Electrophoretic mobilities of the complexes between sodium dodecyl sulfate and various peptides or proteins determined by free solution electrophoresis using coated capillaries

Microchip Electrophoresis Assay for Calmodulin Binding Proteins

The calcium signaling protein calmodulin regulates numerous intracellular processes. We introduce a sensitive microchip assay to separate and detect calmodulin binding proteins. The assay utilizes an optimized microchip electrophoresis protein separation platform with laser-induced fluorescence detection. Fluorescence-labeled calmodulin modified with a photoreactive diazirine crosslinker allowed selective detection of calmodulin binding proteins. We demonstrate successful in-vitro crosslinking of calmodulin with two calmodulin binding proteins, calcineurin and nitric oxide synthase. We compare the efficacy of commonly applied electrophoretic separation modes: microchip capillary zone electrophoresis, microchip micellar electrokinetic chromatography/gel electrophoresis, and nanoparticle colloidal arrays. Out of the methods tested, polydymethylsiloxane/glass chips with microchip zone electrophoresis gave the poorest separation, whereas sieving methods in which electro-osmotic flow was suppressed gave the best separation of photoproducts of calmodulin conjugated with calmodulin binding proteins.

Comparison of separation modes for microchip electrophoresis of proteins

Separation of a set of model proteins was tested on a microchip electrophoresis analytical platform capable of sample injection by two different electrokinetic mechanisms. A range of separation modes-microchip capillary zone electrophoresis, microchip micellar electrokinetic chromatography, and nanoparticle-based sieving-was tested on glass and polydimethylsiloxane/glass microchips and with silica-nanoparticle colloidal arrays. The model proteins calmodulin (18 kiloDalton), bovine serum albumin (66 kDa), and concanavalin (106 kDa) were labeled with Alexa Fluor 647 for laser-induced fluorescence detection. The best separation and resolution were obtained in a silica-nanoparticle colloidal array chip.

Effect of detergent on electromigration of proteins: CE of very low density lipoprotein receptor modules and viral proteins

The different electrophoretic behavior of the members of two groups of proteins with respect to the absence or presence of detergent additives in the BGE was explored. Recombinant soluble concatemers of repeat 3 of the very low density lipoprotein (VLDL)-receptor fused at their N-terminus to maltose-binding protein (MBP) exhibited different electrophoretic mobilities in borate buffer (pH 8.3) in the absence and in the presence of dodecyl-PEG ether (D-PEG). This enabled the separation of the receptor fragments from MBP after enzymatic cleavage. In the presence of SDS, the mobilities of all proteins approached the same values with increase in detergent concentrations. In contrast, viral capsid proteins of a human rhinovirus (HRV) exhibited different migration in the presence of the additive. For the receptor proteins, extreme apparent high plate numbers were observed when the SDS concentration in the sample and the separation buffer differed. This effect might be erroneously interpreted as a high efficiency. However, it is due to the conductivity boundaries caused by the sample and leads to a total loss of separation.

Stacking and separation of fluorescent derivatives of amino acids by micellar electrokinetic chromatography in the presence of poly(ethylene oxide)

A new approach for the analysis of large-volume naphthalene-2,3-dicarboxaldehyde (NDA) derivatives of amino acids by micellar electrokinetic chromatography (MEKC) in conjunction with a purple light-emitting diode-induced fluorescence detection is described. In order to optimize resolution, speed, and stacking efficiency, a discontinuous condition is essential for the analysis of NDA-amino acid derivatives. The optimum conditions use 2.0M TB (pH 10.0) buffer containing 40mM sodium dodecyl sulfate (SDS) to fill the capillary, deionized water to dilute samples, and 200mM TB (pH 9.0) containing 10mM SDS to prepare 0.6% poly(ethylene oxide) (PEO). Once high voltage is applied, PEO solution enters the capillary via electroosmotic flow and SDS micelles interact and thus sweep the NDA-amino acid derivatives having smaller electrophoretic mobilities than that of SDS micelles in the sample zone. When the aggregates between SDS micelles and NDA amino acid derivatives enter PEO zone, they are stacked due to decrease in electric field and increases in viscosity. Under the optimum conditions, the concentration and separation of 0.53-microL 13 NDA-amino acid derivatives that are negatively charged has been demonstrated by using a 60-cm capillary, with the efficiencies 0.3-9.0x10(5) theoretical plates and the LODs at signal-to-noise ratio 3 ranging from 0.30 to 2.76nM. When compared to standard injection (30-cm height for 10s), the approach allows the sensitivity enhancements over the range of 50-800 folds for the derivatives. The new approach has been applied to the analysis of a red wine sample, with great linearity of fluorescent intensity against concentrations (R(2)>0.98) and the RSD (three repetitive runs in one day) values of the migration times for the ten identified amino acids less than 2.8%.

