The separation of hypericin's enantiomers and their photophysics in chiral environments

We report the first separation of the enantiomers of hypericin. Their steady-state optical spectra and ultrafast primary photoprocesses are investigated in chiral environments. Within experimental error, there is no difference between the two enantiomers in any of the systems considered. This is consistent with the emerging picture that the rich and extended absorption spectrum of hypericin is not a result of ground-state heterogeneity. It is also consistent with the observation that the spectra and photophysics of hypericin are generally insensitive to environments in which it does not aggregate.     

全烷基化和部分烷基化三氟乙酰化的α-环糊精手性毛细管色谱柱分离对映体

合成了全戊基化和部分戊基化三氟乙酰化的两种α-环糊精手性固定相.制备出柱效在3500塔片/m以上的高效手性毛细管柱,分离了一些对映体及不对称合成中的中间体,如α-苯乙胺,α-苯丙胺、酒石酸酯和缩酮等.结果表明,三氟乙酰化的α-环糊精衍生和的对手性醇、胺和酯等对映体的选择性比全戊基化的环糊精高.对映体中引入的衍生化基团对选择性也有明显影响.     

Open tubular capillary columns with basic templates made by the generalized preparation protocol in capillary electrochromatography chiral separation and template structural effects on chiral separation capability

Some open tubular (OT) molecule imprinted polymer (MIP) silica capillary columns have been prepared using atenolol, sulpiride, methyl benzylamine (MBA) and (1-naphthyl)-ethylamine (NEA) as templates by the pre-established generalized preparation protocol. The four MIP thin layers of different templates showed quite different morphologies. The racemic selectivity of each MIP column for the template enantiomers was optimized by changing eluent composition and pH. The template structural effects on chiral separation performance have been examined. This work verifies the versatility of the generalized preparation protocol for OT-MIP silica capillary columns by extending its boundary toward templates with basic functional group moieties. This study is the very first report to demonstrate a generalized MIP preparation protocol that is valid for both acidic and basic templates. The chiral separation performances of atenolol and sulpiride by the MIPs of this study were found better than or comparable to those of atenolol and sulpiride obtained by non-MIP separation techniques and those of some basic template enantiomers obtained by MIP based techniques.     

Synthesis of chiral carbosilane dendrimers with l-cysteine and N-acetyl-l-cysteine on their surface and their application as chiral selectors for enantiomer separation by capillary electrophoresis

The synthesis of chiral carbosilane dendrimers functionalized with cysteine and N-acetylcysteine groups is presented. These dendrimers were obtained through thiol–ene addition reactions and their application as chiral selectors in capillary electrophoresis was investigated. Four drugs used as model compounds were analyzed under different experimental conditions observing that the use of a first generation dendrimer containing 4 terminal N-acetyl-l-cysteine groups enabled the enantiomeric discrimination of razoxane with a discrimination power similar to that obtained with other powerful chiral selectors such as cyclodextrins.     

New derivatives of cyclodextrins as chiral selectors for the capillary electrophoretic separation of dichlorprop enantiomers

-, β- and γ-cyclodextrins (CDs), as well as some of their chemical derivatives, have been tested as chiral resolving agents for the capillary zone electrophoretic resolution of the racemic herbicide dichlorprop, (±)-2-(2,4-dichlorophenoxy)propionic acid, of which only the (+)-isomer is herbicidally active. The complexation constants of the herbicide enantiomers with the cyclodextrin host molecules have been calculated from the electrophoretic migration time data at variable cyclodextrin concentration. The experimental results showed that several of the investigated CDs allowed dichlorprop enantiomer resolution. In particular, a newly synthesised ethylcarbonate derivative of β-CD showed the best enantiomer resolution properties among the tested compounds, while the remaining ones showed inferior or no performances at all. The calculated inclusion constants allowed identification of the best conditions for enantioresolution, and an explanation of the different complexation properties of the investigated compounds has been proposed on the basis of molecular modeling.     

HPLC separation of dihydropyridine derivatives enantiomers with emphasis on elution order using polysaccharide‐based chiral columns

The separation of enantiomers of five chiral dihydropyridine derivatives was studied on five different polysaccharide‐based chiral HPLC columns with various normal‐phase (NP), polar organic, and reversed‐phase eluents. Along with the successful separation of analyte enantiomers, the emphasis of this study was on enantiomer elution order (EEO) with various columns and mobile phase composition. The interesting phenomenon of reversal of EEO, recently reported in the case of amlodipine (AML) depending on the concentration of formic acid in acetonitrile, was also confirmed with NP eluents. Under RP conditions at relatively low water content, the EEO of AML could also be reverted by varying the concentration of formic acid in the mobile phase. However, at higher water content the same parameter did not affect the EEO, but only induced gradual decrease in resolution up to complete co‐elution of enantiomers. Additionally, in organic‐aqueous mobile phases retention factors decreased with increasing water content but only up to 20% (v/v), while above this concentration the expected typical RP behavior was observed. The presence of the commonly used additive diethylamine in the mobile phase seems important for observing a reversal in EEO with increasing concentration of formic acid. The reversal of the EEO was characteristic of AML only and was not observed for any of other dihydropyridines included in this study.     

