Chapter 8 High Performance Capillary Electrophoresis of Carbohydrates and Glycoconjugates

Immobilization of peroxidase glycoprotein on gold electrodes modified with mixed epoxy-boronic Acid monolayers

The development of bioelectronic enzyme applications requires the immobilization of active proteins onto solid or colloidal substrates such as gold. Coverage of the gold surface with alkanethiol self-assembled monolayers (SAMs) reduces nonspecific adsorption of proteins and also allows the incorporation onto the surface of ligands with affinity for complementary binding sites on native proteins. We present in this work a strategy for the covalent immobilization of glycosylated proteins previously adsorbed through weak, reversible interactions, on tailored SAMs. Boronic acids, which form cyclic esters with saccharides, are incorporated into SAMs to weakly adsorb the glycoprotein onto the electrode surface through their carbohydrate moiety. To prevent protein release from the electrode surface, we combine the affinity motif of boronates with the reactivity of epoxy groups to covalently link the protein to heterofunctional boronate-epoxy SAMs. The principle underlying our strategy is the increased immobilization rate achieved by the weak interaction-induced proximity effect between slow reacting oxyrane groups in the SAM and nucleophilic residues from adsorbed proteins, which allows the formation of very stable covalent bonds. This approach is exemplified by the use of phenylboronates-oxyrane mixed monolayers as a reactive support and redox-enzyme horseradish peroxidase as glycoprotein for the preparation of peroxidase electrodes. Quartz crystal microbalance, atomic force microscopy, and electrochemical measurements are used to characterize these enzymatic electrodes. These epoxy-boronate functional monolayers are versatile, stable interfaces, ready to incorporate glycoproteins by incubation under mild conditions.

Recent developments in capillary electrophoresis and capillary electrochromatography of carbohydrate species

This review article is concerned with the recent developments in capillary electrophoresis (CE) and capillary electrochromatography (CEC) of carbohydrates. The literature shows that CE possesses impressive potential in the analysis of carbohydrates. On the other hand, CEC has just started to show promise in the analysis of carbohydrates. Advances in separation and detection approaches of derivatized and underivatized carbohydrates are discussed based on the available literature. In addition, important applications are illustrated.

Use of reversed polarity and a pressure gradient in the analysis of disaccharide composition of heparin by capillary electrophoresis

A capillary electrophoresis method with reversed polarity, combining both the application of a voltage and a pressure gradient between the buffer vials, was developed for the analysis of eight heparin-derived delta-disaccharides obtained by enzymatic depolymerization. A 60 mM formic acid buffer at pH 3.40 was selected as running electrolyte, with an applied voltage of -15 kV and an over-imposed pressure gradient (3.45.10(-3) MPa) for 6 min from inlet to outlet starting at 20 min. Figures of merit such as run-to-run and day-to-day precision, and limits of detection were established. The electrophoretic method was applied to the analysis of depolymerization products of different kinds of heparins. The composition of the depolymerization buffer was selected in order to reduce baseline distortions in the electrophoretic separation, thus a buffer solution containing 20 mM Tris, 50 mM sodium chloride, and 3 mM calcium chloride at pH 7.10 was used. Percentages of molar disaccharide compositions for unfractionated heparins from porcine, bovine and ovine intestinal mucosa, and bovine lung were determined. In addition, low-molecular-mass heparins from bovine and porcine intestinal mucosa were analysed as well.

Capillary zone electrophoresis of proteins with fused-silica capillaries having polymers and surfactants adsorbed onto surfactant moieties previously covalently bound to the capillary column surface

