Oligomers of "extended viologen", p-phenylene-bis-4,4'-(1-aryl-2,6-diphenylpyridinium), as candidates for electron-dopable molecular wires

We report the synthesis and spectral characterization of the first five members of an oligomeric series built from alternating p-connected 1,4-benzene and 1,4-pyridinium rings, 1[n]-4[n], n=1-5, with p-phenylene-bis-4,4'-(1-aryl-2,6-diphenylpyridinium) ("extended viologen") as the repeating unit. The lengths of these rodlike molecules range from 2 to 9 nm. The monomer was obtained from p-phenylene-bis-4,4'-(2,6-diphenylpyrylium) (5) and p-phenylenediamine (6) or p-aminoacetanilide (9). Higher oligomers were synthesized by stepwise elaboration of the monomer by reactions with the appropriate bis-pyrylium (5) or pyrylium-phenylene-pyridinium (8) salts. Eight different counterions were used, and dodecamethylcarba-closo-dodecaborate was found to offer particularly favorable solubility characteristics. Ultraviolet absorption spectra of the oligomers suggest that the individual extended viologen segments interact only weakly, as a result of the strongly twisted orientation of the benzene rings that separate them. The UV spectrum of the monomer was interpreted by comparison with semiempirical INDO/S calculations performed at a DFT optimized geometry.     

One-electron reduction of an "extended viologen" p-phenylene-bis-4,4'-(1-aryl-2,6-diphenylpyridinium) dication

One-electron reduction of the "extended viologen" dication 1 yields the red cation radical 2, characterized by strong near-IR absorption. It has been generated and studied by pulse radiolytic, electrochemical, redox titration, UV-visible, and electron paramagnetic resonance spectroscopic methods. All results are in agreement with a fully delocalized electronic structure for 2.     

Nonaqueous capillary electrophoretic separation of polyphenolic compounds in wine using coated capillaries at high pH in methanol

Nonaqueous capillary electrophoretic separation of a group of flavonoids (quercetin, myricetin, catechin, epicatechin) and resveratrol in wine was investigated in methanol at high pH. Malonate background electrolyte (pH* 13.5, ionic strength I = 14.2 mmol/L) provided highly repeatable separations of the analytes. Tests of untreated and coated (poly(glycidylmethacrylate-co-N-vinylpyrrolidone)) capillaries showed the analysis to be faster (6.5 min vs. 25 min) and the repeatability better in the coated capillaries. The coating procedure was simple and highly repeatable and the coating was stable during 40-45 runs. Determination of the last migrating peaks (epicatechin, resveratrol and catechin) was achieved merely by evaporating the wine samples and reconstituting the residue in methanol. For determination of the first migrating peaks (quercetin and myricetin) the samples were submitted to solid-phase extraction in C8 cartridges.     

Nonaqueous capillary electrophoresis of imatinib mesylate and related substances

In the present study, nonaqueous capillary electrophoretic separation of imatinib mesylate (IM) and related substances, N-(5-amino-2-methylphenyl)-4-(3-pyridyl)-2-pyrimidinamine (PYA), N-(4-methyl-3-(4-(pyridin-3-yl)pyrimidin-2-ylamino)phenyl)-4-((piperazin-1-yl)methyl) benzamide (NDI) and 4-chloromethyl-N-(4-methyl-3-((4-(pyridin-3-yl) pyrimidin-2-yl) amino) phenyl) benzamide (CPB) was developed. The influential factors affecting separation, including type and concentration of the electrolyte, applied voltage, and buffer modifier were investigated. Baseline separation of the studied analytes was obtained using a buffer of 50 mM Tris and 50 mM methanesulfonic acid in methanol at a apparent pH (pH*) of 1.65. To enhance the sensitivity, large-volume sample stacking was employed for online concentration. The strongest analytical signal with a suitable separation was achieved when the injection time was 100 s. The linearity ranges of PYA and NDI were 0.100-2.50 μg mL(-1) , and that of CPB was 0.125-2.50 μg mL(-1) , with good coefficients (r(2) > 0.9948). The relative standard deviations of intra- and interday were satisfactory. Under the optimized conditions, seven batches of the synthesized samples were analyzed and CPB was detected in two batches. Owing to its simplicity, effectiveness, and low price, the developed method is promising for quality control of IM.     

