Gas chromatographic determination of the interconversion energy barrier for dialkyl 2,3-pentadienedioate enantiomers

The enantiomers of dialkyl 2,3-pentadienedioate undergo interconversion during gas chromatographic separation on chiral stationary phases. In this paper the on-column apparent interconversion kinetic and thermodynamic activation data were determined for dimethyl, diethyl, propylbutyl and dibutyl 2,3-pentadienedioate enantiomers by gas chromatographic separation of the racemic mixtures on a capillary column containing a polydimethylsiloxane stationary phase coupled to 2,3-di-O-methyl-6-O-tertbutyldimethylsilyl-beta-cyclodextrin. A deconvolution method was used to determine the individual enantiomer peak areas and retention times that are needed to calculate the interconversion rate constants and the energy barriers. The apparent rate constants and interconversion energy barriers decrease slightly with an increase in the alkyl chain length of the dialkyl 2,3-pentadienedioate esters. The optimum conformation of the dialkyl 2,3-pentadienedioate molecules, their separation selectivity factors and apparent interconversion enthalpy and entropy data changes with the alkyl chain length. The dependence of the apparent interconversion energy barrier (deltaG(app)(a-->b), deltaG(app)(b-->a)) on temperature was used to determine the apparent activation enthalpy (deltaH(app)(a-->b), deltaH(app)(b-->a)) and apparent entropy (deltaS(app)(a-->b), deltaS(app)(a-->b)) (where a denotes the first and b second eluted enantiomer). The comparison of the activation enthalpy and entropy (deltaS(app)(a-->b), deltaS(app)(a-->b)) indicated that the interconversion of dialkyl 2,3-pentadienedioate enantiomers on the HP-5+Chiraldex B-DM column series is an entropy driven process at 160 degrees C. Data obtained for dimethyl 2,3-pentadienedioate enantiomers on the HP-5+Chiraldex B-DM column series at 120 degrees C (deltaG(app)(a-->b) = 123.3 and deltaG(app)(b-->a) = 124.4 kJ mol(-1)) corresponds (at the 95% confidence interval) with the value of deltaG(#) = 128+/-1 kJ mol(-1) found at this temperature by gas chromatography using a two-dimensional stop flow technique on an empty capillary column [V. Schurig, F. Keller, S. Reich, M. Fluck, Tetrahedron: Asymmetry 8 (1997) 3475].     

2-芳基丙酸类手性药物色谱拆分的热力学研究

以0.5%和1.0%(体积分数)正丙醇-50 mmol/L磷酸盐缓冲液(pH 6.41)为流动相,温度74~313 K,在Chiral-AGP柱手性固定相上,考察了萘普生和布洛芬对映体在手性柱上的保留和分离行为。在实验范围内,温度升高对分离不利,随着温度的升高,对映体的保留时间、分离度和选择性因子都减少;萘普生对映体的分离度均比布洛芬大;流动相含1.0％正丙醇时,萘普生对映体和布洛芬对映体达到完全分离应控制的最高温度分别为298和288 K。用ln k对1/T作图得到的Van’t Hoff曲线都具有良好的     

High-performance liquid chromatographic enantioseparation of alpha-substituted glycine analogs on a quinine-based anion-exchanger chiral stationary phase under variable temperature conditions

The retention of enantiomers of chiral analytes, i.e. alpha-substituted glycine analogs, on a quinine-based anion-exchanger chiral stationary phase was studied in the temperature range of 5-70 degrees C and at different mobile phase compositions, using isocratic elution in the reversed-phase mode. By variation of both mobile phase composition and temperature, baseline separations could be achieved for these enantiomers. Separation could be optimized more quickly by adjusting the column temperature rather than the mobile phase composition. The dependence of the natural logarithms of retention and selectivity factors (lnk' and lnalpha) on the inverse of temperature, 1/T (van't Hoff plots) was used to determine thermodynamic data on the enantiomers. Calculated thermodynamic constants (Delta(DeltaH degrees ), Delta(DeltaS degrees ) and Delta(DeltaG degrees )) were applied to promote an understanding of the thermodynamic driving forces for retention in this chromatographic system. The elution sequence of the enantiomers in most cases was determined.     

