over a wide range of column temperatures. These compounds are used for the treatment of onychomycosis. The selectivity data obtained were assessed using a chiral recognition model, based on the formation of complexes between the solute molecule and the cyclodextrin cavity. Gibbs Helmholtz parameters (Δ(ΔH), Δ(ΔS)) between R- and S- enantiomers were determined from the logarithm of the separation factor, α, versus the reciprocal of the temperature plots. The thermodynamic results predicted that the enantioselectivity mechanism was related to both the solute's bulkiness and the asymmetric carbon atom configuration.     

Analysis of vitamin E in food and phytopharmaceutical preparations by HPLC and HPLC-APCI-MS-MS

Summary The analysis of α, β, γ, δ-tocopherols, trienols, α-tocopheryl acetate and nicotinate (vitamin E) in complex matrices was carried out using a new liquid chromatographic (HPLC) method giving better separation efficiency, selectivity and sensitivity than that described in the literature. The use of normal-phase (NP)-HPLC on silica gel with issoctane-diisopropylether-1,4-dioxane as optimized mobilepphase yielded higher resolution than conventional reversed-phase (RP)-HPLC using methanol mobile phase. Identification of peaks was by UV-absorbance at 295 nm, diode array, or fluorescence detection (λ _ex = 295 nm,λ _ex = 330 nm). The latter was found to be more selective and ten times more sensitive than UV-absorbance detection. A quadrupole, ion-trap mass spectrometer with an atmospheric-pressure ionization (APCl) interface was used to detect vitamin E constituents in the femtomole range. With collision-induced dissociation (CID) in the ion source, which gave characteristic fragmentation, the identity of the investigated compounds could be confirmed. Plots of peak area versus amount injected allowed quantitation of α, β, γ, δ-tocopherols and-trienols, α-tocopheryl acetate and nicotinate in real samples such as peanut, almond, spinach, spelt grain bran, latex and tablets. The method described offers fast identification and quantitation of vitamin E constituents of complex biological origin. Dedicated to Professor Dr. Heinz Engelhardt on the occasion of his 65^th birthday.     

Prediction of the chromatographic behaviour for a series of diuretic compounds

Summary The proportion of organic modifier and the pH of the acetonitrile-water mixtures used as mobile phases were optimized in order to separate a group of diuretic compounds covering a wide range of physyco-chemical properties. The Linear Solvation Energy Relationship (LSER) formalism based either on the multiparameter π^*, β and α scales or the single solvent polarity parameterE _T ^N , have been used to predict their chromatographic behaviour as a function of the percentage of acetonitrile in the eluent. Moreover, correlation established between retention and pH of the aqueous-organic mobile phases have been used to predict the chromatographic behaviour of the diuretic compounds studied as a function of the eluent pH. Linear correlation between a function of the eluent pH. Linear correlation between the chromatographic retention and theE _T ^N polarity parameter of mobile phases containing different percentages of organic modifier has been obtained Based on the knowledge of the acid-base dissociation constant the relation between retention and mobile phase pH has also been linearized. These relationship allowed an important reduction of the experimental retention data needed for developing a given separation and a great improvement in chromatographic optimization schemes.     

Chiral discrimination ofN-(dansyl)-dl-amino acids on human serum albumin stationary phase: Effect of a mobile phase modifier

Summary The effect of perchlorate anion as mobile phase modifier on the separation factor, α, forN-(dansyl)-dl-norvaline andN-(dansyl)-dl-tryptophan on a human serum albumin (HSA) column was studied by varying the concentration,c, of the chaotropic agent and the column temperatureT. Gibbs-Helmholtz parameters Δ(ΔH) and Δ(ΔS) between thed andl enantiomers were determined from linear van't Hoff plots of lnα against 1/T. Thermodynamic results indicated that the enhancement of the separation factor observed asc was increased was enthalpically controlled owing to stereoselective H-bonding interactions. Such behavior was used to optimize the chromatographic conditions for separation ofN-(dansyl)-amino acids on HSA.     