Nonaqueous and aqueous capillary electrophoresis of synthetic polymers

In this work, the use of capillary electrophoresis (CE) to analyze synthetic polymers is reviewed including works published till February 2004. The revised works have been classified depending on the CE mode (e.g., free solution capillary electrophoresis, capillary gel electrophoresis, etc.) and type of buffer (i.e., nonaqueous, aqueous and hydro-organic background electrolytes) employed to separate synthetic macromolecules. Advantages and drawbacks of these different separation procedures for polymer analysis are discussed. Also, physicochemical studies of complex polymer systems by CE are reviewed, including drug release studies, synthetic polyampholytes, dendrimers, fullerenes, carbon nanotubes and associative copolymers.     

Silver (I)-Mediated Separations by Nonaqueous Capillary Electrophoresis: Nonaqueous Argentation Electrophoresis

This study represents the first application of Ag(I) charge transfer complexation in nonaqueous capillary electrophoresis. This method applies the principles of argentation chromatography to nonaqueous electrophoretic separations and is termed “nonaqueous argentation electrophoresis”. Since the separations are performed in 100% nonaqueous media, the advantages of nonaqueous solvents, such as enhanced solubility and flexibility in selectivity enhancement, compared to an aqueous or mixed hydroorganic solvent, are realized. A variety of compounds were separated. Qualitatively, the separation of eleven sulfonamides in 100% acetonitrile is shown to improve greatly upon the addition of Ag(I). These results also show that nonaqueous argentation electrophoresis provides fast, well-resolved separations of compounds, such as N-containing heterocyclics, that can selectively complex with Ag(I). Migration data and separation selectivities of these compounds by nonaqueous argentation electrophoresis were compared to previous aqueous argentation electrophoresis results. Selectivities were found to be significantly different for the two separation media. Ag(I) complexation provides an effective means of manipulating selectivity in nonaqueous capillary electrophoresis.     

Nonaqueous capillary electrophoresis method for the analysis of gleevec and its main metabolite in human urine

The viability of nonaqueous capillary electrophoresis (NACE) was investigated for determination of gleevec and its main metabolite in human urine using a fused-silica capillary. Baseline separation of the studied solutes was obtained using a nonaqueous solution composed of 12 mM ammonium acetate and 87.6 mM acetic acid in methanol-acetonitrile (ACN) (80:20, v:v) providing analysis time shorter than 3 min. Different aspects including stability of the solutions, linearity, accuracy and precision were studied in order to validate the method in the urine matrix. Detection limits of 24 microg L(-1) for gleevec and its metabolite were obtained. A robustness test of the method was carried out using the Plackett-Burman fractional factorial model with a matrix of 15 experiments. The developed method is simple, rapid and sensitive and has been used to determine gleveec and its metabolite at clinically relevant levels in human urine. Before NACE determination, a solid-phase extraction (SPE) procedure with a C18 cartridge was necessary. Real determination of these analytes in two patient urines were done.     

Effect of surfactant species and electrophoretic medium composition on the electrophoretic behavior of neutral and water‐insoluble linear synthetic polymers in nonaqueous capillary zone electrophoresis

We have recently demonstrated the separation of neutral and water‐insoluble linear synthetic polymers in nonaqueous capillary zone electrophoresis (NACZE) using a cationic surfactant of cetyltrimethylammonium chloride (CTAC). In this study, eight ionic surfactants were investigated for the separation of four synthetic polymers (polystyrene, polymethylmethacrylates, polybutadiene, and polycarbonate); only three surfactants (CTAC, dimethyldioctadecylammonium bromide, and sodium dodecylsulfate) caused their separation. The order of the interaction between the polymers and the surfactants depended on both the surfactant species and the composition of the electrophoretic medium. Their investigation revealed that the separation is majorly affected by the hydrophobic interactions between the polymers and the ionic surfactants. In addition, the electrophoretic behavior of polycarbonate suggested that electrostatic interaction also affects the selectivity of the polymers.     

