Separation Of Geometric Isomers Research Articles

Immobilization of two dendritic organic phases onto silica and their molecular shape recognition for polycyclic aromatic hydrocarbons, tocopherols and carotenoid isomers

BackgroundMolecular shape selectivity, based on the size and shape parameters of the molecule, such as length and planarity, is a separation process that can be used for compounds with restricted shapes, such as isomers. The separation of geometric isomers is challenging because these compounds have similar physicochemical properties but differ slightly in molecular shape. The ability to separate and quantify these isomers is important in high performance liquid chromatography (HPLC), which is one of the most widely used techniques in separation science today, because the shape of the molecule has a strong influence on biological processes. ResultsWe prepared symmetrical discoidal dendrimeric organomolecule gelators (GSDM) and o-phenylenediamine-derived low-molecular-weight dendrimeric organomolecule gelators (G1) and bonded them to silica surfaces. The dendritic organic compound-grafted silica (SiO2@GSDM and SiO2@G1) was used as HPLC stationary phases for the separation of shape-restricted isomers of polycyclic aromatic hydrocarbons (PAHs), carotenoids and tocopherols. The two phases exhibit a very high molecular shape selectivity compared to the commercially available alkyl phases. There are differences in molecular shape selectivity between the two stationary phases. Changes in the chemical structure of dendritic organic compounds can alter the orientation of the molecules, as well as changes in the molecular recognition ability. It was found that SiO2@GSDM has high molecular linear selectivity for PAHs at different temperatures, even at 50 °C. The planar selectivity of SiO2@GSDM was better for triphenylene and o-terphenyl benzenes compared to SiO2@G1. SignificanceThis separation behavior may be attributed to the combined effect of weak interaction centers, which allowed the effective separation of bioactive and shape-restricted isomers through multiple interactions. Furthermore, SiO2@GSDM showed better separation of tocopherols and carotenoids, suggesting that the backbone and ordered structure of organic molecular gelators is an effective way to improve the shape selectivity of the molecules, whereas the molecular orientation of the functional groups influences the separation mechanism of the shape-restricted isomers.

C60‐Mediated Molecular Shape Sorting: Separation and Purification of Geometrical Isomers

AbstractA supramolecular crystallization‐based approach has been developed for the shape‐dependent separation of geometrical isomers under near‐ambient conditions. Difficulties to separate such isomers arise because of their very similar physical properties. The present approach relies on the ability of C60 to preferentially form solvate crystals with molecules of a specific geometry. Subsequently, these molecules are released upon mild heating to regenerate pure C60. By taking isomers of xylene and trimethylbenzene (TMB) as examples, we show that one of the isomers can be extracted from the rest with very high purity. To separate TMB isomers, a new C60–1,3,5‐TMB solvate was developed, which led to the isolation of isomer purities greater than 99.6 %. Versatility, a low operating temperature of approximately 100 °C, a separation efficiency of more than 10 weight % of C60 per cycle, and reagent recyclability makes this a promising molecular shape‐sorting approach.

HYDROPHILIC INTERACTION PLANAR CHROMATOGRAPHY OF GEOMETRICAL ISOMERS OF SELECTED Co(III) COMPLEXES

The retention behavior of fourteen geometrically isomeric Co(III) complexes: three pairs of neutral facial-meridional (fac-mer) isomers, one pair of cis-trans isomeric complexes of neutral type, and three pairs of cationic cis-trans isomers have been investigated on thin-layer of silica gel. The effect of the composition of the mobile phases consisting of different ratios of water, methanol, and sodium chloride on retention of the complexes has been explored. In the broad range of mobile phase compositions, a successful separation of geometrical isomers was obtained. A retention order of isomeric complexes consistent with the mobile phase composition was found. The best resolution of isomers was achieved by using solvent systems containing 10% water. On the basis of the results obtained, possible separation mechanisms were considered. Hydrophilic-interaction chromatography was assumed to be the mechanism determining separation in the case of mobile phases containing small amounts of both water and salt.