Recent advances in the preparation and application of monolithic capillary columns in separation science

Novel column technologies involving various materials and efficient reactions have been investigated for the fabrication of monolithic capillary columns in the field of analytical chemistry. In addition to the development of these miniaturized systems, a variety of microscale separation applications have achieved noteworthy results, providing a stepping stone for new types of chromatographic columns with improved efficiency and selectivity. Three novel strategies for the preparation of capillary monoliths, including ionic liquid-based approaches, nanoparticle-based approaches and “click chemistry”, are highlighted in this review. Furthermore, we present the employment of state-of-the-art capillary monolithic stationary phases for enantioseparation, solid-phase microextraction, mixed-mode separation and immobilized enzyme reactors. The review concludes with recommendations for future studies and improvements in this field of research.     

Preparation of open tubular molecule imprinted polymer capillary columns with various templates by a generalized procedure and their chiral and non‐chiral separation performance in CEC

A generalized preparation procedure of open tubular (OT) molecule imprinted polymer (MIP) columns is proposed for a number of templates with acidic functionality such as profen drugs and others. The template (S‐enantiomer) was mixed with methacrylic acid, ethylene glycol dimethacrylate and 4‐styrenesulfonic acid, dissolved in a porogen mixture of ACN/2‐propanol (9/1), and incubated in a pretreated and silanized fused silica capillary by the thermal non‐covalent polymerization procedure. The whole preparation procedure was exactly the same for all the MIP capillaries except for the selection of template. Nevertheless, the morphologies of the MIP layers were markedly variant depending upon the choice of template. The separation efficiency of each OT‐MIP column for chiral separation of R‐ and S‐enantiomers was examined and tuned to obtain the best separation efficiency by changing the chromatographic parameters such as eluent composition and pH. Different optimized conditions were obtained for different OT‐MIP columns. Nevertheless, a unified eluent could be used to obtain still quite satisfactory results. Non‐chiral separation of the MIP columns were also examined in the unified eluent with two sets of test mixtures, that is, a mixture of alkylbenzenes and a mixture of small polar solutes. The chiral and non‐chiral separation of this study resulted in very good separation efficiencies. This work is the very first study for the generalization of preparation of OT‐MIP columns for a number of templates.     

High-performance liquid chromatographic separation of imidazolinone herbicide enantiomers and their methyl derivatives on polysaccharide-coated chiral stationary phases

Many chiral pesticides exhibit enantioselectivity in biotransformation and ecotoxicity in the environment. A significant class of chiral pesticides is imidazolinone herbicides, of which enantioselectivity has not been well studied. Development of efficient chiral separation methods is the first step for allowing characterization of enantioselectivity in environmental processes. In this study, we attempted to resolve enantiomers of imidazolinone herbicides using reversed-phase and normal-phase high-performance liquid chromatography with polysaccharide-type chiral columns. Enantiomers of imazethapyr, imazaquin, and imazamox were separated on a Chiralcel OD-R column using 50mM phosphate buffer-acetonitrile as mobile phase. Enantiomers of imazapyr, imazapic, imazethapyr, imazamox and imazaquin were resolved on a Chiralcel OJ column using n-hexane (0.1% trifluoroacetic acid)-alcohol as mobile phase. The enantiomers of five methyl derivatives of imidazolinone herbicides were also resolved on the Chiralcel OJ column. The Deltak' values revealed a structure-enantioselectivity relationship for the separation behaviors of the enantiomers on the OJ column. The described method was successfully applied for chiral analysis of two imidazolinone herbicides (imazapyr and imazaquin) in spiked soil samples.     

The separation of the enantiomers of diquats by capillary electrophoresis using randomly sulfated cyclodextrins as chiral selectors

Diquats, derivatives of the widely used herbicide diquat, represent a new class of functional organic molecules. A combination of their special electrochemical properties and axial chirality could potentially result in their important applications in supramolecular chemistry, chiral catalysis, and chiral analysis. However, prior to their practical applications, the diquats have to be prepared in enantiomerically pure forms and the enantiomeric purity of their P- and M-isomers has to be checked. Hence, a chiral capillary electrophoresis (CE) method has been developed and applied for separation of P- and M-enantiomers of 11 new diquats. Fast and better than baseline CE separations of enantiomers of all 11 diquats within a short time 5–7 min were achieved using acidic buffer, 22 mM NaOH, 35 mM H₃PO₄, pH 2.5, as a background electrolyte, and 6 mM randomly sulfated α-, β-, and γ-cyclodextrins as chiral selectors. The most successful selector was sulfated γ-cyclodextrin, which baseline separated the enantiomers of all 11 diquats, followed by sulfated β-cyclodextrin and sulfated α-cyclodextrin, which baseline separated enantiomers of 10 and nine diquats, respectively. Using this method, a high enantiopurity degree of the isolated P- and M-enantiomers of three diquats with a defined absolute configuration was confirmed and their migration order was identified.     

