Application of tetraoxadiaza-crown ether derivatives as chiral selector modifiers in capillary electrophoresis

Two new diaza-crown ether derivatives (R-1, RS-1) have been synthesized from 1,4,10,13-tetraoxa-7,16-diazacyclooctadecane and tested as potential chiral selectors in capillary electrophoresis (CE) for the chiral separation of five amino acid derivatives. The individual use of the selectors did not lead to chiral differentiation. However, they enhanced the enantioselective effect of different cyclodextrins in dual selector systems. In this paper, we report the effect of different substituted diaza-crown ether derivatives on the separation results obtained in dual systems with cyclodextrins.     

Capillary electrophoresis separation of peptide diastereomers that contain methionine sulfoxide by dual cyclodextrin‐crown ether systems†

A dual‐selector system employing achiral crown ethers in combination with cyclodextrins has been developed for the separation of peptide diastereomers that contain methionine sulfoxide. The combinations of the crown ethers 15‐crown‐5, 18‐crown‐6, Kryptofix® 21 and Kryptofix® 22 and β‐cyclodextrin, carboxymethyl‐β‐cyclodextrin, and sulfated β‐cyclodextrin were screened at pH 2.5 and pH 8.0 using a 40/50.2 cm, 50 μm id fused‐silica capillary and a separation voltage of 25 kV. No diastereomer separation was observed in the sole presence of crown ethers, while only sulfated β‐cyclodextrin was able to resolve some peptide diastereomers at pH 8.0. Depending on the amino acid sequence of the peptide and the applied cyclodextrin, the addition of crown ethers, especially the Krpytofix® diaza‐crown ethers, resulted in significantly enhanced chiral recognition. Keeping one selector of the dual system constant, increasing concentrations of the second selector resulted in increased peak resolution and analyte migration time for peptide‐crown ether‐cyclodextrin combinations. The simultaneous diastereomer separation of three structurally related peptides was achieved using the dual selector system.     

有机改性剂对氯代酚毛细管区带电泳分离的影响

在分离19种氯代酚的过程中,考察了不同的有机添加剂对其毛细管区带电泳分离的影响,发现除了缓冲溶液的pH值外,缓冲溶液添加剂对氯代酚的电泳分离也有较大影响。这种影响与添加剂和氯代酚形成氢键的能力有关。     

Protein separation by capillary gel electrophoresis: A review

Capillary gel electrophoresis (CGE) has been used for protein separation for more than two decades. Due to the technology advancement, current CGE methods are becoming more and more robust and reliable for protein analysis, and some of the methods have been routinely used for the analysis of protein-based pharmaceuticals and quality controls. In light of this progress, we survey 147 papers related to CGE separations of proteins and present an overview of this technology. We first introduce briefly the early development of CGE. We then review the methodology, in which we specifically describe the matrices, coatings, and detection strategies used in CGE. CGE using microfabricated channels and incorporation of CGE with two-dimensional protein separations are also discussed in this section. We finally present a few representative applications of CGE for separating proteins in real-world samples.     

Carbon nanotubes as separation carrier in capillary electrophoresis

The utility and versatility of carboxylic single-walled carbon nanotubes (c-SWNT) in capillary electrophoresis (CE) is demonstrated, using as model solutes homologues and structural isomers. In the case of homologues of caffeine and theobromine, distinct changes in the electrophoretic parameters occur at a critical concentration of c-SWNT in the run buffer. It is suggested that the c-SWNT of a definite concentration could form a network in the run buffer as a pseudostationary phase on the basis of the unique tubule structure, providing a different separation from sodium dodecyl sulfate (SDS) micelles. In the case of structural isomers of catechol and hydroquinone, differing from the homologues, it is mainly attributable to the functional groups on the c-SWNT that have an effect on the electrophoretic behaviors by forming intermolecular hydrogen bonding with analytes. Furthermore, aggregated c-SWNT serve as anticonvective media and minimize solute diffusion contributing to zone broadening. The presence of charged c-SWNT suppressed the electrodiffusion and decreased the adsorption between capillary wall and solutes, which led to better peak shapes of isomers.     