Chromatographic resolution, chiroptical characterization and urinary excretion of the enantiomers of sulindac

Summary The chromatographic resolution of the enantiomers of sulindac has been achieved using a Chiralpak AD CSP (10 μm, 250×4.6 mm) with a mobile phase of hexane: ethanol (85∶15 v/v) containing trifluoroacetic acid (0.05% v/v) at a flow rate of 1.0 mL min⁻¹. Under these conditions the enantiomers eluted with separation and resolution factors of 1.43 and 2.46 respectively. Semipreparative isolation of the enantiomers and their characterization by circular dichroism spectroscopy and NMR, in the presence of a chiral shift reagent, indicated that the elution order was (−)-(S)- before (+)-(R)-sulindac. The enantiomeric composition of sulindac in urine following administration of the racemic drug to man was determined by sequential achiral-chiral chromatography. Achiral analysis was carried out using a Spherisorb S5 ODS2 stationary phase (5 μm, 250×4.6 mm) and a mobile phase of aqueous acetic acid (2% v/v; pH 3.5): acetonitrile: THF (50∶48∶2 by volume) at a flow rate of 1.0 mL min⁻¹. The HPLC eluate containing sulindac (retention time 4.9 min) was collected and following workup, the enantiomeric composition of the drug was determined using the CSP. Over the 24 h collection period sulindac was excreted predominantly as theR-enantiomer, but the enantiomeric composition was found to vary markedly with time which is presumably associated with the complex metabolism of the drug.     

Structural and calorimetric investigations on nonaqueous liquid crystals and gel phases in the binary system K-stearate/glycerol

The K-stearate/glycerol (KC₁₈/Gl) binary system was studied at mole fractions of stearate of x _KC18 = 0.10, 0.25, 0.30 and 0.50. Small- and wide-angle X-ray diffraction (XRD) measurements were combined with differential scanning calorimetry (DSC) measurements at different temperatures. The investigations were intended to verify the previously published phase diagram and were targeted at the confirmation of the gel-like (G₁) phase and the isotropic (I) phase. The XRD and DSC measurements lead to the conclusion that the G₁ phase as well as the I phase, the existence of which had been proposed from texture observations, do in fact not exist. Consequently, a correction of the preliminary phase diagram is given. This corrected phase diagram reveals the crystalline phase (C) ⇆ gel phase (G) ⇆ hexagonal phase (H_α) ⇆ isotropic, micellar phase phase transitions for low KC₁₈ concentrations of x _KC18 = 0.15–0.3 and the C ⇆ G ⇆ lamellar phase (L_α) phase transitions for concentrations about or higher than x _KC18 = 0.35. The C, G, L_α and H_α phases have been further characterized by structural parameters (characteristic d values) as a function of temperature. The phase transitions C ⇆ G, G ⇆ L_α and G ⇆ H_α correlate with sharp shifts in the d value of the first small-angle reflections. Received: 20 April 1999 Accepted: 28 July 1999     

Substrate and positional selectivity in electrophilic substitution reactions in pyrrole, furan, thiophene, and selenophene derivatives and related benzoannelated systems

Data on the relative reactivities (substrate selectivity) of five-membered heterocycles in electrophilic substitution reactions and positional selectivity (α : β ratio) in these reactions were analyzed. Unlike the substrate selectivity (pyrrole ≫ furan > selenophene > thiophene) determined by the position of heteroatoms in the Periodic Table, the positional selectivity decreases in the order corresponding to the change in the relative stability of the onium states of the elements (O⁺ < Se⁺ ≤ S⁺ < N⁺) and reflects the predominant role of heteroatoms in the stabilization of σ complexes formed upon β-substitution. These differences in the positional selectivity of the parent heterocycles have a substantial effect on the orientation in electrophilic substitution reactions in their derivatives and the corresponding benzoannelated systems. This interpretation was confirmed by ab initio quantum chemical calculations (RHF/6–31G(d) and MP2/6– 31G(d)//RHF/6–31G(d)) and density functional theory calculations (B3LYP/6–31G(d)). Quantum chemical calculations were performed by the above-mentioned methods for model N-R-pyrroles (R = Me, Et, Prⁱ, Bu^t, CH=CH₂, C≡CH, Ph, PhSO₂, and 4-O₂NC₆H₄) and their α- and β-protonated σ complexes. The results of these calculations demonstrated that it is the steric factors and charges on the β-C, α-C, and N atoms and the substituents at the N atom (the kinetic control), as well as the nature of the electrophile, rather than the difference in the relative stabilities of the onium states of N⁺ (which depends on the nature of the substituent at the N atom and reflects the role of the heteroatom in stabilization of σ complexes formed via β-substitution; the thermodynamic control) that are responsible for the type of orientation (α or β) that prevails. Dedicated to Academician V. I. Minkin on the occasion of his 70th birthday. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 837–846, April, 2005.     

