Biomedical subjects
Publications and source records attributed to Martin G Schmid.
This review gives an overview of recent developments in CZE, EKC, and CEC covering the literature since the year 2004. Since there appeared a special issue on applications, this review focuses on the progress in electromigration techniques and new methodological developments. New techniques, new chiral selectors as well as new chiral stationary phases for CEC are discussed.
Particle-loaded monoliths containing a polymethacrylamide backbone were prepared by suspending a silica-based chiral phase in the mixture of the monomers followed by in-situ polymerization in the capillary. As chiral selector l-4-hydroxyproline chemically bonded to 3 microm silica particles was used following the separation principle of ligand-exchange. Electrolytes containing Cu(II) ions were used. Amino acid enantiomers were separated by capillary-LC and CEC, whereby the latter showed the better resolution properties. For the chiral separation of alpha-hydroxy acids the EOF was reversed by copolymerizing diallyldimethylammonium chloride instead of vinylsulfonic acid as charge providing agent. Short columns of 6 cm were found to be sufficient in the case of CEC for baseline separations of amino acids with alpha values up to 5.
This work deals with the application of stereoselective antibodies against L-T3 as a tailor-made chiral selector in micro-HPLC. The separations were performed in microbore columns using commercially available anti-L-T3 antibodies chemically bonded to 5 microm silica gel. The enantiomers of T3 were baseline separated under mild continuous isocratic elution conditions using 10 mM phosphate buffer, pH 7.4. The D-enantiomer eluted with the void volume, while the L-enantiomer was retained by the antibody phase and eluted second. An indirect competitive and non-competitive enzyme linked immunosorbent assay (ELISA) was used for testing the stereoselectivity of anti-L-T3 antibodies.
Novel particle-loaded monolithic capillary electrochromatography (CEC) phases for chiral separations were prepared via ring-opening metathesis polymerization (ROMP) within the confines of fused silica columns with 200 microm i.d. using norborn-2-ene (NBE), 1,4,4a,5,8,8a-hexahydro-1,4,5,8,exo,endo-dimethanonaphthalene (DMN-H6) as monomers, 2-propanol and toluene as porogens, RuCl2(PCy3)2(CHPh) as initiator and silica-based particles containing the chiral selector. By suspending silica particles bearing the chiral selector in the polymerization mixture, particle-based monoliths are easily prepared. This approach has several advantages compared to particle-based separation media: (i) the concept of particle-based monoliths is broadly applicable, as any silica-based chiral phase can be used; (ii) they are inexpensive to prepare; and (iii) the manufacturing process is very simple, no sophisticated packing procedures or the preparation of end frits are required. To show the usefulness of this concept for chiral CEC, the chiral separation performance of particle-loaded CEC monoliths bearing teicoplanin aglycone, chemically bonded to 3 microm silica gel, was investigated for a set of glycyl-dipeptides. Particle-loaded ROMP CEC monoliths showed good separation performance for glycyl-dipeptides.
This work deals with the application of BSA and canine serum albumin (CSA) for enantioseparation of tryptophan derivatives with CE. The aim of this work was the investigation of the influence of different functional groups of tryptophan derivatives on enantioseparation. CSA as a chiral selector was tested to compare its selector properties with those of BSA. The enantiomers of the tryptophan derivatives were separated by adding BSA or CSA to the BGE. The influence of pH, temperature, BSA and CSA concentration and organic modifiers was investigated. It was found that the stereoselectivity for the different tryptophan derivatives is dependent on the albumin species. It turned out that the different functional groups of the derivatives showed a significant influence on stereoselectivity.
The preparation and application of dynamically coated ligand-exchange chromatography phases for enantioseparation is described. The phases were prepared by pumping a solution of N-decyl-L-4-hydroxyproline, N-hexadecyl-L-4-hydroxyproline, or N-2-hydroxydodecyl-L-4-hydroxyproline through a commercially available monolithic RP-18 column. These coatings are stable against desorption for months at ambient temperature when aqueous mobile phases are used. The columns were applied to the chiral separation of amino acids, glycyl dipeptides and diastereomeric dipeptides, and tripeptides. The chiral selector can be removed or changed easily by washing the column with ACN or methanol. Ultrafast separations in the range of seconds were achieved using high flow rates.
Almost half of the drugs in use today are chiral. It is well established that the pharmacological activity is mostly restricted to one of the enantiomers (eutomer). There can be qualitative and quantitative differences in the activity of the enantiomers. In many cases, the inactive enantiomer (distomer) shows unwanted side effects or even toxic effects. Even if the side effects are not that drastic, the distomer has to be metabolized and this represents an unnecessary burden for the organism. Therefore, the development of methods for the separation of enantiomers, both on analytical and preparative scale, has become increasingly important. Chromatographic techniques such as thin layer chromatography (TLC), gas chromatography (GC), supercritical fluid chromatography (SFC), and above all high-performance liquid chromatography (HPLC) have been used for enantiomer separation for about two decades. More recently, electromigration techniques, such as capillary electrophoresis and capillary electrochromatography, have been shown to be powerful alternatives to chromatographic methods. This review gives a short overview of different chiral separation principles and their application. Several new developments are discussed.
