Practice and theory of enantioselective complexation gas chromatography

Rapid chiral assay of amino compounds using diethyl squarate

The versatility and importance of chiral compounds make it urgent to develop fast and efficient methods to detect the absolute configuration, enantiomeric excess(ee), and concentration of chiral compounds. In this study, we demonstrate that commercially available diethyl squarate can rapidly react with various types of chiral amino compounds and exhibit characteristic ultraviolet (UV) and circular dichroism (CD) signals. The UV and CD signals can determine the total concentration of the two enantiomers and ee value of the sample, respectively. The probe showed a broad substrate scope, applicable to 39 tested chiral amino compounds, including chiral amino acids, amino alcohols, and amines. Additionally, the probe accurately detected 10 samples of phenylalanine, phenylglycinol, and phenethylamine with the error range less than 8%, demonstrating the practicability of this method.

A dual-channel colorimetric and ratiometric fluorescence chemosensor for detection of Hg2+ ion and its bioimaging applications

A new colorimetric and ratiometric fluorescence chemosensor 4-((3-(octadecylthio)-1,4-dioxo-1,4-dihydronaphthalen-2-yl)amino)benzenesulfonamide (4DBS) was synthesized and investigated for the selective detection of Hg²⁺ in DMSO-H₂O (9:1, v/v) solution. The chemosensor was efficiently synthesized in two steps via Michael-like addition and nucleophilic substitution reactions. The ratiometric fluorescence turn-on response was obtained towards Hg²⁺, and its fluorescence emission peak was red-shifted by 140 nm with an associated color change from light maroon to pale yellow due to the intramolecular charge transfer effect. The formed coordination metal complex was further evaluated by FT-IR, ¹H NMR, and quantum chemical analyses to confirm the binding mechanism. The detection process was sensitive/reversible, and the calculated limit of detection for Hg²⁺ was 0.451 µM. Furthermore, 4DBS was effectively utilized as a bioimaging agent for detection of Hg²⁺ in live cells and zebrafish larvae. Additionally, 4DBS showed distinguishing detection of Hg²⁺ in cancer cells in comparison with normal cells. Thus, 4DBS could be employed as an efficient bioimaging probe for discriminative identification of human cancer cells.

Mitochondria-targeted dual-channel colorimetric and fluorescence chemosensor for detection of Sn2+ ions in aqueous solution based on aggregation-induced emission and its bioimaging applications

Several fluorescence and colorimetric chemosensory for Sn²⁺ detection in an aqueous media have been reported, but applications remain limited for discriminative Sn²⁺ detection in live human cells and zebrafish larvae. Herein, a mitochondria-targeted Sn²⁺ "turn-on" colorimetric and fluorescence chemosensor, 2CTA, with an aggregation-induced emission (AIE) response was developed. The sensing of Sn²⁺ was enabled by a reduction-enabled binding pathway, with the conversion of -C˭O groups to -C-OH groups at the naphthoquinone moiety. The color changed from light maroon to milky white in a buffered aqueous solution. The chemosensor 2CTA possessed the excellent characteristics of good water solubility, fast response (less than 10 s), and high sensitivity (79 nM) and selectivity for Sn²⁺ over other metal ions, amino acids, and peptides. The proposed binding mechanism was experimentally verified by means of FT-IR and NMR studies. The chemosensor 2CTA was successfully employed to recognize Sn²⁺ in live human cells and in zebrafish larvae. In addition, a colocalization study proved that the chemosensor had the ability to target mitochondria and overlapped almost completely with MitoTracker Red. Furthermore, a bioimaging study of live cells demonstrated the discriminative detection of Sn²⁺ in human cancer cells and the practical applications of 2CTA in biological systems.

Chiral Fluorescent Recognition by Naphthalimide

Chirality plays a very important role in medicine, biochemistry and other fields. Because the enantiomers of chiral drugs often show different pharmacology activity, metabolism, and toxicity, therefore, the recognition of chiral molecules is very important, and has become a hot spot and frontier of modern chemical research. In this paper, a new method for recognizing chiral molecular based on naphthalimide dye(NA)⊂cucurbit[5]uril(CB[7]) assembly is developed. NA as guest can be combined with the host CB[7] to form a 1:1 NA⊂CB[7] assembly. Furthermore, this assembly was used as a fluorescent probe to recognize D/L-phenylalanine and D/L-phenylalaninol by fluorescence titration. When D-phenylalanine or D-phenylalaninol was added to NA⊂CB[7] assembly, the fluorescent intensity of assembly was partially quenched, but when L-phenylalanine or L-phenylalaninol was added to NA⊂CB[7], the fluorescence intensity of the assembly almost unchanged. Herein, chiral recognition platform based on achiral NA⊂achiral CB[7] was constructed.

An overview of recent progress in solvent systems, additives and modifiers of counter current chromatography

Solvent systems are critical to counter current chromatography (CCC). Appropriate additives and modifiers can be used to improve the separation efficiency of CCC.