Influence of the chain length of K-soaps on the phase diagram and structural parameters of nonaqueous liquid crystals and the gel phase in K-soap/glycerol binary systems

 Analogously to the aqueous K-soap/water systems already examined, the five glycerol · (Gl)-containing systems KC _n /Gl (n = 12, 14, 16, 18, 22) also built up hexagonal (H_α), lamellar (L_α), isotropic micellar (S), gel-like (G) and crystalline phases (C). These phases were identified by texture observations with a polarizing microscope, by differential scanning calorimetry measurements and by X-ray diffraction investigations. The appertaining phase regions were plotted in the binary phase diagram. Binary Gl-containing K-soap systems have the following properties. The H_α phase is built up at low soap concentration. The L_α phase is formed at high soap concentrations. The temperature of the phase transition H_α ⇆ S runs through a maximum. Increasing the chain lengths of the soaps shifts the formation of the H_α phase to lower soap concentration. A strong correlation between the chain length of K-soaps and the d values of L_α, H_α, G and C phases is found. Based on the comparison of the X-ray diffractograms of the G phase a structural model is proposed. The G phase consists of two groups of domains with two different dimensions. Received: 9 August 1999/Accepted in revised form: 20 September 1999     

The kinetics of ethanol adsorption from the aqueous phase onto zeolite NaZSM-5

The kinetics of the isothermal adsorption of ethanol from an aqueous solution onto a hydrophobic zeolite of the NaZSM-5 type in the temperature range 298–333 K was investigated. Specific shape parameters of the adsorption degree curves were determined. The changes in the specific shape parameters of the adsorption degree curves with temperature were determined. The kinetic parameters of ethanol adsorption (E_a,ln A) were determined by the initial rate, the saturation rate and the maximum rate methods as well as from the Johnson, Mehl and Avramy equation. The kinetic model of ethanol adsorption kt=[1−(1−α)^1/3] was determined by the “model fitting” method. Ethanol adsorption from aqueous solution onto NaZSM-5 is a kinetically controlled process limited by the rate of three-dimensional movement of the boundary layer of the adsorption phase. A model for the mechanism of ethanol adsorption onto NaZSM-5 is suggested on the basis of the kinetic model. Ethanol molecules in aqueous solution are associated in clusters. The activation energy of the adsorption process corresponds to the energy required for the detachment of an ethanol molecule from a cluster and its adsorption onto the zeolite.     

Interactions of native and modified cyclodextrins with some B-vitamins

Interactions of native and modified α- and β-cyclodextrins with nicotinic acid, pyridoxine and pyridoxal were studied by isothermal titration calorimetry, solution calorimetry, and ¹H NMR spectroscopy at 298.15 K and pH 6.8. Weak 1:1 complex formation was found only between α-cyclodextrin and nicotinic acid. The stability constant and corresponding thermodynamic parameters of complex formation (Δ_c G, Δ_c H and Δ_c S) were calculated using the calorimetric data. The ¹H NMR data indicate the shallow insertion of the carboxylic group of the nicotinic acid molecule into α-CD cavity. For all other compounds the weak interactions, not accompanied by complex formation, were characterized by the enthalpic virial coefficients calculated on the basis of McMillan-Mayer approach. The obtained thermodynamic parameters were analyzed in the terms of influence of the solutes’ structure on the selectivity of intermolecular host-guest interactions.     