Chiral separations by host-guest complexation with cyclodextrin and crown ether in capillary zone electrophoresis

Summary Capillary zone electrophoresis using cyclodextrins and a chiral crown ether as buffer constituents was studied for the enantiomeric separation of drugs and amino acids. Based on results obtained from separation of racemic -amino acids both chiral selectors are compared with respect to resolution, efficiency and retention time. For (±)-Quinagolide effects of buffer composition and temperature are examined using -cyclodextrin as chiral agent. Optimum conditions were pH 2.5 at 30 mmol L^–1 -cyclodextrin. A linear dependence of retention on -cyclodextrin concentration allowed calculation of formation constants of the host-guest complexes. Buffer concentration and temperature also influence resolution. The application of a chiral crown ether to the separation of optical isomers in capillary zone electrophoresis is described for the first time. Chiral recognition of solutes depends on the formation of protonated alkyl amines and separation is attributed to the formation of diastereomeric host-guest complexes with different interactions for each enantiomer. The effects of crown ether concentration on resolution are presented.     

Silica nanoparticles for separation of biologically active amines by capillary electrophoresis with laser-induced native fluorescence detection

This paper describes the analysis of biologically active amines by capillary electrophoresis (CE) in conjunction with laser-induced native fluorescence detection. In order to simultaneously analyze amines and acids as well as to achieve high sensitivity, 10 mM formic acid solutions (pH < 4.0) containing silica nanoparticles (SiNPs) were chosen as the background electrolytes. With increasing SiNP concentration, the migration times for seven analytes decrease as a result of increase in electroosmotic flow (EOF) and decrease in their electrophoretic mobilities against EOF. A small EOF generated at pH 3.0 reveals adsorption of SiNPs on the deactivated capillary wall. The decreases in electrophoretic mobilities with increasing SiNP concentration up to 0.3x indicate the interactions between the analytes and the SiNPs. Having a great sensitivity (the limits of detection at a signal-to-noise ratio (S/N) = 3 of 0.09 nM for tryptamine (TA)), high efficiency, and excellent reproducibility (less than 2.4% of the migration times), this developed method has been applied to the analysis of urinal samples with the concentrations of 0.50 +/- 0.02 microM, 0.49 +/- 0.04 microM, and 74 +/- 2 microM for TA, 5-hydroxytryptamine, and tryptophan, respectively. The successful examples demonstrated in this study open up a possibility of using functional nanoparticles for the separation of different analytes by CE.     

Amphiphilic silica nanoparticles as pseudostationary phase for capillary electrophoresis separation

Amphiphilic silica nanoparticles surface-functionalized by 3-aminopropyltriethoxysilane (APTES) and octyltriethoxylsilane (OTES) were successfully prepared and characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectrometry (FT-IR) and thermogravimetry (TG) techniques. The potential use of these bifunctionalized nanoparticles as pseudostationary phases (PSPs) in capillary electrophoresis (CE) for the separation of charged and neutral compounds was evaluated in terms of their suitability. As expected, fast separation of representative aromatic acids was fulfilled with high separation efficiency, because they migrate in the same direction with the electroosmotic flow (EOF) under optimum experimental conditions. Using a buffer solution of 30mmol/L phosphate (pH 3.0) in the presence of 0.5mg/mL of the synthesized bifunctionalized nanoparticles, the investigated basic compounds were baseline-resolved with symmetrical peaks. Due to the existence of amino groups on the surface of nanoparticles, "silanol effect" that occurs between positively charged basic analytes and the silanols on the inner surface of capillary was greatly suppressed. Furthermore, the separation systems also exhibited reversed-phase (RP) behavior when neutral analytes were tested.     

On-line concentration and separation of indolamines, catecholamines, and metanephrines in capillary electrophoresis using high concentration of poly(diallyldimethylammonium chloride)