Chiral separation of bupivacaine enantiomers by capillary electrophoresis partial-filling technique with human serum albumin as chiral selector

Capillary electrophoresis (CE) is a powerful technique for enantiomer separations due to its intrinsic high separation efficiencies, speed of analysis, low reagent consumption and small sample requirements. However, some chiral selectors present strong background UV absorption providing high detection limits. The present paper deals with the application of the partial-filling technique to the separation of bupivacaine enantiomers by capillary electrophoresis using human serum albumin (HSA) as chiral selector. In this procedure the cationic surfactant cetyltrimethylammonium bromide (CTAB) was used as a dinamic capillary coating in order to reduce the electro-osmotic flow and detect both bupivacaine enantiomers out of the chiral selector plug. Several experimental conditions such as CTAB concentration, pH, HSA concentration and plug length, background electrolyte concentration, temperature and voltage were studied. Under the selected conditions it is possible to detect the separated enantiomers out of the HSA plug in less than 4 min using 50 mM Tris pH 8 as background electrolyte with 50 microM CTAB, at 30 degrees C and using a separation voltage of 25 kV. The proposed methodology was then validated for analytical purposes and applied to the analysis of pharmaceutical preparations commercially available. The results obtained with the proposed methodology were in good agreement with those declared by the manufacturers. The simplicity, sample throughput, accuracy, reproducibility and low cost of the proposed method make it suitable for the control of the enantiomeric composition of bupivacaine in pharmaceuticals.     

Gas chromatographic separation of triglycerides according to their degree of unsaturation on glass capillary columns

Triglycerides are separated according to their molecular weight and their degree of unsaturation on glass capillary columns by use of the on-column injection technique. The columns employed are silylated SE-30 columns of 4.5, 8, and 10m length with a film thickness of ca. 0.15 μm. The paper gives several examples of chromatograms of natural and hydrogenated triglycerides as well as of mixtures of pure, known triglycerides before and after interesterification.     

Recent advances in chiral separation principles in capillary electrophoresis and capillary electrochromatography

This review summarizes recent developments in chiral separation in capillary zone electrophoresis (CZE), electrokinetic chromatography (EKC), and capillary electrochromatography (CEC) covering literature published since the year 2000. New chiral selectors and innovative approaches for CE and CEC are introduced. Recent progress in column technology for CEC is highlighted and the development of new chiral stationary phases is discussed. This review is not dedicated to list applications but will focus on new developments.     

Chiral separation of anticholinergic drug enantiomers in nonaqueous capillary electrophoresis

Nonaqueous capillary electrophoretic (NACE) method for the separation of nine structurally similar chiral anticholinergic drugs was developed. The eight drug enantiomers were separated on baseline within 18 min using 20mM phosphoric acid and 10 mM NaOH, containing 10 mM heptakis(2,3-dimethyl-6-sulfato)-4beta-cyclodextrin (HDMS-beta-CD) in methanol. The results were compared with those obtained in the high performance liquid chromatography system.     

Derivatized cyclodextrins for the separation of enantiomers in capillary electrophoresis

The use of cyclodextrin derivatives for the efficient separation of enantiomeric drugs is described. Hydroxypropylation, methylation or carboxymethylation of the cyclodextrin not only result in a better solubility of the cyclodextrin in aqueous solutions, but also favor, via additional hydrogen bonding, the stabilization of one of the cyclodextrin-analyte complexes. The influence of the background electrolyte on peak shape is also described here. Carboxymethylated cyclodextrin can be used in similar manner to uncharged cyclodextrins at low pH values (below 4). At pH values above 5, however, its charge also allows the separation of uncharged enantiomers as in a micellar-like system.     

The separation of enantiomers on modified cyclodextrin columns: Measurements and molecular modeling

The enantiomers of 2-chloropropionic acid methyl ester, cis-pinane, 2-bromoethylbenzene, 2-bromobutane, 2-hydroxybutane trifluoroacetyl ester, and styrene oxide have been resolved on an octakis-(3-O-butyryl-2,6-di-O-pentyl)-γ-cyclo-dextrin capillary column, and the separation of the styrene oxide enantiomers has also been studied on columns coated with octakis-(3-O-trifluoroacetyl-2,6-di-O-pentyl)-cyclodextrin, octakis-(2,3,6-tri-O-pentyl)-γ-cyclodextrin, heptakis-(3-O-trifluoroacetyl-2,6-di-O-pentyl)-β -cyclodextrin, and heptakis-(2,3,6-tri-O-methyl)-β-cyclodextrin. Thermodynamic parameters (ΔG, ΔH, and ΔS) were determined from variable temperature measurements. The inclusion complexes containing styrene oxide were also studied by molecular modeling techniques. It has been found that a combined molecular mechanics–molecular dynamics approach may be a valuable tool for rationalizing the qualitative trends observed in the experimental separation factors. For the inclusion complexes considered here it is shown that the orientation of the guest relative to the cyclodextrin host is determined by the size and polarity of the cyclodextrin.