Complete chromatographic separation of steroids, including 17α and 17β-estradiols, using a carbazole-based polymeric organic phase in both reversed and normal-phase HPLC

Poly(2-N-carbazolylethyl acrylate) with terminal trimethoxysilyl groups was prepared as an organic phase and immobilized onto silica. The retention behavior of the column packed with this carbazole-based polymer-immobilized silica (Sil-CEA) was investigated by using various estrogenic steroids and corticoids in both reversed-phase and normal-phase liquid chromatography. As a result, complete separation was confirmed for eight kinds of steroids with Sil-CEA. The most specific separation with Sil-CEA can be emphasized by the high separation factor (e.g., α = 1.39 in methanol–water (7:3, v/v) at 35 °C) for 17α and 17β-estradiols, one of the most difficult pairs of isomers in chromatographic separation, whereas for two kinds of commercially available polymeric ODS columns as references α = 1.01, only, under the same conditions. Because the excellent separation and retention order with Sil-CEA was maintained even in a normal-phase mobile phase such as a hexane–2-propanol, it is estimated that the CEA phase has multiple interaction mechanisms through stronger interactions such as dipole–dipole, carbonyl–π, and hydrogen bonding interactions than the hydrophobic effect expected with ODS.     

A calorimetric study of the acid dissociation of the conjugate acids of poly(N-vinylimidazole) and polyallylamine

Thermodynamic parameters for acid dissociation of the conjugate acids of poly(N-vinylimidazole) and polyallylamine have been determined in the presence of sodium chloride and sodium nitrate. Even though the plots of ΔG ⁰ against the degree of dissociation, α, are highly dependent on the added salt concentration levels, the concentration effect has never been observed for the corresponding ΔH ⁰ versus α plots. The effect on the ΔG ⁰ versus α plots has been attributed to the entropy change of the counterions between a polyelectrolyte phase and a bulk solution phase. The α dependency of ΔH ⁰ is affected remarkably by the kinds of cationic polymers and counter-anions. Each ΔH ⁰ value at completely neutralized conditions is quite close to the corresponding ΔH ⁰ value of the monomer analog. The difference in the ΔH ⁰ values at fully charged conditions has been explained by the heats due to The ion-pair formation of chloride anion to the conjugate acids of poly(N-vinylimidazole) and polyallylamine has been supported by ³⁵Cl NMR measurement. It has also been suggested that chloride anions bind the basic polymer molecules even at fully neutralized conditions. Received: 2 June 1999/Accepted in revised form: 19 July 1999     

An electroconductivity study on degree of counterion binding or dissociation of a-sulfonatomyristic acid methyl ester micelles in water as a function of temperature

 For a sodium salt of α-sulfonatomyristic acid methyl ester (14SFNa), one of the α-SFMe series surfactants, the differential conductivity (∂κ/∂C) _T , _P vs. square root of concentration (√C) was employed in order to determine not only CMC but also the limiting molar conductance (Λ⁰) and the molar conductance of micellar species (Λ^M). Based on the data of the degree of counterion binding to micelles (β) determined previously at different temperatures ranging 15–50 °C at every 5 °C, the experimental values of the degree of dissociation (ionization) of a micelle (α_EX) were calculated by regarding as α_EX=1−β. The ratio Λ^M/Λ⁰ corresponding to the ratio of slopes below and above CMC in the curve of specific conductivity (κ) vs. concentration (C), which has been often assumed to be the degree of ionization of micelles (α), was compared with the present α_EX. However, the ratio Λ^M/Λ⁰ (=α) was found to have a correlationship with α_EX (=1−β) as α_EX≈0.40×(Λ^M/Λ⁰), or strictly, α_EX=0.40 (Λ^M/Λ⁰)+0.08, indicating that the simple ratio of the slopes below and above CMC in κ vs. C curve is not true for α_EX=1−β. On the other hand, the method proposed by Evans gave a value closer to α_EX compared with the simple ratio. Received: 17 September 1996 Accepted: 8 April 1997     

TD-DFT Calculation on UV-Vis Spectra of the Complex 8-((Trimethoxysilyl)methylthio)quinoline⋅ZnCl<Subscript>2</Subscript>

The electronic structure and absorption spectra properties of the complex 8-((trimethoxysilyl)methylthio)quinoline⋅ZnCl₂ in the gas phase and in acetonitrile (MeCN) have been investigated by means of DFT/TD-DFT calculations. Calculation results indicate that the broad and weak experimentally observed absorption bands of the complex in MeCN at 335.6 nm originates from spin-forbidden singlet-triplet transitions, but the other experimentally observed absorption bands at 318.5 nm, 310.6 nm and 237.5 nm arise from spin-allowed singlet-singlet transitions. Inclusion of MeCN as solvent leads to dramatic changes in the electronic structures and energy levels of the frontier molecular orbitals of the complex, and hence transition mechanisms of the absorption bands are also changed. For the complex, whether in the gas phase or in MeCN, the metal Zn does not participate in the transitions involved, in the gas phase the calculated lowest-energy absorption band of the complex comes from π→π ^∗ mixed with n→π ^∗ transitions with LLCT (ligand-to-ligand charge transfer) character, while in MeCN, the calculated lowest-energy absorption band is of LLCT/ILCT (intra-ligand charge transfer) character.     

