Effects of the gradient profile, sample volume and solvent on the separation in very fast gradients, with special attention to the second-dimension gradient in comprehensive two-dimensional liquid chromatography

Gradient elution provides significant improvement in peak capacity with respect to isocratic conditions and therefore should be used in comprehensive two-dimensional LC×LC, both in the first and in the second dimension, where, however, gradients are limited to a short time period available for separation, usually 1 min or less. Gradient conditions spanning over a broad mobile phase composition range in each second-dimension fraction analysis are used with generic "full in fraction" (FIF) gradients. "Segment in fraction" (SIF) gradients cover a limited gradient range adjusted independently to suit changing lipophilicity range of compounds transferred to the second dimension during the first-dimension gradient run and to provide regular coverage of the two-dimensional retention space. Optimization of the gradient profiles is important tool for achieving high two-dimensional peak capacity and savings of the separation time in comprehensive LC×LC. Calculations based on the well-established gradient-elution theory can be used to predict the elution times and bandwidths in fast gradients, taking into account increased probability of pre-gradient or post-gradient elution. The fraction volumes transferred into the second dimension may significantly affect the second-dimension bandwidths, especially at high elution strength of the fraction solvent, which may cause even band distortion or splitting in combined normal-phase (HILIC)-RP systems, but also in some two-dimensional RP-RP systems. In the present work, the effects of the fast gradient profile, of the sample volume and solvent on the elution time and bandwidths were investigated on a short column packed with fused-core porous-shell particles, providing narrow bandwidths and fast separations at moderate operating pressure.     

Some new information on the formation of substituted 4‐amino‐1‐substituted phenyl‐1H‐pyrazoles from β‐enaminones and diazonium tetrafluoroborates

Upon reaction of 2‐methyl‐, 3‐ethoxycarbonyl, and 4‐ethoxycarbonylbenzenediazonium tetrafluoroborate with 1‐cyclopropyl‐3‐phenylaminohex‐2‐en‐1‐one 3‐cyclopropylcarbonyl‐1‐(substituted phenyl)‐5‐ethyl‐4‐phenylamino‐1H‐pyrazoles are formed. On the other hand, the reaction of 1‐cyclopropyl‐3‐phenylaminohex‐2‐en‐1‐one and 5‐methylaminohept‐4‐en‐3‐one with sterically more demanding 2‐ethoxycarbonylbenzenediazonium tetrafluoroborate does not give the corresponding pyrazoles but the probable intermediates on the route to the pyrazoles: 1‐cyclopropyl‐3‐phenyliminohexane‐1,2,4‐trione 2,4‐bis(2‐ethoxycarbonylphenylhydrazone) and 3‐methyliminoheptane‐2,4,5‐trione 2,4‐bis(2‐ethoxycarbonylphenylhydrazone), respectively. All the compounds were identified on the basis of ¹H‐ and ¹³C‐NMR spectra. The structure of 1‐cyclopropyl‐3‐phenyliminohexane‐1,2,4‐trione 2,4‐bis(2‐ethoxycarbonylphenylhydrazone) was confirmed by means of ¹⁵N‐NMR spectra and X‐ray. The bis(2‐ethoxycarbonylphenylhydrazones) were found to show atropoisomerism due to a hindered rotation around the bond between the carbons of imino group and the hydrazono group next to carbonyl. In the case of the crystalline cyclopropyl derivative, the unit cell was found out to contain two molecules of opposite chirality. J. Heterocyclic Chem., (2011).     

A non-radioactive assay for precise determination of intracellular levels of imatinib and its main metabolite in Bcr-Abl positive cells

Multidrug resistance (MDR) is often associated with overexpression of the P-glycoprotein (P-gp, ABCB1). It was demonstrated that the P-gp mediated efflux decreases the drug concentration in cancer cells which results in the failure of chemotherapy. However, the MDR phenotype in cancer cells obviously involves various mechanisms. Therefore, if we want to estimate a contribution of the P-gp expression to the MDR phenotype, a clear quantitative relationship between the intracellular drug level and cell sensitivity must be established. To achieve this goal, a sensitive and non-radioactive assay for precise determination of intracellular levels of imatinib and its main metabolite N-desmethyl imatinib (CGP 74588) has been developed. The assay is based on an optimised extraction of cells with 4% formic acid after their separation from the growth medium by centrifugation through a layer of silicone oil. Cell extracts are subsequently analyzed by LC/MS/MS. Calibration curves were linear from 1 to 500 nmol/l for imatinib and from 2 to 500 nmol/l for CGP 74588, with correlation coefficients (r²) better than 0.998 and 0.996, respectively. The limit of quantitation (LOQ) was 1 nmol/l for imatinib and 2 nmol/l for CGP 74588. Our method has been successfully applied to the determination of intracellular levels of imatinib in sensitive K562 and their resistant variant, K562/Dox cells.     

Theoretical study on the protonation of cucurbit[6]uril

Abstract  

The most probable structures of the cucurbit[6]uril·H₃O⁺ and cucurbit[6]uril·(H₃O⁺)₂ cationic complex species have been derived by quantum mechanical DFT calculations. In these two complexes, each of the H₃O⁺ ions is bound by three strong linear hydrogen bonds to three carbonyl oxygen atoms of the parent macrocycle.     