Nonaqueous capillary electrophoresis for the analysis of selected tropane alkaloids in a plant extract

Summary The potential of nonaqueous capillary electrophoresis has been investigated for the separation of structurally similar tropane alkaloids. The effects of the organic solvent and of electrolyte composition on separation selectivity, migration times, and efficiency are described. The addition of trifluoroacetic acid to the separation buffer was found beneficial for manipulation of the order of migration of the two positional isomers littorine and hyoscyamine. Replicate injections under nonaqueous conditions gave migration time and peak area data of excellent precision. The application of the optimized conditions to the analysis of hyoscyamine and scopolamine in genetically transformed root cultures ofDatura candida x D. aurea is presented.     

Application of 1-Alkyl-3-methylimidazolium-Based Ionic Liquids as Background Electrolytes in Nonaqueous Capillary Electrophoresis for the Analysis of Coptidis Alkaloids

Interest in ionic liquids (ILs) for their potential in analytical chemistry is increasing because they are environmentally benign and are good separation solvents. The aim of the presented investigation was to verify whether ILs would be a suitable background electrolyte (BGE) in nonaqueous capillary electrophoresis (NACE) for organic cations analysis of the closely related analogues. In this study, a novel and very simple NACE method has been established for analyzing seven quaternary alkaloids in Coptis rhizome using 1-alkyl-3-methylimidazolium tetrafluoroborate-based ionic liquid as BGE. The effects of the alkyl group, imidazolium counterion (anionic part), along with the concentration of IL, are investigated and discussed. Baseline separation, high efficiencies, and symmetrical peaks of the seven alkaloids were obtained. The separation mechanism could be hydrophobic and hydrogen-bonding interactions between the alkaloids and the imidazolium cations. The optimum conditions were 70 mM 1-decyl-3-methylimidazolium tetrafluoroborate (1D-3MI-TFB) methanol solution (apparent pH 2.66) and 30 kV applied voltage. The detection was performed at 254 nm. Seven quaternary alkaloids in Coptis rhizome were separated within 14 min. The proposed NACE separation procedure is highly reproducible and can be applied in the qualitative and quantitative analysis of Coptidis alkaloids.     

Nonaqueous capillary electrophoresis method for the analysis of tamoxifen, imipramine and their main metabolites in urine

The viability of nonaqueous capillary electrophoresis (NACE) was investigated for the simultaneous determination of tamoxifen, imipramine and their main metabolites (4-hydroxytamoxifen and desipramine, respectively). Baseline separation of the studied solutes was obtained on a 57 cm × 75 μm capillary using a nonaqueous solution composed of 17 mM ammonium acetate and 1.25% acetic acid in 80:20 (v:v) methanol-acetonitrile, temperature and voltage 22 °C and 15 kV, respectively, and hydrodynamic injection. Paroxetine was used as internal standard. Different aspects including linearity, accuracy, ruggedness and precision was studied. Detection limits between 9.0 and 15.0 μg L⁻¹ were obtained for all the studied compounds. The developed method is simple, rapid and sensitive and has been used to determine tamoxifen, imipramine and their metabolites at clinically relevant levels in human urine. Before NACE determination, a solid phase extraction (SPE) procedure with a C₁₈ cartridge was necessary. Real determination of these analytes in three females urines were done.     

Comparison of the separation of aziridine isomers applying heptakis(2,3-di-O-methyl-6-sulfato)beta-CD and heptakis(2,3-di-O-acetyl-6-sulfato)beta-CD in aqueous and nonaqueous systems

Aziridines are attracting interest as protease inhibitors, which might be used, e.g., for treatment of parasitic diseases. Within the framework of greater projects dealing with the search of new selective protease inhibitors, a huge number of aziridines with two stereogenic centers will be synthesized. Thus, a fast and reliable screening method for the evaluation of the isomeric composition is needed. Robust baseline separations were obtained using heptakis(2,3-di-O-acetyl-6-sulfato)beta-CD (HDAS) in acidic methanol and sulfated beta-CD in acidic phosphate buffer. With HDAS the resolutions were higher and migration times shorter. Thus, the method will be used as a screening method for further isomeric mixtures of aziridines.     

