Immobilization of phospholipid-avidin on fused-silica capillaries for chiral separation in open-tubular capillary electrochromatography

Chromatographic Studies of Protein-Based Chiral Separations

The development of separation methods for the analysis and resolution of chiral drugs and solutes has been an area of ongoing interest in pharmaceutical research. The use of proteins as chiral binding agents in high-performance liquid chromatography (HPLC) has been an approach that has received particular attention in such work. This report provides an overview of proteins that have been used as binding agents to create chiral stationary phases (CSPs) and in the use of chromatographic methods to study these materials and protein-based chiral separations. The supports and methods that have been employed to prepare protein-based CSPs will also be discussed and compared. Specific types of CSPs that are considered include those that employ serum transport proteins (e.g., human serum albumin, bovine serum albumin, and alpha1-acid glycoprotein), enzymes (e.g., penicillin G acylase, cellobiohydrolases, and α-chymotrypsin) or other types of proteins (e.g., ovomucoid, antibodies, and avidin or streptavidin). The properties and applications for each type of protein and CSP will also be discussed in terms of their use in chromatography and chiral separations.

Enantioseparations in open-tubular capillary electrochromatography: Recent advances and applications

This review highlights recent advances and applications in open-tubular capillary electrochromatography (OT-CEC) for enantioseparations during the last decade. Although extensive research has been conducted in the area of separations by use of CEC, and a big number of reviews have been published, there is not a review on exclusively the use of chiral stationary phases (CSPs) in OT-CEC for enantioseparations. Therefore, in this review, the design and synthesis of different CSPs are presented, and their potential applications in OT-CEC for enantioseparations are discussed. The different approaches to CSP development include chiral nanomaterials, porous layers, molecular imprinting, sol-gel technique, polyelectrolyte multilayer coating, polymeric coating and others.

Chiral separations by capillary electrophoresis using proteins as chiral selectors

Capillary electrophoresis (CE) methods using proteins as the chiral selectors have been developed for the separation of enantiomeric mixtures. For chiral separations in protein-based CE, two methods were utilized. One is affinity capillary electrochromatography (ACEC), and the other is affinity CE (ACE). This chapter deals with the advantages and disadvantages of ACEC and ACE. Furthermore, enantioseparations utilizing ACEC based on packed α(1)-acid glycoprotein-immobilized silica gels, immobilized avidin to fused silica capillaries and ACE based on penicillin G-acylase dissolved in the running buffer are described in detail.

Enantioselective capillary electrochromatography: recent developments and new trends

Since its development in the early 1970s, CEC has been studied quite extensively, but unfortunately its use is still mostly located at an academic level. Reasons for this are the limited availability of commercially available stationary phases (SPs) and columns, along with some practical limitations, such as column fragility, lack of column robustness and reproducibility. Nevertheless, CEC maintains a place among the separation techniques, probably because of its unique feature to combine two separation principles. Also in the field of chiral separations, CEC is often used as a separation technique and already showed its potential for this kind of analyses. This overview will focus on the recent applications, i.e. between 2006 and 2010, in enantioselective analysis by means of CEC. For the selected applications, the used SPs (chiral selectors) and their potential for future method development or screening purposes will be evaluated and critically discussed.

Recent advances of enantioseparations in capillary electrophoresis and capillary electrochromatography

A comprehensive survey of recent developments and applications of capillary electromigration techniques for enantioseparations from January 2006 to June 2010 is presented. The techniques include capillary electrophoresis, chip capillary electrophoresis and capillary electrochromatography. The separation principles and the chiral recognition mechanisms are discussed. Additionally, on-line preconcentrations in chiral capillary electrophoresis are also reviewed.