Fused-silica capillary columns having cationic surfactant moieties (CSM) covalently attached to the capillary inner walls were introduced for the separation of proteins by capillary zone electrophoresis (CZE). The CSM capillary coating proved to be useful in the separation of basic and acidic proteins when modified hydroxypropylcellulose (HPC), namely epoxybutane-HPC (EBHPC), was adsorbed to the primary CSM coating, yielding a hybrid coating (i.e., coating consisting of a covalently bound ligand and an adsorbed ligand). The EBHPC layer rendered the capillary surface highly hydrophilic, thus permitting the separation of basic proteins with relatively high plate counts. Also, the CSM coating was useful for the separation of acidic proteins when the capillary wall had, in addition, a negatively charged polymer adsorbed to the surface, e.g., hyaluronic acid. In general, neutral and charged polymeric compounds could be readily adsorbed by the CSM coating, thus altering the zeta potential of the capillary and diminishing solute adsorption to the capillary surface. In other words, the sign of the zeta potential of the capillary surface could be tailored to be of the same sign as the charge of the analytes. Under these conditions, little or no solute-wall interaction could be observed due to electrostatic repulsion. Although the capillary surface was charged, the presence of adsorbed polymer suppressed or, in most cases, even eliminated the electroosmotic flow (EOF).

Capillary electrochromatography of derivatized mono- and oligosaccharides

An octadecyl-silica (ODS) stationary phase with light surface coverage of octadecyl ligands was introduced for capillary electrochromatography (CEC) at moderate electroosmotic flow (EOF) velocity. The ODS stationary phase was intentionally produced with light surface coverage in order to ensure a moderate EOF velocity across the packed capillary column, thus allowing relatively rapid analysis time. Despite the fact that the stationary phase leaves 75% of the surface silanols unreacted, fused-silica capillary columns packed with this ODS stationary phase exhibited reversed-phase behavior toward neutral alkylbenzene homologous solutes using hydroorganic eluents. Closely related p-nitrophenylglycosides including some p-nitrophenyl-monosaccharides and p-nitrophenyl-maltooligosaccharides were readily separated on the ODS capillary column within a relatively short analysis time. Also, alpha- and beta-anomers of some p-nitrophenyl-monosaccharides were readily separated in the presence of a small amount of borate buffer in the hydroorganic eluent.

Recent developments in capillary electrophoresis of carbohydrate species

Carbohydrates are ubiquitous species involved in many life processes. Because of the multilateral roles of carbohydrates, their analysis has come to have increasing importance. As shown in this review, capillary electrophoresis in its various modes of operation has proved very useful in the analysis of carbohydrate species including mono- and oligosaccharides, glycoproteins, glycopeptides and glycosaminoglycans. Advances in separation approaches and applications as well as advances in detection including sensitive and selective pre-column derivatization are described. In summary, this comprehensive review is a supplement to previous reviews and covers the published work in 1996 and the first half of 1997.

Fast separation of underivatized carbohydrates by coelectroosmotic capillary electrophoresis

A method for the rapid quantitative analysis of underivatized acidic sugars, monosaccharides and disaccharides using coelectroosmotic capillary electrophoresis was developed. Indirect UV detection at 254 nm using sorbate as background electrolyte was employed for monitoring the analytes. A highly alkaline pH value of the electrolyte system was chosen in order to achieve an electrophoretic mobility of the saccharides towards the anode. A dynamic reversal of the electroosmotic flow and, by this means, a codirectional movement of the negatively charged analytes and the electroosmotic flow is accomplished by employing a polycationic surfactant (hexadimethrine bromide), which is added to the background electrolyte. To further improve the resolution of specific carbohydrates, acetone is used as organic modifier. A practical application of the developed method for the fast determination of fructose, glucose, and sucrose in various soft drinks is provided.

Pulsed electrochemical detection of nonchromophoric compounds following capillary electrophoresis

Pulsed electrochemical detection (PED) following capillary electrophoresis (CE) has been applied to the direct detection of polar aliphatic compounds. These compounds typically have poor optical detection properties and are considered to be electroinactive under constant applied potentials. The voltammetric responses at microelectrodes of model compounds were studied under various buffers and pH conditions. The detection of unsubstituted carbohydrates requires highly alkaline conditions, whereas amine-containing compounds (e.g., glycopeptides, peptides, and amino acids) and thiocompounds can best be detected at mildly alkaline (i.e., pH 9.0) and mildly acidic (i.e., pH 5.5) conditions, respectively. In a comparative study of pulsed amperometric detection (PAD) and integrated pulsed amperometric detection (IPAD), IPAD is determined to be better suited to manage the large oxide-formation backgrounds, which accompany oxide-catalyzed detections of amine- and sulfur-containing compounds. IPAD results in more stable baselines, eliminates oxide-induced artifacts, and often yields lower limits of detection. Mass detection limits under optimal conditions are typically 10 fmole or less. This paper delineates PED waveform parameters for the optimal detection of underivatized nonchromophoric compounds of biological significance in various operating buffers and electrolytes.