Determination of the interconversion energy barrier of 2,3-pentadienedioic acid enantiomers by HPLC. 2. On-column interconversion

In this paper, an HPLC method is used to determine the enantiomerization barrier of 2,3-pentadienedioic acid enantiomers. The racemate of 2,3-pentadienedioic acid was separated by HPLC on a chiral CHIROBIOTIC T column with a 90:10 (100:0.5:0.5 MeOH/HOAc/TEA)/H2O mobile phase. Peak areas of enantiomers prior to (A(+)0, A(-)0) and after the separation (A(+), A(-)), were used for calculation of the rate constants and the enantiomerization barrier, as determined by computer-assisted peak deconvolution of the peak clusters on the chromatograms. The kinetic equation for irreversible reactions was used to determine the apparent enantiomerization rate constants and the interconversion energy barrier. The dependence of the apparent enantiomerization barrier (deltaG1(app), deltaG-1(app)) on temperature was used to determine the apparent activation enthalpy (deltaH1(app), deltaH(-1)app) and entropy (deltaS1(app), deltaS-1(app)) for the interconversion of 2,3-pentadienedioic acid enantiomers, where the coefficients 1 and -1 designate the interconversions (+) --> (-) and (-) --> (+), respectively.     

Temperature and enantioseparation by macrocyclic glycopeptide chiral stationary phases

Seventy-one chiral compounds were separated on four macrocyclic glycopeptide chiral selectors: teicoplanin, its aglycone, ristocetin A and vancomycin, using three possible separation modes: reversed phase with methanol/buffer mobile phases, normal phase with hexane/ethanol mobile phases and polar ionic mode (PIM) with 100% methanol mobile phase with trace amounts of acid and/or base. These 148 separations were studied in a 5-45 degrees C temperature range. Peak efficiencies always increased with temperature, but in only 17% of the separations studied a small increase of the enantioresolution factor was observed. In the majority (83%) of the cases, the enantioresolution decreased or even vanished when temperature increased. All 148 Van't Hoff plots were linear showing that the selector did not change in the temperature range studied. The calculated enthalpy and entropy variations showed that the interaction of the solute with the stationary phase was always enthalpy driven with normal and reversed mobile phases. It could be enthalpy as well as entropy driven with PIM mobile phases strongly dependent on the solute. The plots of delta(deltaH) versus delta(deltaS) were linear in most cases (enthalpy entropy compensation). This observation cannot be used to give clear information on chiral recognition mechanisms, but it allowed identifying specific stationary phase-solute interactions because the points corresponding to the respective thermodynamic parameters were clearly delineated from the general compensation lines.     

Retention mechanism of fatty alcohol ethoxylates in reversed-phase liquid chromatography

Fatty alcohol ethoxylates (FAEs) are widely used nonionic surfactants that have distributions in both alkyl and poly(ethylene oxide) (PEO) chain length. Generally, two-dimensional liquid chromatography technique is required for the complete characterization of both distributions. By selecting a proper stationary and mobile phase condition, however, we can obtain fully resolved chromatograms of a FAE sample (Brij 30) with respect to both alkyl and PEO chain length by using a single reversed-phase C18 column and aqueous acetonitrile mobile phase. FAEs show a peculiar reversed-phase liquid chromatography (RPLC) retention behavior with an aqueous-organic mobile phase, the retention mechanism of which has not been fully elucidated. For a fixed alkyl chain length, FAEs with higher-molecular-mass PEO block elutes first and the van't Hoff plot of the retention factor shows a curvature. The unique retention behavior can be understood from the opposite thermodynamic characteristics associated with RPLC retention of PEO block and alkyl chain: the sorption process of PEO to the non-polar stationary phase shows deltaH(o) > 0 and deltaS(o) > 0 while the alkyl chain shows deltaH(o) < 0 and deltaS(o) < 0 in contrast. The relative magnitude of the two contributions can change the elution order of the FAE. Therefore the often found, inverted elution order of FAEs (the early elution of FAEs with longer PEO block) is due to the positive enthalpic interaction of PEO blocks, which is a characteristic of the hydrophobic interaction. And the curvature of the van't Hoff plots was analyzed assuming the temperature dependent thermodynamic variables.     