This paper tackles a simple and efficient method for the simultaneous separation and stacking of neurotransmitters in capillary electrophoresis with UV detection. By using poly(diallyldimethylammonium chloride) (PDDAC) as a buffer additive, the high and reversed EOF are observed. Moreover, the mobility of indolamines and catecholamines decreases as the PDDAC concentration increases. Based on the difference in mobility in the presence and absence of PDDAC, the analytes were simply stacked between the boundary of the sample zone and the background electrolyte containing PDDAC. The separation of 14 analytes including indolamines, catecholamines, and metanephrines was accomplished within 33 min under optimal conditions (1.2% PDDAC and 5 mM formic acid at pH 4.0), and the values of relative standard deviation of their migration time were less than 3.1%. By applying stacking methods for fourteen analytes, we observed: (a) the sample injection volume of sample is up to 216 nL, (b) the limits of detection at signal-to-noise of 3 range from 15.4 to 122.1 nM, and (c) the sensitivity enhancements, compared to normal injection (12 nL), range from 110- to 220-fold. Under the optimal stacking conditions, the present method has been applied to analyze of vanillomandelic acid, 5-hydroxyindole-3-acetic acid, dopamine, tryptamine, and 3-indoxyl sulfate in urine samples.     

The complexation of alkali metal ions by crown ethers,aza crown ethers,and cryptands in propylene carbonate

The reactions between alkali metal ions and crown ethers, aza crown ethers, and cryptands in propylene carbonate were studied by potentiometric and calorimetric titrations. The most stable complexes formed by macrocyclic and macrobicyclic ligands are when the ligand and cation dimensions are comparable. On comparing the complex stabilities of crown ethers and aza crown ethers of the same size, crown ethers were, on the whole, found to form the most stable complexes, with the exception of the lithium cation. Enthalpic factors are responsible. Substitution of the amino group protons of the aza crown ethers by benzyl groups leads to a high increase in values of the reaction enthalpy. This effect is partly compensated by entropic contributions. The bulky benzyl groups reduce the ligand solvent interactions and induce a ligand conformation with the lone pair of electrons from the nitrogen donor atoms which are more or less directed inside the cavity. The thermodynamic data for the transfer from methanol to propylene carbonate indicate that the ligands containing nitrogen show specific interactions with methanol.This paper is dedicated to Professor H. Strehlow on the occasion of his 70th birthday.     

毛细管电泳分离几种神经递质的研究

研究了不同毛细管预处理方法对组胺、5-羟色胺及儿茶酚胺类神经递质电泳分离的影响，采用优化的毛细管预处理方法及电泳分离条件基线分离了组胺、5-羟色胺、去甲肾上腺素和肾上腺素。利用多巴胺和5-羟色胺在毛细管内壁吸附效应的不同，对电泳淌度极为相近的多巴胺和5-羟色胺进行了分离和鉴定。以PC22细胞为研究对象，证实了该细胞中所含大量神经递质是多巴胺，不是5-羟色胺。     

Spermine-graft-dextran non-covalent copolymer as coating material in separation of basic proteins and neurotransmitters by capillary electrophoresis

Spermine-graft-dextran (Spe-g-Dex) copolymer was synthesized and used as a non-covalent coating for the separation of proteins and neurotransmitters by capillary electrophoresis. The coating was obtained via flushing the capillary with 1.0% Spe-g-Dex copolymer solution for 2 min. Electroosmotic flow (EOF) was strongly suppressed, ranging from −1.60 × 10⁻⁹ to 3.65 × 10⁻⁹ m² V⁻¹ s⁻¹. Effect of experimental conditions, such as the copolymer concentration, the concentration and pH of the background electrolyte (BGE), on the Spe-g-Dex coating was investigated. Separation of lysozyme, cytochrome c, ribonuclease A and α-chymotrypsinogen yielded high separation efficiencies ranging from 141 000 to 303 000 plates/m and recoveries from 85.4% to 98.3% at pH 4.0 (284.0 mM sodium acetate–acetic acid buffer, I = 50 mM). Run-to-run repeatabilities and day-to-day, and capillary-to-capillary reproducibilities were all below 1.7%. In addition, Spe-g-Dex coating allowed the successful separation of five neurotransmitters, 5-hydroxytryptamine, dopamine, epinephrine, isoprenaline, dobuamine at pH 4.0 with high separation efficiencies of 290 000–449 000 plates/m.     

Characterization of single-stranded DNA separation by capillary gel electrophoresis

We have investigated the effect of polymer gel reconditioning, the shape of the capillary, the applied electric field, and the capillary length for single-stranded DNA. The polyethylene oxide gel had deformed under the high electric field causing the degradation of the separation power. By the reintroduction of the fresh polyethylene oxide gel for the next run, one-base resolution was recovered. It turned out that the tip of the capillary at the injection side needed to be clean and symmetric for much improved resolution. Changing DNA motion by the pulsed electric field resulted in the separation of DNA far more than 500 bases.     