Capillary electrophoresis and electrochemical detection of underivatized oligo- and polysaccharides with surfactant-controlled electroosmotic flow

Complex polysaccharide mixtures were analyzed directly without derivatization by capillary zone electrophoresis in strongly alkaline solutions and electrochemical detection at a Cu electrode. The positively charged surfactant cetyltrimethylammonium bromide was included in the electrophoresis medium in order to reverse the electroosmotic flow and permit elution to be in order of increasing polysaccharide size. Carbohydrate samples analyzed by this approach included linear maltoses, enzymatically hydrolyzed starch, and commercially available dextrans of up to an average molecular weight of 18,300. Detection by constant-potential oxidation at a Cu electrode was very sensitive, with detection limits for individual carbohydrates generally below the femtomole level.

The influence of buffer composition on separation efficiency and resolution in capillary electrophoresis of 8-aminonaphthalene-1,3,6-trisulfonic acid labeled monosaccharides and complex carbohydrates

The effect of buffer conditions -- varying in salt type, pH, and concentration -- on the separation of 8-aminonaphthalene-1,3,6-trisulfonic acid (ANTS)-labeled monosaccharides and complex-type carbohydrates was investigated. Different buffer systems for high and low electroosmotic flow conditions were chosen: a phosphate and a citrate background electrolyte, each at pH 2.5, a phosphate buffer, pH 9.0, and a borate buffer at pH 9.5. All buffer systems displayed differences in resolution and selectivity. Phosphate and borate buffer demonstrated the greatest selectivity changes for ANTS-labeled carbohydrates. While separation in the phosphate system relies mainly on differences in the charge-to-mass-ratio, additional selectivity can be achieved with borate complexation of glycoconjugates. The use of borate buffers improved monosaccharide separations whereas complex carbohydrates showed a loss in resolution. The citrate background electrolyte at low pH caused no significant changes in the separation performance. The pH 9.0 phosphate buffer showed a reversed migration order of the ANTS conjugates with a decreased resolution, compared to the pH 2.5 phosphate buffer, due to the strong electroosmotic flow generated under high pH conditions. An ovalbumin-derived oligosaccharide library demonstrates the significance of buffer selectivity for complex carbohydrate separations. The separation in the acidic phosphate and the alkaline borate buffer generates a different pattern and only the combination of both buffer systems allows an appropriate assessment of sample complexity.

Capillary gel electrophoresis separation of high-mannose type oligosaccharides derivatized by 1-aminopyrene-3,6,8-trisulfonic acid

In this paper we report the capillary gel electrophoresis separation of 1-aminopyrene-3,6,8-trisulfonic acid (APTS) labeled oligosaccharides, released enzymatically from bovine pancreatic ribonuclease B. The released and labeled high-mannose structures were identified by spiking the separated peaks with the appropriate commercially available individual oligosaccharides. Baseline separation of the three positional isomers of the mannose-7 and mannose-8 oligosaccharides was attained. Comparison of the electrophoretic mobilities of the high-mannose type branched carbohydrates to the linear molecules of maltooligosaccharides (glucose oligomers) have been shown using different gel concentrations in the running buffer system. We observed that increasing gel concentration in the running buffer causes an increase in the relative mobility values of the high-mannose type carbohydrate molecules compared to the linear glucose oligomers. Analysis of our data indicated that this increase in relative migration time was not due to sieving, but seemed to be related to the mannose content and hydrodynamic volume of the branched glycans as well as to the viscosity of the separation medium.