Retention mechanism of poly(ethylene oxide) in reversed-phase and normal-phase liquid chromatography

The retention behavior of low- and high-molecular-mass poly(ethylene oxide) (PEO) in reversed-phase (RP) and normal-phase (NP) liquid chromatography was investigated. In RPLC using a C18 bonded silica stationary phase and an acetonitrile-water mixture mobile phase, the sorption process of PEO to the stationary phase showed deltaH(o) > 0 and deltaS(o) > 0. Therefore, PEO retention in RPLC separation is an energetically unfavorable, entropy-driven process, which results in an increase of PEO retention as the temperature increases. In addition, at the enthalpy-entropy compensation point the elution volume of PEO was very different from the column void volume. These observations are quite different from the RPLC retention behavior of many organic polymers. The peculiar retention behavior of PEO in RPLC separation can be understood in terms of the hydrophobic interaction of this class of typical amphiphilic compounds with the non-polar stationary phase, on the one hand, and with the aqueous mobile phase, on the other. The entropy gain due to the release of the solvated water molecules from the PEO chain and the stationary phase is believed to be responsible for the entropy-driven separation process. On the other hand, in NPLC using an amino-bonded silica stationary phase and an acetonitrile-water mixture mobile phase, PEO showed normal enthalpy-driven retention behavior: deltaH(o) < 0 and deltaS(o) < 0, with the retention decreasing with increasing temperature and PEO eluting near the column void volume at the enthalpy-entropy compensation point. Therefore, high-resolution temperature gradient NPLC separation of high-molecular-mass PEO samples can be achieved with relative ease. The molecular mass distribution of high-molecular-mass PEO was found to be much narrower than that measured by size-exclusion chromatography.     

GC separation of 2-substituted ethyl propionate enantiomers on permethylated and 2,6-dimethyl-3-pentyl beta- and gamma-cyclodextrin stationary phases

In this work, the capillary gas chromatographic enantiomer separation of eight congeneric compounds with general formulae CH3-HCX-COOC2H5 (where X = Cl, Br, I, CN, OH, OC2H5, OC6H5 and NHCOCF3) on four different permethyl- and 2,6-di-O-methyl-3-O-pentyl- beta- and gamma-CD stationary phases has been studied. The separation of enantiomers was evaluated in terms of the interactions of the X substituent of studied derivatives, as well as the nature of the 3-O-alkyl group in the 2,6-di-O-methyl-3-O-alkyl-CDs and the CDs cavity size. The differences in thermodynamic data [deltaH and -deltaS] obtained for studied compounds and the selectivity of modified beta- and gamma-cyclodextrin phases in gas chromatographic separation were evaluated. deltaH values were compared with a deltaH value of an achiral standard (ethyl propionate, where X = H) in order to obtain the contribution of a particular substituent to the overall interaction energy. It was shown that the variation in the enantiomeric separation with temperature and the retention order of these compounds on a given cyclodextrin capillary column depends on the nature of the substituents bonded to stereogenic carbon atom. It was found that the temperature dependencies of selectivity factors, In a on 1/T, were both linear as well as non-linear, inter alia depending on the number of glucopyranose units of the CD derivatives. The enantiospecific thermodynamic data [delta21(deltaH)] and [-delta21(deltaS)] which characterize the chiral recognition in the separation system were used to gain more insight into the mechanistic aspects of the enantioseparations on permethylated and 2,6-di-O-methyl-3-O-pentyl-beta- and gamma-cyclodextrins.     

Enantioseparation of phenylalanine analogs on a quinine-based anion-exchanger chiral stationary phase: structure and temperature effects

Application of a cinchona alkaloid-based chiral anion-exchanger stationary phase for the direct high-performance liquid chromatographic enantioseparation of N-protected unusual phenylalanine analogs is reported. The N-benzyloxycarbonyl, N-3,5-dinitrobenzyloxycarbonyl, N-benzoyl and N-3,5-dinitrobenzoyl derivatives were well separable with high resolution. To achieve optimal separation of the enantiomers, the chromatographic conditions and temperature were varied. Linear van't Hoff plots were observed in the studied temperature range, 278-343 K, and the apparent changes in enthalpy, delta(deltaH degrees), entropy, delta(delta S degrees), and Gibbs free energy, delta(delta G degrees), were calculated. The values of the thermodynamic parameters depended on the nature of the N-acyl groups, on the structures of the compounds, and especially on the nature of the substituent on C3 of phenylalanine.     