Biotin-avidin binding kinetics measured by single-molecule imaging

The high affinity of avidin for biotin has made it useful for many bioanalytical applications involving the immobilization of proteins, vesicles, and other biomolecules to surfaces. To understand the formation and stability of the resulting biotin-avidin complex, it is useful to know the kinetics of the binding reaction, especially for situations where the complex is formed at a liquid-solid interface typically used in sensor or separation applications. In this work, a single-molecule fluorescence method is developed for measuring the kinetics and affinity constant for the binding of neutravidin, a deglycosylated variant of avidin, to surface-immobilized biotin. Biotin was immobilized using succinimidyl ester chemistry onto amine sites on glass surfaces. The surface density of biotin was controlled by the extreme dilution of 3-aminopropyltriethoxysilane into a monolayer of 2-cyanoethyltriethoxysilane. The resulting biotin binding sites are spaced apart by micrometer distances, and this avoids crowding effects and makes the resolution of single molecules possible. The binding and unbinding of individual tetramethylrhodamine-labeled neutravidin molecules is measured in situ by total-internal-reflection fluorescence (TIRF) microscopy imaging. Single-molecule detection and counting is readily achieved by this measurement, where quantitative control is established by determining the probabilities of false positive and negative events based on the intensity distributions of background and single-molecule spots and by comparing the bound molecule populations with the independently measured density of binding sites on the surface. The kinetics of binding and unbinding are evaluated by intermittent imaging and counting the number of bound neutravidin molecules versus time, following introduction of a neutravidin solution or its replacement by buffer over the low-density biotinylated surface. The neutravidin binding kinetics were found to be fast, essentially diffusion-controlled, while the stability of the complex and its dissociation rate appear to be influenced by the chemistry of biotin immobilization.

Recent accomplishments in the field of enantiomer separation by CEC

The present review intends to summarize recent developments in the field of enantioselective separations and analysis by CEC. It covers studies published in English language in common peer-reviewed journals within the period between 2003 and 2006. Both, methods making use of chiral mobile phase additives as well as chiral stationary phases for electrochromatographic enantiomer separations, are reviewed. Achievements that have been made on the various column technologies, such as open-tubular, particle-packed, inorganic, organic and particle-fixed (hybrid-type) monolithic as well as molecularly imprinted polymer phases, are discussed.

Use of a fritless dual tapered column and a low flow interface for capillary electrochromatography–mass spectrometry

To avoid problems associated with the use of sintered frits to retain packing material, tapered columns were investigated for use with capillary electrochromatography-mass spectrometry (CEC-MS) analysis. Taking the advantage that negatively charged stationary phase particles have a net velocity directed towards the buffer reservoir (inlet) over a wide range in pH, a fritless CEC column with a single taper tip was prepared for CEC-MS analysis. During CEC-MS analysis, the tapered end was immersed in the buffer reservoir and the unmodified end was pointed toward the ionization source. For better sensitivity, this single tapered CEC column was coupled to ESI/MS using a low flow sheath liquid interface. With this setup, occasional blockage of the ESI sprayer by stationary phase particles was observed. In addition, significant dead volume was observed because the unmodified tip could not be inserted into the very end of the sprayer of the low flow sheath liquid interface. To circumvent these problems, a dual tapered CEC column was prepared. This fritless dual tapered column CEC-MS approach alleviated the problems of frit, sprayer blockage and extensive dead volume.

Recent progress in polar stationary phases for CEC

This review summarizes most of the recent developments in the preparation and application of polar stationary phases for CEC covering the literature published since the year 2004. These polar stationary phases have been adopted for separation of analytes by the modes of packing column CEC, open-tubular CEC (o-CEC) and monolithic column CEC. Currently, development of o-CEC using biomolecules, such as protein and DNA, as the immobilized ligands is highlighted partly due to the simplicity of preparation. Furthermore, monolithic columns have been extended quickly, particularly inorganic materials-based monoliths, such as silica, zirconia, hafnium, etc., as an alternative to packed columns have been developed quickly.

Chiral resolution of tryptophan derivatives by CE using canine serum albumin and bovine serum albumin as chiral selectors

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.