Fluorescence recognition of chiral amino alcohols by using a novel ionic liquid sensor

A novel task-specific ionic liquid derived from l-phenylalaninol was prepared as an enantioselective fluorescent sensor for the first time.

Enantiomeric separation of volatile organics by gas chromatography for the in situ analysis of extraterrestrial materials: kinetics and thermodynamics investigation of various chiral stationary phases

The performances of several commercial chiral capillary columns have been evaluated with the aim of determining the one most suitable for enantiomeric separation in a gas chromatograph onboard a space probe. We compared the GC-MS response of three capillary columns coated with different chiral stationary phases (CSP) using volatile chiral organic molecules which are potential markers of a prebiotic organic chemistry. The three different chiral capillary columns are Chirasil-Val, with an amino acid derivative CSP, ChiralDex-β-PM, with a CSP composed of dissolved permethylated β-cyclodextrins in polysiloxane, and Chirasil-Dex, with a CSP made of modified cyclodextrins chemically bonded to the polysiloxane backbone. Both kinetics and thermodynamics studies have been carried out to evaluate the chiral recognition potential in these different types of columns. The thermodynamic parameters also allow a better understanding of the driving forces affecting the retention and separation of the enantiomers. The Chirasil-Dex-CSP displays the best characteristics for an optimal resolution of the chiral compounds, without preliminary derivatization. This CSP had been chosen to be the only chiral column in the Sample Analysis at Mars (SAM) experiment onboard the current Mars Science Laboratory (MSL) mission, and is also part of the Mars Organic Molecules Analyzer (MOMA) gas chromatograph onboard the next Martian mission ExoMars. The use of this column could also be extended to all space missions aimed at studying chirality in space.

Investigation of novel immobilized 3-(perfluoroalkanoyl)-(1R)-camphorate nickel complexes in enantioselective complexation gas chromatography

Novel 3-(perfluoroalkanoyl)-(1R)-camphorate nickel complexes immobilized to poly(dimethylsiloxane) phases are presented. Immobilized 3-(perfluoroalkanoyl)-(1R)-camphorate nickel complexes with a trifluoromethyl (CF(3); nickel(II)-bis[(1R,4S)-3-trifluoromethanoyl-10-propylenoxycamphor]-polysiloxane Ni(tfpc)(2)@PS) and a heptafluoropropyl-substituent (C(3)F(7); nickel(II)-bis[(1R,4S)-3-heptafluorobutanoyl-10-propylenoxycamphor]-polysiloxane Ni(hfpc)(2)@PS) were synthesized, characterized and immobilized to polysiloxane. Ni(hfpc)(2)@PS was immobilized with a selector content of 38% and their enantioseparation ability was compared to selector concentrations of 4% and 20%. The influence of the perfluorinated moiety as well as the effect of the selector concentration on enantioseparations were investigated. Based on different functionalized organic compounds the quality of enantioseparation was analyzed and compared.

Online coupling of enantioselective capillary gas chromatography with proton nuclear magnetic resonance spectroscopy

The hyphenation of enantioselective capillary gas chromatography and mass spectrometry is not always sufficient to distinguish between structural isomers, thus requiring peak identification by NMR spectroscopy. Here the first online coupling of enantioselective capillary gas chromatography with proton nuclear resonance spectroscopy is described for the unfunctionalized chiral alkane 2,4-dimethylhexane resolved on octakis(6-O-methyl-2,3-di-O-pentyl)-gamma-cyclodextrin at 60 degrees C. NMR allows constitutional and configurational isomers (diastereomers and enantiomers) to be distinguished. Enantiomers display identical spectra at different retention times, which enable an indirect identification of these unfunctionalized alkanes. The presented method is still at an early development stage, and will require instrumental optimization in the future.

Chiral separations

The main goal of this review is to provide a brief overview of chiral separations to researchers who are versed in the area of analytical separations but unfamiliar with chiral separations. To researchers who are not familiar with this area, there is currently a bewildering array of commercially available chiral columns, chiral derivatizing reagents, and chiral selectors for approaches that span the range of analytical separation platforms (e.g., high-performance liquid chromatography, gas chromatography, supercritical-fluid chromatography, and capillary electrophoresis). This review begins with a brief discussion of chirality before examining the general strategies and commonalities among all of the chiral separation techniques. Rather than exhaustively listing all the chiral selectors and applications, this review highlights significant issues and differences between chiral and achiral separations, providing salient examples from specific classes of chiral selectors where appropriate.

Recent advances in gas chromatography for solid and liquid stationary phases containing metal ions

This review is devoted to the application of metal complexes as column packings and liquid stationary phases in gas chromatography. Particular attention is paid to the stationary phases with nitrogen-containing functional groups (e.g., amine and ketoimine) and beta-diketonates on the modified silica surface. The review also concerns the results of the research on metallomesogenes and chiral stationary phases. The factors influencing the retention mechanism in complexation gas chromatography are discussed. Practical application of the metal chelate-containing chromatographic packings for analytical separation of organic substances is considered.