Alkyl nitrates as achiral and chiral solute probes in gas chromatography. Novel properties of a beta-cyclodextrin derivative and characterization of its enantioselective forces.

Achiral and chiral interactions of alkyl nitrates (R-O-NO2) with heptakis-(3-O-acetyl-2,6-di-O-pentyl)-beta-cyclodextrin (LIPODEX-D) in the gas phase were investigated chromatographically. Two major outcomes can be summarized. First, LIPODEX-D shows very fast temperature-dependent variations of the selectivity up to changes in the order of elution for C1-C5 alkyl nitrates. These changes in selectivity reveal that LIPODEX-D possesses different shape selectivities for small alkyl nitrates at different temperatures (40-80 degrees C), i.e. with increasing isothermal separation temperature extended (chain-like) alkyl nitrates have increased retention relative to bulky alkyl nitrates. The observations are highly reproducible and might indicate conformational changes of the cyclodextrin, however, an ultimate proof would require further spectroscopic investigation. Secondly, the chiral separations of systematically varied sets of C4-C11 alkyl nitrates allowed the thermodynamic characterization of enantiodiscriminating interactions. Quantitative evidence is provided showing that the presence of an ethyl group at the asymmetric carbon atom of an n-alkyl nitrate gives a strong enthalpic contribution to the resulting enantioselectivity. The Gibbs free energy differences -deltaR,S(deltaG) decrease systematically three to six times if the ethyl group is either shortened or enlarged by only a -CH2-increment. The results are based on two separate thermodynamic approaches, i.e. the determination of thermodynamic quantities (-deltaR,S(deltaG), -deltaR,S(deltaH), -deltaR,S(deltaS), Tiso) and a theoretical concept of enthalpy-entropy compensation. The data from our laboratory experiments also indicate that van der Waals interactions are responsible for chiral discrimination.     

Extraordinary chiral discrimination in inclusion gas chromatography. thermodynamics of enantioselectivity between a racemic perfluorodiether and a modified gamma-cyclodextrin

The enantiomers of the perfluorodiether "compound B" [2-(fluoromethoxy)-3-methoxy-1,1,1,3,3-pentafluoropropane], a decomposition product of the inhalational anesthetic sevoflurane [2-(fluoromethoxy)-1,1,1,3,3,3-hexafluoropropane], were separated by gas chromatography on octakis(3-O-butanoyl-2,6-di-O-n-pentyl)-gamma-cyclodextrin (Lipodex E), dissolved in polysiloxane PS 255 (30% w/w), with an unexpectedly high separation factor of alpha = 10.6 at 26 degrees C. Using the concept of the retention increment R', non-enantioselective and enantioselective contributions to retention were separated and thus reliable thermodynamic parameters of enantioselectivity, i.e. - deltaS,R(deltaG) = 5.7 (0.05) kJ/mol at 303 K, - deltaS,R(deltaH) = 20.1 (0.64) kJ/mol, deltaS,R(deltaS) = -47.4 (2.0) J/K mol and T(isoenant) = 424 (30) K or approximately 150 degrees C, were determined by temperature-dependent measurements. The enantiomeric bias represents the largest values ever measured in enantioselective gas chromatography. An equation is presented which allows calculation of the non-enantioselective contributions to retention from measurements at two arbitrary concentrations of Lipodex E in polysiloxane. Surprisingly, the enantioselectivity is greatly reduced when employing the beta-cyclodextrin analogue and breaks down completely with the alpha-cyclodextrin analogue of Lipodex E.     