ChemInform Abstract: Selective and Convenient Enzymatic Separation of Geometric Isomers of N-Protected α-Dehydrophenylalanine Esters.

Selective enzymatic hydrolysis of a mixture of (Z)- and (E)-N-protected-ΔPhe-OMe with α-chymotrypsin A took place to give (Z)-ΔPhe-OH and (E)-APhe-OMe.

Separation of isomers of dienoic acids by electromigration techniques

AbstractDifferent capillary electromigration techniques were employed to resolve geometrical isomers of sorbic acid, decadienoic acid, and ethyl sorbate. Since these substances differ in their polarity, shape, and size, various electromigration approaches were investigated to separate the four geometrical isomers of each compound. With capillary electrophoresis (CE) modified with a cyclodextrin (β‐CD) the four isomers of sorbic acid were separated using a buffer that consists of 60 mM tetraborate and 8 mg/mL β‐CD. The separation of decadienoic acid geometrical isomers was not possible, even at elevated tetraborate and cyclodextrin concentrations. The four isomers of decadienoic acid were successfully separated using micellar electrokinetic chromatography (MEKC) with a buffer consisting of 30 mM tetraborate and 100 mM SDS and microemulsion electrokinetic chromatography (MEEKC). Ethyl sorbate is the least polar of all the studied substances and its isomers could not be separated by MEKC or MEEKC. The resolution was improved and isomers were fully separated using capillary electrochromatography (CEC) with ODS stationary phase and a mobile phase consisting of 10 mM boric acid in 50% acetonitrile. Minor differences in the polarity and the shape of isomers and high resolving power of the applied techniques were sufficient for separation of very similar compounds. We have shown that versatile electromigration techniques can be applied for separation of geometrical isomers of dienoic acids and their esters.

Impurity profiling of non-nucleoside reverse transcriptase inhibitors by HPLC using a porous graphitic carbon stationary phase

Porous graphitic carbon (PGC) HPLC columns have been marketed for several years and have been shown to be superior to octadecyl silica columns for the separation of geometric isomers and closely related compounds. This makes them ideally suited to the determination of related substances and egradation products of drugs. In this study, PGC was used to improve the separation of related substances and degradation products of DPC 961, a new Non Nucleoside Reverse Transcriptase Inhibitor in development for the treatment of HIV infections. The porous graphitic carbon column dramatically improved the separation ofcis/trans isomeric related substances. In addition, the PGC column separated DPC 961 degradants which were not resolved from the main drug peak on a silica-based octadecyl column.

Preparation and evaluation of an imidazole-coated capillary column for the electrophoretic separation of aromatic acids

An imidazole-coated capillary column for electrophoresis has been prepared by means of organosilanization. With mesityl oxide as neutral marker, the results indicated that the electroosmotic flow of the bonded phase displays a dramatic difference in pH dependence in comparison with that of the bare fused-silica column. The presence of positive charges on the coating surface and the anionic exchange property, due to the cationic property of the imidazole group at pH values below 6, allows the separation of geometric isomers that are very similar in ionic mobility. Separation parameters including buffer composition and concentration, pH, applied voltage, and the influence of other additives were investigated. By using acetate buffer (100 m M, pH 5.2) and an applied voltage of −15 kV with UV detection at 212 nm, the separation of 11 aromatic acids including mono-, di-, tri- and tetra-carboxylic acids could be achieved in less than 14 min. The average plate number was 3×10 5/m. With acetate buffer (25 m M, pH 5.5) and an applied voltage of −25 kV, the addition of silver nitrate or β-cyclodextrin significantly improved the resolution of some more highly charged carboxylic acids.

Chromatographic and electrophoretic analysis of biomedically important retinoids.