Nonaqueous versus aqueous capillary electrophoresis of alpha-helical polypeptides: effect of secondary structure on separation selectivity

The CE separation of alpha-helical polypeptides composed of 14-31 amino acid residues has been investigated using aqueous and nonaqueous BGEs. The running buffers were optimized with respect to pH. Generally, higher separation selectivities were observed in nonaqueous electrolytes. This may be explained by a change in the secondary structure when changing from water to organic solvents. Circular dichroism spectra revealed a significant increase in helical structures in methanol-based buffers compared to aqueous buffers. This change in secondary structure of the polypeptides contributed primarily to the different separation selectivity observed in aqueous CE and NACE. For small oligopeptides of two to five amino acid residues no significant effect of the solvent was observed in some cases while in other cases a reversal of the migration order occurred when changing from aqueous to nonaqueous buffers. As these peptides cannot adopt secondary structures the effect may be attributed to a shift of the pKa values in organic solvents compared to water.     

Nonaqueous capillary electrophoresis with laser-induced fluorescence detection: a case study of comparison with aqueous media

A novel method based on separation by nonaqueous capillary electrophoresis (NACE) combined with laser-induced fluorescence (LIF) detection was developed and compared with classic aqueous modes of electrophoresis in terms of resolution of solutes of interest and sensitivity of the fluorescence detection. Catecholamines derivatized with 4-chloro-7-nitro-2,1,3-benzoxadiazole (NBD-Cl) were chosen as test analytes for their subtle fluorescence properties. In aqueous systems, capillary zone electrophoresis (CZE) was not suitable for the analysis of test analytes due to complete fluorescence quenching of NBD-labeled catecholamines in neat aqueous buffer. The addition of micelles or microemulsion droplets into aqueous running buffer can dramatically improve the fluorescence response, and the enhancement seems to be comparable for micellar electrokinetic chromatography (MEKC) and microemulsion electrokinetic chromatography (MEEKC). As another alternative, NACE separation was advantageous when performing the analysis under the optimum separation condition of 20 mM sodium tetraborate, 20 mM sodium dodecyl sulfate (SDS), 0.1% (v/v) glacial acetic acid, 20% (v/v) acetonitrile (ACN) in methanol medium after derivatization in ACN/dimethyl sulfoxide (DMSO) (3:2, v/v) mixed aprotic solvents containing 20 mM ammonium acetate. Compared with derivatization and separation in aqueous media, NACE-LIF procedure was proved to be superior, providing high sensitivity and short migration time. Under respective optimum conditions, the NACE procedure offered the best fluorescence response with 5-24 folds enhancement for catecholamines compared to aqueous procedures. In addition, the mechanisms of derivatization and separation in nonaqueous media were elucidated in detail.     

Nonaqueous capillary electrophoresis for rapid and sensitive determination of fangchinoline and tetrandrine in radix Stephaniae tetrandrae and its medicinal preparations

A simple, rapid, and sensitive non-aqueous capillary electrophoresis procedure with head-column field-amplified sample stacking concentration for the analysis of fangchinoline and tetrandrine is established. Optimum separation and stacking conditions were obtained when the sample was injected at 8 kV for 50 s after preliminary pressure injection of ethanol (16.9 kPa) for 0.6 s and separated with the buffer containing 50 mM ammonium acetate, 0.5% (v/v) acetic acid, and 50% (v/v) acetonitrile in methanol medium at 24 kV applied voltage. The analytes were detected by UV at 214 nm. The two bisbenzylisoquinoline alkaloids can be separated within 6 min and quantified with high sensitivity. The detection limits were 0.30 ng mL(-1) for fangchinoline and 0.34 ng mL(-1) for tetrandrine, which indicated that the sensitivities were at least 1000-fold enhanced over those reported in the literature as obtained by UV detection. The method was applied to the analysis of fangchinoline and tetrandrine in Radix Stephaniae tetrandrae and its medicinal preparations with good results.     