联苯类对映体在纤维素类手性固定相上的高效液 相色谱法直接拆分

涂敷纤维素-三(3,5-二甲基苯基氨基甲酸酯)于自制的球形氨丙基硅胶上,制备了手性固定相。用该固定相直接拆分了一系列外消旋联苯类保肝药物。考察了柱温和流动相中乙醇含量对样品的保留和立体选择性的影响,升高柱温或增大流动相中乙醇含量都会使对映体的保留因子和分离因子降低。氨考察榈的分离因子的对数与温度的倒数成线性关系。确定了对映体从流动相传递到固定相的吉布斯自由能变之差值、焓变之差值及熵变之差值。     

Brush-type chiral stationary phase for enantioseparation of acidic compounds. Optimization of chiral capillary electrochromatographic parameters

The capillary electrochromatographic separations of three acidic enantiomers (carprofen, coumachlor and warfarin) were studied on a capillary column packed with 5 microm (3R,4S)-Whelk-O 1 chiral stationary phase. The influence of several experimental parameters (mobile phase pH, type of background electrolyte, acetonitrile ratio, temperature, applied voltage and ionic strength) on electroosmotic flow velocity, retention factor, selectivity factor, efficiency, resolution and effectiveness of chiral separation was evaluated. It was notable that the optimum resolution of the acidic enantiomers was achieved at pH 3.0 phosphate buffer, suggesting that capillary electrochromatography in the ion-suppressed mode can be applied for chiral separations of a range of acidic compounds.     

含磷手性化合物在多聚糖类手性固定相上的手性分离

在纤维素 三(3,5 二甲基苯基氨基甲酸酯)(ChiralcelOD)和直链淀粉 三(3,5 二甲基苯基氨基甲酸酯)(ChiralpakAD H)手性固定相上,采用高效液相色谱正相条件,分离了系列含磷手性化合物。考察了流动相中有机改性剂的种类及浓度对手性分离的影响;研究了化合物的结构与保留及对映体选择性的关系;并探讨了手性识别机理。     

Enantiomeric separation of chiral pesticides by high-performance liquid chromatography on an amylose tris-(S)-1-phenylethylcarbamate chiral stationary phase

Amylose tris-(S)-1-phenylethylcarbamate chiral stationary phase (CSP) was prepared. The direct enantiomeric separation of chiral pesticides on this CSP had been studied by HPLC. The mobile phase was n-hexane-isopropanol at a flow rate of 1.0 mL/min. The effects of isopropanol content and column temperature on retention and enantioselectivity were investigated. Thirty-two samples were tested, of which ten interacted enantioselectively with the CSP. Five samples were completely resolved and another five underwent near-baseline or partial resolution. The enantiomers were identified by a circular dichroism detector. Linear van't Hoff plots were established and the thermodynamic parameters were thus calculated.     

Novel approach to the study of the chiral discrimination mechanism in a series of dansyl amino acids.

With poly(octadecylsiloxane) as the liquid chromatographic stationary phase, phosphate buffer as the mobile phase, a series of D- and L-dansyl amino acids as solutes, and beta-cyclodextrin as the chiral selector, a study was conducted of the hydrophobic effect on both the solute complexation with the chiral selector and chiral discrimination mechanisms by varying the sucrose concentration in the mobile phase and the column temperature. The number of sucrose molecules excluded during the solute complexation with the chiral selector proved to be a good marker of the solute inclusion in the cavity. Gibbs Helmotz parameters delta(deltaH) and delta(deltaS) between D- and L-enantiomers were determined from plots of the logarithm of the intrinsic selectivity versus the reciprocal of the temperature. The results obtained predicted that the enantioselectivity was related to the bulkiness of the solute. This numerical approach is a valuable tool in the exploration of the steric effects implied in the formation of the host-guest complex.     

Thermodynamic study on the gas chromatographic separation of the enantiomers of aromatic alcohols using heptakis(2,3-di-O-methyl-6-O-tert-butyldimethylsilyl)-beta-cyclodextrin as a stationary phase

Gas chromatographic separation of the enantiomers of nineteen structurally related aromatic alcohols was investigated as a function of temperature using a heptakis(2,3-di-O-methyl-6-O-tert-butyldimethylsilyl)-beta-cyclodextrin-coated capillary column. Thermodynamic parameters were determined and compared with those obtained with the nonchiral, reference stationary phase, OV-1701. While the -deltaH and -deltaS values for the more retained enantiomers of all nineteen alcohols are comparable on the chiral stationary phase used, the -delta(deltaH) and -delta(deltaS) values are considerably different. Of all the solutes tested, enantiodiscrimination was the greatest for the 2,6-difluoro-alpha-methylbenzyl alcohol.     