The determination of retinol (vitamin A) and its metabolites, as well as synthetic retinoids, in biological samples is a challenging task due to the sensitivity of these compounds to light, heat and oxygen, high protein binding, separation of geometric isomers and determination of low endogenous levels. Numerous procedures for sample preparation have been published for biological fluids and tissues, consisting of solvent extraction, solid-phase extraction (off-line) and HPLC with column switching (on-line solid-phase extraction). The last-mentioned technique has several advantages, including a high degree of automation, no evaporation of extraction solvents, protection from light and higher sensitivity. Due to the favourable UV characteristics of most retinoids, HPLC with UV detection is most often employed, and photodiode array detection is becoming more and more popular. Fluorescence and electrochemical detection have found only a limited field of application, but the use of LC-MS resulted in a few highly sensitive methods. Reconsideration of GC through the use of better deactivated columns and cold on-column injection and evaluation of new promising separation methods, such as supercritical fluid chromatography and capillary electrophoresis, have shown preliminary encouraging results, but appear to reach the required sensitivity only by coupling to MS. Therefore, HPLC with UV detection is still the method of choice for highly sensitive and selective retinoid determination, as well as for high sample throughput and robustness.

Liquid Chromatographic Separation of Geometrical Isomers Using Spherical Carbon Packings Prepared from Spherical Cellulose Particles

Abstract The packings of spherical carbon beads were prepared by graphitizing the spherical cellulose particles. A types of carbon spherical packings were obtained by various degrees of graphitization. These packings were characterized by examining their chromatographic properties. The higher the degree of graphitization is, the higher the π-electron recognition became, although there was no change in the selection for a methylene group. The mechanism for recognizing the π-electron and the steric selectivity is described and discussed.

Separation of geometrical isomers of sesquiterpene and diterpene alcohols on bare silica by carbon dioxide supercritical fluid chromatography

The application of supercritical fluid chromatography (SFC) with modified carbon dioxide to the separation of geometrical isomers of several sesquiterpene or diterpene alcohols is reported. The chromatographic system consists of bare silica (250×4.6 mm i. d.) with carbon dioxide-methanol mobile phase and UV detection. A mixture of sesquiterpene alcohols, (2E-6E)-farnesol, α-bisabolol, (E)-nerolidol and (Z)-nerolidol, and also of two diterpene alcohols, (R)- and (S)-geranyllinalol, was separated by SFC using a mixture of carbon dioxide modified with 0.5–1.0% methanol as mobile phase.

Investigation of specific interaction between olefins and cyano-silica modified with copper(II) complexes

Silica gel packings containing copper salt bonded to cyano functional groups on the surface of silica particles were synthesized. It was found that such packings are capable of specific interactions with olefins. The interactions were diversified enough to allow the separation of geometrical isomers. Moreover, it was found that, because of the character of the cyano group, the differences between the packing with free −CN groups and the packing containing −CN groups bonded into a complex with Cu(II) were less marked than in the case of diphenylphosphine and thiol groups investigated earlier.

HPLC Separation of Geometric Isomers of Carbamyl Peptides

Abstract Carbamylated peptides that were studied showed much improved resolution on C-18 reversed phase HPLC columns compared to the parent peptides. A number of dipeptides were carbamylated with ethyl-, n-propyl- or isopropyl isocyanates. The three carbamyl derivatives of each dipeptide could easily be resolved. Carbamylated dipeptides with reversed amino acid sequences were also easily separated. Methionine-enkephalin, leucine-enkephalin, Angiotensins I, II and III and substance P were carbamylated with isocyanates derived from certain carcinostatic 2-chloroethyl nitrosoureas. 1-(2-Chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU), 1-(2-chloroethyl)-3-(cis-4-hydroxycyclohexyl)-1-nitrosourea (cis-4-hydroxy-CCNU) and 1-(2-chloroethyl)-3-(trans-4-hydroxycyclohexy1)-1-nitroso urea (trans-4-hydroxy-CCNU) gave carbamylated derivatives of each peptide and each mixture of derivatives from a single parent peptide could be resolved. Conditions were found in each case whereby baseline resolution of the corresponding ...