A Validated Chiral LC Method for the Enantiomeric Separation of Repaglinide on Amylose Based Stationary Phase

A simple, isocratic, normal phase chiral HPLC method was developed and validated for the enantiomeric separation of repaglinide, (S)-(+)-2-ethoxy-4-N [1-(2-piperidinophenyl)-3-methyl-1-butyl] aminocarbonylmethyl] benzoic acid, an antidiabetic in bulk drug substance. The enantiomers of repaglinide were resolved on a ChiralPak AD-H (amylose based stationary phase) column using a mobile phase consisting of n-hexane: 2-propanol:trifluoroacetic acid (95:5:0.2 v/v/v) at a flow rate of 1.0 mL min⁻¹. The resolution between the enantiomers was found to be not >3.5 in optimized method. The presence of trifluoroacetic acid in the mobile phase played an important role, in enhancing chromatographic efficiency and resolution between the enantiomers. The developed method was extensively validated and proved to be robust. The calibration curve for (R)-enantiomer showed excellent linearity over the concentration range of 900 ng mL⁻¹ (LOQ) to 6,000 ng mL⁻¹. The limit of detection and limit of quantification for (R)-enantiomer were 300 and 900 ng mL⁻¹, respectively. The percentage recovery of the (R)-enantiomer ranged between 98.3 and 101.05% in bulk drug samples of repaglinide. Repaglinide sample solution and mobile phase were found to be stable up to 48 h. The developed method was found to be enantioselective, accurate, precise and suitable for quantitative determination of (R)-enantiomer in bulk drug substance.     

A comparative study of234U/238U alpha-activity ratios determined by alpha-spectrometry and calculated from mass spectrometric data

²³⁴U/²³⁸U α-activity ratios determined by α-spectrometry (AS) and those calculated from the atom ratio data using the half-life values are compared in some of the isotopic reference materials of uranium and a few other uranium samples. For α-spectrometry, electrodeposited sources were prepared and a large area passivated ion implanted (IPE) detector (450 mm²) was used for recording the α-spectra. The isotopic composition of U was determined by thermal ionisation mass spectrometry (TIMS) and the recommended half-life values of²³⁴U and²³⁸U were used to calculate the α-activity ratio. It is observed that²³⁴U/²³⁸U α-activity ratios calculated from the atom ratio data are consistently high, with a mean difference of about 5%, when compared to the α-spectrometry results. This discrepancy warrants confirmation by a few more laboratories and suggests redetermination of the half-life values of²³⁴U and²³⁸U.     

DFT calculation of α-cyclodextrin dimers. contribution of hydrogen bonds to the energy of formation

The structures and energies of formation of α-cyclodextrin (α-CD) dimers formed according to the “head-to-head” (HH), “head-to-tail” (HT), and “tail-to-tail” (TT) modes, harmonic vibrational frequencies, and intensities of IR bands of the IR transitions were calculated by the DFT/PBE density functional method with full geometry optimization without symmetry restrictions. The spectral data were transformed into spectral patterns. An α-CD molecule can exist in two isomeric forms close in energy, namely, α-CD⁻ and α-CD⁺, with different directions of the ring intramolecular hydrogen bonds. Among the three α-CD⁻ dimers, the highest dimerization energy (E _d/kcal mol⁻¹) belongs to the HH⁻ (68.9), TT⁻ (43.4), and HT⁻ (24.8) dimers. The strength of the α-CD⁺ dimers decreases in the series: TT⁺ (56.7), HT⁺ (49.4), and HH⁺ (42.4). The energies E _H of hydrogen bonds were calculated from the low-frequency shifts of bands of stretching vibrations of the OH groups involved in the formation of these hydrogen bonds. The E _H value for each dimer correlates with E _d. A possibility of formation of intermolecular hydrogen bonds is a driving force of association of α-CD molecules in aqueous solutions. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1289–1296, August, 2006.