Extraction and DFT study on the complexation of H3O+ ion with tetrakis(2-ethoxyethoxy)-tetra-p-tert-butylcalix[4]arene

Abstract  

Extraction experiments in the two-phase water/nitrobenzene system and γ-activity measurements were used to determine the stability constant of protonated tetrakis(2-ethoxyethoxy)-tetra-p-tert-butylcalix[4]arene in nitrobenzene saturated with water. Density functional theory (DFT) calculations were applied to derive the most probable structure of the tetrakis(2-ethoxyethoxy)-tetra-p-tert-butylcalix[4]arene·H₃O⁺ complex species.     

A novel fused-silica capillary dropping mercury electrode with long drop-time and its application on determination of critical micelle concentration

A novel long drop time mercury electrode has been constructed from common fused-silica capillary (50 μm I.D., 360 μm E.D.). Proposed device provides reproducible mercury drops with typical lifetime 40-120 s. The electrode was used for a set of electrocapillary measurements aimed at determination of critical micelle concentration of anionic surfactants by a convection controlled drop-time technique. A critical micelle concentration of sodium dodecyl sulfate 5.6 ± 0.4 mmol L⁻¹ and 4.3 ± 0.4 mmol L⁻¹ were obtained in 1 mmol L⁻¹ and 5 mmol L⁻¹ phosphate buffer (pH 7.0), respectively. The values were comparable to those obtained from conductometric measurements under the same conditions (7.0 ± 0.1 mmol L⁻¹ and 5.2 ± 0.1 mmol L⁻¹, respectively) and the difference was explained in accordance with theory of hemi-micelle formation.     

Past, present, and future of applications of electroanalytical techniques in analytical and physical organic chemistry

After a brief introduction describing the main milestones in the development of DC polarography (DCP) as well as linear sweep (LSV) and cyclic voltammetry (CV), attention is paid to first applications of DCP in the reduction of organic compounds. In aqueous solutions the electron transfer (ET) is often accompanied by a proton transfer. Limiting currents in DCP enable investigation of kinetics of chemical reactions preceding ET. Dependence of the limiting current on concentration of a reagent, like H⁺ or OH^{??/sup> ions, enables determination of rate constants (k) of very fast reactions, with k values between 104 and 1010?L?mol???s??, comparable with those studied by relaxation methods. Application of CV is most advantageous for investigation of rates of chemical reactions following the ET. Comparison of analytical applications of DCP (reductions) with those of LSV (oxidations) is given. Apart from fast reactions taking place before the ET in the solution in the vicinity of the electrode surface, DCP can also be used for investigation of slower reactions, taking place in the bulk of the solution. Data obtained by DCP for determination of equilibrium (K) and rate (k) constants of reactions of organic compounds. Hence, DCP can be used in physical organic chemistry of solutions, in some cases complementing data obtained by UV–vis spectrophotometry. Some examples of determinations of K and k are given. Uncertain future of practical analytical applications of DCP and LSV is discussed as is the brighter future of applications in physical organic chemistry. As the main factor limiting successful applications is considered the limited opportunity for education of future research advisors in academia and group supervisors in industry.}     

Photochemistry of HI on argon and water nanoparticles: hydronium radical generation in HI·(H2O)n

Photochemistry of HI molecules on large Ar(n) and (H(2)O)(n), n ～ 100-500, clusters was investigated after excitation with 243 nm and 193 nm laser radiation. The measured H-fragment kinetic energy distributions pointed to a completely different photodissociation mechanism of HI on water than on argon clusters. Distinct features corresponding to the fragment caging (slow fragments) and direct exit (fast fragments) were observed in the spectra from HI photodissociation on Ar(n) clusters. On the other hand, the fast fragments were entirely missing in the spectrum from HI·(H(2)O)(n) and the slow-fragment part of the spectrum had a different shape from HI·Ar(n). The HI·(H(2)O)(n) spectrum was interpreted in terms of the acidic dissociation of HI on (H(2)O)(n) in the ground state, and hydronium radical H(3)O formation following the UV excitation of the ionically dissociated species into states of a charge-transfer-to-solvent character. The H(3)O generation was proved by experiments with deuterated species DI and D(2)O. The experiment was complemented by ab initio calculations of structures and absorption spectra for small HI·(H(2)O)(n) clusters, n = 0-5, supporting the proposed model.     

HNC/HCN ratio in acetonitrile, formamide, and BrCN discharge

Time-resolved Fourier transform (FT) spectrometry was used to study the dynamics of radical reactions forming the HCN and HNC isomers in pulsed glow discharges through vapors of BrCN, acetonitrile (CH(3)CN), and formamide (HCONH(2)). Stable gaseous products of discharge chemistry were analyzed by selected ion flow tube mass spectrometry (SIFT-MS). Ratios of concentrations of the HNC/HCN isomers obtained using known transition dipole moments of rovibrational cold bands v(1) were found to be in the range 2.2-3%. A kinetic model was used to assess the roles the radical chemistry and ion chemistry play in the formation of these two isomers. Exclusion of the radical reactions from the model resulted in a value of the HNC/HCN ratio 2 orders of magnitude lower than the experimental results, thus confirming their dominant role. The major process responsible for the formation of the HNC isomer is the reaction of the HCN isomer with the H atoms. The rate constant determined using the kinetic model from the present data for this reaction is 1.13 (±0.2) × 10(-13) cm(3) s(-1).