HP-β-环糊精对盐酸舍曲林异构体手性选择包合和色谱保留行为影响的研究

研究了不同色谱条件下HP-β-环糊精(HP-β-CD)作为反相高效液相色谱(RP-HPLC)手性添加剂对盐酸舍曲林异构体的手性选择性和色谱保留行为的影响。通过保留因子(k)的倒数1/k对［HP-β-CD］的良好线性关系证明HP-β-CD与盐酸舍曲林异构体形成了包合比为1∶1的包合物,并通过1/k对［HP-β-CD］的直线求得其结合常数。同时系统研究了pH值、缓冲液浓度(离子强度)、乙腈溶剂强度及温度对结合常数的影响,对结合常数与它们的关系做出了定量的描述;计算了手性分离过程中的热力学参数,并结合所计算的结     

Determination of interconversion barriers by dynamic gas chromatography: epimerization of chalcogran

The four stereoisomers of chalcogran 1 ((2RS,SRS)-2-ethyl-1,6-di-oxaspiro[4.4]nonane), the principal component of the aggregation pheromone of the bark beetle pityogenes chalcographus, are prone to interconversion at the spiro center (C5). During diastereo- and enantioselective dynamic gas chromatography (DGC), epimerization of 1 gives rise to two independent interconversion peak profiles, each featuring a plateau between the peaks of the interconverting epimers. To determine the rate constants of epimerization by dynamic gas chromatography (DGC), equations to simulate the complex elution profiles were derived, using the theoretical plate model and the stochastic model of the chromatographic process. The Eyring activation parameters of the experimental interconversion profiles, between 70 and 120 C in the presence of the chiral stationary phase (CSP) Chirasil-beta-Dex, were then determined by computer-aided simulation with the aid of the new program Chrom-Win: (2R,5R)-1: deltaG(++) (298.15 K) = 108.0 +/-0.5 kJ mol(-1), deltaH(++) = 47.1+/-0.2 kJ mol(-1), deltaS(++) = -204+/-6 JK(-1) mol(-1): (2R,5S)-1: deltaG(++) (298.15 K) = 108.5+/-0.5 kJ mol(-1), deltaH(++) = 45.8+/-0.2 kJ mol(-1), deltaS(++) = -210 +/-6 J K mol(-1); (2S,5S)-1: deltaG(++) (298.15 K)= 108.1+/-0.5 kJ mol(-1), deltaH(++) = 49.3+/-0.3 kJ mol(-1), deltaS(++) = -197+/-8 J K(-1) mol(-1); (2S,5R)-1: deltaG(++) (298.15 K)=108.6+/-0.5 kJ mol(-1), deltaH(++) = 48.0+/-0.3 kJ mol(-1), deltaS(++) = -203+/-8 J K(-1) mol(-1). The thermodynamic Gibbs free energy of the E/Z equilibrium of the epimers was determined by the stopped-flow multidimensional gas chromatographic technique: deltaG(E/Z) (298.15 K)= -0.5 kJ mol(-1), deltaH(E/Z) = 1.4 kJ mol(-1) and deltaS(E/Z) = 6.3 J K(-1) mol(-1). An interconversion pathway proceeding through ring-opening and formation of a zwitterion and an enol ether/alcohol intermediate of 1 is proposed.     

Chiral separations of pesticide enantiomers by high-performance liquid chromatography using cellulose triphenylcarbamate chiral stationary phase

The direct chiral separations of pesticide enantiomers by high-performance liquid chromatography by applying self-prepared cellulose triphenylcarbamate chiral stationary phase are performed. The mobile phase is n-hexane modified by isopropanol as a polar modifier. Nine chiral pesticides (benalaxyl, vinclozolin, diclofop-methyl, tebuconazole, quizalofop-ethyl, hexaconazole, lactofen, isocarbophos, and paclobutrazol) show enantioselectivity on the chiral stationary phase. An online circular dichrorism detector is used for identifying the pesticide enantiomers. The influences of the volume content of isopropanol and column temperature on the separations are investigated. The thermodynamic parameters related to the chiral distinguish mechanisms are also calculated.