Molecular Theory of Liquid Adsorption Chromatography

Abstract A statistical-mechanical theory, based on a lattice model, has been developed to address the molecular mechanism of retention and selectivity in both normal-phase and reversedphase liquid adsorption chromatography. The model is a natural “competitive-equilibrium” one, where possible contributions from solvent-solvent and solute-solvent interactions, and, hence, from solution nonideality, are not neglected. Homogeneous and heterogeneous adsorbent surfaces, single-solvent and binary mixed-solvent mobile phases, and solute molecules of different size and shape are treated. Practical applications of the theory are presented to demonstrate its utility and significance. For homogeneous adsorbents and neat solvents, the molecular energetics of retention and selectivity are examined, with special emphasis on the effects of solute size and shape, and, relatedly, the modes of solute adsorption. Separations of geometrical isomers and homologous series in real and simulated chromatographic processes are inve...

An improved preparation of some insect sex attractants: Synthesis and separation of geometrical isomers by formation of urea complexes

Wittig condensations of aldehydes or ketones with phosphonium salts are frequently used methods for the syntheses of straight-chain or branched alkenes. Suitable choice of reaction conditions may provide the desired geometrical isomer as reaction product. However, we have found that mixtures of geometrical isomers can be conveniently separated on a large scale by the relatively inexpensive method of urea inclusion complex formation. The recovery of both isomers from the separation procedure is almost quantitative. Urea inclusion complexes are formed preferentially withE isomers. Formation of the inclusion complex is affected by the skeletal structure and by the nature of the terminal functional group. By applying this method of separation at a convenient stage, several insect pheromones were prepared without the necessity of a stereoselective step. The technique was used for syntheses of (Z)-11-hexadecenal and (Z)-9-tetradecenal (components of theHeliothis virescens pheromone) and for the synthesis of candidate attractants possessing a trisubstituted double bond.

The separation of geometrical isomers and mixed ligand forms of cobalt(III) and chromium(III) β-diketonates by high-pressure liquid chromatography

Pairs of geometrical isomers of octahedral cobalt(III) and chromium(III) chelates of trifluoroacetylacetone, benzoylacetylacetone and 2,2-dimethylhexane-3,5-dione can be separated by adsorption h.p.l.c. on silica. Trends in elution and required solvent systems are noted and chelate elution is substantiated by mass spectral analysis of collected peaks. Mixed ligand chromium β-diketonates of trifluoroacetylacetone and hexafluoroacetylacetone are resolved in both isocratic and gradient elution modes. Detection with a chromium-specific d.c. argon plasma is reported; mass spectrometry is applied to elucidate elution patterns.

Monoethylenic fatty acids of a partially hydrogenated herring oil

AbstractA Canadian Atlantic herring oil hydrogenated for margarine use to an iodine value of 76 and melting point of 32.5 C was found to have 30% saturated acids and 66% monounsaturated fatty acids. The monounsaturated fatty acids could be analytically determined ascis andtrans isomers by open tubular gas liquid chromatography.Trans acids were 33% of the C16 and C18 monounsaturated acids, and 32 and 28%, respectively, of the C20 and C22 monounsaturated acids. After separation of geometric isomers by Florisil‐silver nitrate chromatography the positional isomers in each class were determined by oxidative fission. The double bond positions of the originalcis fatty acids were largely retained in bothcis andtrans isomers, but additional isomers were observed, especially in thetrans fatty acids.

Separation of geometrical isomers of unsaturated fatty acids by the method of circulation gas chromatography

The method of circulation gas chromatography was used for the separation of the geometrical isomers of 9-methyloctadecenoate and 9,12-methyloctadecadienoate on packed columns. The efficiency of separation achieved is comparable with the efficiency of capillary columns.

La Préparation et l’isomérisme géometrique des Acides Cinnamiques Méthylthio-α Substitués

An improved method of preparation and the separation of geometrical isomers of cis and trans α-methylthio-cinnamic acid derivatives are described. The observation on the NMR spectra and the determination of geometrical structure of these compounds by various spectrometric methods are also included.