The chiral separation of halogenated amino acids by ligand-exchange CE is described. Halogenated amino acids attracted increasing interest in recent years because of their physiological activities. Different chiral selectors, as there are L-4-hydroxyproline, L-histidine, and N-alkyl derivatives of L-4-hydroxyproline in form of their copper(II) complexes, are compared for their chiral recognition ability for halogenated amino acids. The influence of various parameters, such as selector concentration, pH, organic modifier, and field strength, on the resolution was investigated. All halogenated amino acids investigated were baseline-separated under optimized conditions.
This paper deals with the chiral separation of Fmoc- and Z-derivatives of natural and unnatural sulfur containing amino acids by micro-HPLC. The separations were carried out in microbore columns packed with a new material based on Ristocetin A bonded to 3.5 microm silica gel. The columns were run in the normal phase, polar organic mode as well as in the reversed phase mode, whereby best results were obtained with the reversed-phase mode using mixtures of triethylamine acetate (TEAA) buffer and methanol as mobile phases.
The present study deals with the development of a method for the quantitative determination of lipoic acid in a dietary supplement preparation. A rapid capillary electrophoretic method is developed using UV detection at 208 nm. Although lipoic acid is only weakly UV-absorbing, at this wavelength it could be detected with sufficient sensitivity with an LOD and LOQ of 0.8 and 2.5 microg/ml, respectively. Analysis time was less than 9 min. The compound was extracted from tablets with a recovery of 98.3% and a precision of 2.8% RSD.
Chiral separation of glycyl- and diastereomeric dipeptides and tripeptides was performed by micro-HPLC using macrocyclic antibiotics as chiral selectors. Teicoplanin was compared with teicoplanin aglycone (TAG) regarding selectivity, efficiency and separation time. The stationary phases are based on teicoplanin and TAG chemically bonded to 3.5 mum silica gel. The material was packed into 10 cm x 1 mm stainless steel microcolumns. Different mobile phases were checked using the reversed phase mode. Both teicoplanin and TAG were found to show good chiral separation ability for dipeptides. Glycyl-dipeptides were baseline resolved and most of the diastereomeric dipeptides and tripeptides were separated into their four isomers. In this study, teicoplanin was found to be advantageous compared to TAG regarding separation time, although TAG showed the higher resolution power. Baseline resolution for some glycyl-dipeptides was obtained within 3 min, diastereomeric dipeptides were resolved in 7 min. This method was also shown to be applicable for enantiomer purity control.
Fritless particle-loaded monoliths for chiral capillary electrochromatographic (CEC) separation were prepared. Silica particles containing a chiral selector are suspended in a monomer solution, which is drawn into the capillary followed by in situ polymerization. Thereby the silica-based particles containing the chiral selector are embedded in a nonchiral continuous bed. This kind of chiral stationary phase is inexpensive, easy, and reproducible to prepare and circumvents the preparation of frits. As a model, teicoplanin aglycone as chiral selector bonded to 3 microm silica particles was used. The applicability of this approach is demonstrated by means of the chiral separation of aliphatic and aromatic amino acids and dipeptides. As a further application, the chiral selector ristocetin A bonded to 3 microm silica particles was used for the enantiomeric separation of chiral alpha-hydroxy acids. Since alpha-hydroxy acids migrate toward the anode, a cationic charge-providing agent was copolymerized with the matrix. This served to reverse the direction of the electroosmatic flow (EOF).
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.
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This work deals with investigations on the enantioseparation of glycyl-dipeptides by capillary electrochromatography (CEC) on a capillary packed with teicoplanin aglycone immobilized on 3.5 microm silica gel. The results were compared to those obtained with micro-HPLC using the same chiral stationary phase. Polar organic and reversed-phase mode were checked, whereby the latter showed better results. Out of 12 glycyldipetides investigated, all compounds showed baseline separation with Rs values up to 20. Plate numbers were in the range of 10,000-300,000/m. The choice of organic modifier was found to be crucial. While methanol increased retention time, acetonitrile reduced it. A ternary mixture of ethanol-acetonitrile-aqueous triethylamine acetate solution pH 4.1 was found to be a useful compromise, providing excellent resolution with retention times less than 25 min. Efficiency and resolution were generally found to be higher in CEC than with micro-HPLC.