Nonaqueous capillary electrophoresis coupled with laser-induced native fluorescence detection for the analysis of berberine, palmatine, and jatrorrhizine in Chinese herbal medicines

LIF detection is one of the most sensitive detection methods for CE. However, its application is limited because the analyte is usually required to be derivatized with a fluorescent label. As a result, LIF is seldom used to analyze active ingredients in plants. In this work, we introduce a rapid, simple, and sensitive method of nonaqueous CE (NACE) coupled with laser-induced native fluorescence detection for the simultaneous analysis of berberine, palmatine, and jatrorrhizine. This method skillfully utilizes the native fluorescence of these alkaloids and requires no troublesome fluorescent derivatization. As these alkaloids can fluoresce to some degree, they were simply detected by a commercially available 488 nm Ar+ laser. The native fluorescence of the analytes was greatly enhanced by nonaqueous media. Compared with the reported UV detection method, much lower LOD was achieved (6.0 ng/mL for berberine, 7.5 ng/mL for palmatine, and 380 ng/mL for jatrorrhizine). This method was successfully applied to analyze berberine, palmatine, and jatrorrhizine in two Chinese herbal medicines, Rhizoma coptidis and Caulis mahoniae.     

Influence of the nature of the electrolyte on the chiral separation of basic compounds in nonaqueous capillary electrophoresis using heptakis(2,3-di-O-methyl-6-O-sulfo)-beta-cyclodextrin

The influence on the enantiomeric resolution of the nature of the cationic BGE component (sodium, ammonium or potassium) and that of the anionic component (chloride, formate, methanesulfonate or camphorsulfonate) as well as the concentration of heptakis(2,3-di-O-methyl-6-O-sulfo)-beta-cyclodextrin (HDMS-beta-CD), the selected chiral selector, was studied in nonaqueous capillary electrophoresis (NACE). For this purpose, two D-optimal designs with 33 and 26 experimental points were applied. Three beta-blockers (atenolol, celiprolol and propranolol) and three local anesthetics (bupivacaine, mepivacaine and prilocaine) were selected as basic model compounds. Both cationic and anionic BGE components were found to have a deep impact on the enantiomeric resolution of the investigated analytes but it is the cationic component that has shown the strongest influence. Indeed, in some cases, the change of the latter led to a complete loss of enantioresolution. Based on the observed results, two NACE systems were recommended, namely ammonium formate and potassium camphorsulfonate in a methanolic solution containing HDMS-beta-CD and acidified with formic acid, in order to separate efficiently the enantiomers of basic drugs.     

Characterization of rhinovirus subviral A particles via capillary electrophoresis, electron microscopy and gas-phase electrophoretic mobility molecular analysis: Part I

During infection, enteroviruses, such as human rhinoviruses (HRVs), convert from the native, infective form with a sedimentation coefficient of 150S to empty subviral particles sedimenting at 80S (B particles). B particles lack viral capsid protein 4 (VP4) and the single-stranded RNA genome. On the way to this end stage, a metastable intermediate particle is observed in the cell early after infection. This subviral A particle still contains the RNA but lacks VP4 and sediments at 135S. Native (150S) HRV serotype 2 (HRV2) as well as its empty (80S) capsid have been well characterized by capillary electrophoresis. In the present paper, we demonstrate separation of at least two forms of subviral A particles on the midway between native virions and empty 80S capsids by CE. For one of these intermediates, we established a reproducible way for its preparation and characterized this particle in terms of its electrophoretic mobility and its appearance in transmission electron microscopy (TEM). Furthermore, the conversion of this intermediate to 80S particles was investigated. Gas-phase electrophoretic mobility molecular analysis (GEMMA) yielded additional insights into sample composition. More data on particle characterization including its protein composition and RNA content (for unambiguous identification of the detected intermediate as subviral A particle) will be presented in the second part of the publication.     

Establishment and application of a new method for the determination of kinetic parameters by plug-plug kinetic capillary electrophoresis (ppKCE)

A novel plug-plug kinetic capillary electrophoresis method was established, which can be used to si-multaneously determine the kinetic parameters kon and koff in interaction systems. The method is comparatively simple and some restrictions in conventional ppKCE methods can be effectively avoided. The requirements for resolution and detection sensitivity in this method are much lower than those of conventional ppKCE. The successful determination of the kinetic parameters and the binding constant Kb between citalopram and BSA showed availability of this method. The results were confirmed by us-ing the time ratio method. The application field of kinetic capillary electrophoresis can be expanded with this new method.