Extraction of biogenic amines using sorbent materials containing immobilized crown ethers

Three sorbent materials (A18C6-MS, DA18C6-MS and AB18C6-MS) based on the crown ether ligands, 1-aza-18-crown-6, 1,4,10,13-tetraoxa-7,16-diazacyclo octadecane and 4′-aminobenzo-18-crown-6, respectively, were prepared by the chemical immobilization of the ligand onto mesoporous silica support. The sorbents were characterized by FT-IR, scanning electron microscopy-energy dispersive X-ray microanalysis, elemental analysis and nitrogen adsorption-desorption test. The applicability of the sorbents for the extraction of biogenic amines by the batch sorption method was extensively studied and evaluated as a function of pH, biogenic amines concentration, contact time and reusability. Under the optimized conditions, all the sorbents exhibited highest selectivity toward spermidine (SPD) compared to other biogenic amines (tryptamine, putrescine, histamine and tyramine). Among the sorbents, AB18C6-MS offer the highest capacity and best selectivity towards SPD in the presence of other biogenic amines. The AB18C6-MS sorbent can be repeatedly used three times as there was no significant degradation in the extraction of the biogenic amines (%E > 85). The optimized procedure was successfully applied for the separation of SPD in food samples prior to the reversed-phase high performance liquid chromatography separation.     

Isotopic separation of lithium ions by capillary zone electrophoresis

Separation of ⁶Li and ⁷Li isotopes by CZE was demonstrated. The BGE contained 5 mM 4‐aminopyridine, 0.9 mM oxalic acid, 0.25 mM CTAB, and 0.25% w/v Tween 20 (рН = 9.2). The running conditions were +25 kV at 30°C with indirect photometric detection at 261 nm. Under optimal experimental conditions, the analysis time was less than 21 min. Separation of Li preparations with mole fraction of ⁶Li ranging from 3.44 up to 90.38% was demonstrated.     

Prediction of the separation of phenols by capillary zone electrophoresis

The effect of pH and ionic strength on the migration of neutral acids in capillary zone electrophoresis (CZE) has been studied for several phenols. The mobilities of the phenols and the efficiency of the capillary have been related to the studied factors. The mobility can be related to the pH of the running buffer through the mobility of the phenolate ion, and the conditional acidity pK value of the phenol at the working ionic strength. This allows prediction of the migration of the phenol, solely from its pK_a^′ value (literature pK_a corrected for the ionic strength of the solution) and mobility of the anion, which can be easily calculated from the mobility at a basic pH value and the pK_a^′ value. Combination of the predicted mobility with the efficiency allows estimation of the resolution of the consecutive peaks obtained for a mixture of phenols. This method has been tested for two groups of phenols of environmental interest.     

The GC separation of C2-naphthalene isomers using cyclodextrins and crown ethers

Summary The twelve structural C₂ isomers of naphthalene are resolved on a single capillary column using , , or cyclodextrin as the stationary phase. A change in elution order is seen upon going from to and then cyclodextrin which can be related to the size of the cyclodextrin cavity and the stereochemical fit of the isomer. A crown-ether column and a liquid-crystal column were found to be unable to separate all of the isomers.     

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.     

N-methylformamide as a separation medium in capillary electrophoresis

Summary The organic solvent N-methylformamide (NMF) has been used as a separation medium in capillary electrophoresis. The advantageous properties of this compound are its high dielectric constant, high solubilizing power and low conductivity, as well as its amphiprotic character. It was shown that, unlike for most organic solvents, the electroosmotic flow is substantial. It was found to be possible to utilize NMF without added electrolyte. Field strengths exceeding 1000 volts/cm could be employed, while a low current was maintained and it was thus possible to obtain rapid analyses. Also, the properties of NMF allowed the analysis of substances with a low solubility in aqueous media. These features are exemplified by separation of carboxylic acids and pharmaceuticals. Excellent reproducibility of migration and no sign of electrical breakdown were observed, even under high field strength conditions.Presented at the Sixth International Symposium on High Performance Capillary Electrophoresis, Jan. 31–Feb. 9, 1994, San Diego, USA, Poster #P-113.