Elaborate treatment of retention in chemoselective chromatography--the retention increment approach and non-linear effects

The retention increment approach is described which quantifies the association equilibria of a selectand and a selector in complexation and inclusion chromatography. A thermodynamic treatment of enantioselectivity based on retention phenomena in chromatography including entropy/enthalpy compensation and the isoenantioselective temperature is advanced. Kinetic parameters of enantiomerization are discussed. Non-linear effects, both existent and elusive, are described and proposed, respectively. The enantioselectivity pertaining to catalysis vs. chromatography is compared and a unified nomenclature is proposed. Through an educational effort, this account is aimed at providing a deeper insight into chemoselective aspects of chromatography thereby stimulating further research of both established and speculative phenomena of the most intriguing manifestation of chemoselectivity, that is, of enantioselectivity.

Existence of a low isoenantioselective temperature in complexation gas chromatography

The gas chromatographic enantiorecognition of the two enantiomeric pairs of Z- and E-2-ethyl-dioxaspiro[4,4]nonane (chalcogran), respectively, critically depends on whether the chiral selector nickel(II) bis[3-(heptafluorobutanoyl)-(1R)-camphorate] is chemically linked via a methylene spacer to poly(dimethylsiloxane) (Chirasil-Nickel 1), or is only dissolved in poly(dimethylsiloxane) (Chira-Nickel 2a). On 2a, the enantiomers are separated at ambient temperature with a large apparent enantioseparation factor alpha(app) whereas on Chirasil-Nickel 1 only a low alpha(app) is observed whereby the elution order of the enantiomers is reversed. Concise temperature-dependent studies show that on Chirasil-Nickel 1 a low isoenantioselective temperature, T(isoenant), of 80 degrees C is experimentally observed as the result of enthalpy/entropy compensation. For thermodynamic measurements, the method of the retention-increment R' has been employed. This concept separates achiral contributions to retention due to the poly(dimethylsiloxane) matrix, which are identical for enantiomers, from enantioselective contributions to retention, which are due to complexation of enantiomers with the chiral selector. From Van't Hoff plots, concise thermodynamic data of enantioselectivity, -Delta(D),(L)(DeltaG), -Delta(D,L)(DeltaH), Delta(D,L)(DeltaS) and T(isoenant) were obtained. It was found that minute changes in the structure of the spiroketals as well as miniscule differences of the nature of the chiral selector present in the stationary phase (chemically bonded versus physically dissolved) led to profound differences in enantioselectivity. The observation of a low isoenantioselective temperature T(isoenant) represents are rare phenomenon in enantioselective gas chromatography which both complicates the study of chiral recognition mechanisms and the correlation of the absolute configuration and retention due to the temperature-dependent reversal of the elution order.

Gas chromatographic high-throughput screening techniques in catalysis

Discovering highly efficient catalysts is of great scientific and economical interest. Advances in high-throughput assays in combination with sophisticated analytical techniques have increased the rapidity with which catalysts can be identified and optimized. Understanding how kinetics in the mechanism of catalysis is controlled by structural parameters is essential for a directed design of catalysts. To identify such rate-controlling elementary steps and to develop and refine models, comprehensive experimental kinetic data of a broad variety of substrates are necessary. In the present article concepts of high-throughput screening techniques in catalysis using gas chromatography are reviewed in a survey covering the period from 1998 to 2007. To cover also the origins of concepts and groundbreaking experiments in this research area milestones going back to 1950 are also reviewed. The first part of the review will focus on off-line gas chromatographic analysis, the second part on on-line gas chromatographic analysis covering sequential, parallelized and high-throughput multiplexing gas chromatography. The third part presents recent advances in the integration of chemical transformation and analysis in gas chromatography. The present review article describes the state-of-the-art, scope and limitations, and applications of these different high-throughput screening approaches.

Separation of dialkyl sulfides by metallo-mesogenic stationary phases for complexation gas chromatography

A copper mesogenic side-chain polymer (P-C(15)CuC(18)) was cross-linked onto the capillary wall as a stationary film for gas chromatography (GC) separation of alkyl sulfides. These organic sulfides are of interest for their large health impact because of their wide range of volatiles and high reactivities toward metals. Different GC parameters for optimal separation efficiency are discussed for use with a mesogenic polymer column along with flame photometric detection (FPD). Both the carrier gas flow-rate and column temperature were studied to determine the relationship of plate height to the chemical structure of the solutes, as well as to determine the morphology of the mesogenic polymer. Van 't Hoff plots show phase transitions of the stationary mesophase as the column temperature was varied. The results reveal that the separation mechanism might be based on ligand exchange and polarity interaction between the analytes and the stationary phase, with the vapor pressure of the analytes also being important. The former interaction dominates in the lamellar crystalline phase and the latter interaction dominates in the hexagonal columnar-discotic phase. With high reproducibility for retention time (RSD< or =0.37%) and for peak area (RSD< or =5.16%), the GC-FPD system produced linear calibration graphs (r> or =0.9918) for the determination of 13 sulfides with a detection limit below 2.5 ng.