The state of the art of dynamic coatings

The present review highlights the mechanisms of action and efficiency of three major classes of dynamic coatings so far adopted in capillary electrophoresis: (i) amines to oligo-amines, (ii) neutral synthetic and natural polymers, and (iii) neutral and zwitter-ionic surfactants. Their merits and efficacy have been explored in depth via a novel quantitation technique consisting of eluting, by frontal analysis, any adsorbed proteinaceous material, which can then be correctly quantified as a peak as it moves in front of the detector window. This is achieved by loading sodium dodecyl sulfate (SDS) micelles onto the cathodic side and migrating them electrophoretically into the capillary lumen, where they efficiently sweep any adsorbed polypeptide material. It is found that a common trend, for all quenchers, is linked to a hydrophobicity scale: the more hydrophobic the inhibitor, the better it minimizes potential interactions of macromolecules with the wall. This seems to be true for all the classes of dynamic modifiers tested. Finally, we describe a novel, dynamic to static quencher: it is a quaternary piperazine, bearing a reactive iodine atom at the end of a butyl tail (N(methyl-N-omega-iodo-butyl),N'-methyl piperazine). This molecule first binds to the wall, at alkaline pH values, via ionic and hydrogen bonds. Once docked onto the wall, the reactive tail forms a covalent link with the silica surface, to which it then remains permanently affixed.

Measuring the translational diffusion coefficients of small DNA molecules by capillary electrophoresis

The apparent translational diffusion coefficients of four 20 base pair (bp) DNA oligonucleotides with different sequences have been measured by capillary electrophoresis, using the stopped migration method. The diffusion coefficients of the four oligomers were equal within experimental error, and averaged (120 +/- 10) x 10(-8) cm(2) s(-1) in 40 mM Tris-acetate-EDTA buffer at 25 degrees C. Since this value is nearly identical to the translational diffusion coefficient determined for a different 20-bp oligomer using other methods, the stopped migration method can accurately measure the diffusion coefficients of small DNA oligomers. The apparent diffusion coefficient of a 118-bp DNA restriction fragment was also measured by the stopped migration method. However, the observed value was approximately 25% larger than expected from other measurements, possibly because the diffusion coefficients of larger DNA molecules are somewhat dependent on the ionic strength of the solution.

Quantitative studies on the adsorption of proteins to the bare silica wall in capillary electrophoresis. III: Effects of adsorbed surfactants on quenching the interaction

The efficacy of two classes of surfactants, non-ionic and zwitterionic, in quenching the interaction of proteins with the naked silica wall in capillary electrophoresis, is evaluated. The class of non-ionic detergents is found to be rather inefficient in preventing protein binding to the fused-silica surface, since large amounts (up to 10%) are required for reducing such interactions by 90%. Conversely, zwittergents appear to be much more efficient, since, in the case of sulphobetain SB-16, 90% binding inhibition is achieved at a concentration of surfactant of only 0.3%. In this last case, it is found that the binding inhibition closely follows the values of critical micellar concentrations (CMCs) of the various surfactants, those having the lowest CMC value exhibiting the highest inhibition power. The CMC values also follow a hydrophobicity scale, suggesting that the most hydrophobic zwittergents are the ones that shield more efficiently the silica surface.

Separation of proteins and peptides by capillary electrophoresis in acid buffers containing high concentrations of surfactants

Separations of proteins at acid pH in the presence of a high concentration of surfactant [sodium laurylsulfate (SDS), 50 mmol/l] was investigated. The purpose of using high concentrations of SDS as background electrolyte modifier was threefold: First, the surfactant exerts a washing effect upon the capillary wall thus preventing binding of analytes and possible clogging of the capillary. Second, it was revealed that even under very acid conditions (below pH 3) the surfactant is capable of forming associates with protein analytes which still bear considerable negative charge and can be separated on this basis. Third, the system can be applied not only for protein mixtures sufficiently soluble in neutral to alkaline media (leukocyte lysates, standard proteins), but it can be used also with proteins, that are under such conditions virtually insoluble and their solubilization is possible in acid buffers only (eggshell proteins or collagen CNBr fragments). The result was that adsorption to the capillary wall was minimized and the analytes were separated as negatively charged associates with high efficiency. With collagen fragments partition was possible on the affinity differences of the peptides to the surfactant micelles and inner wall of the capillary. Theoretical plate counts approaching 100,000 were easily achieved even with proteins which under the more conventional operation conditions exhibit considerable sticking to the capillary wall. The other feature of this system is that the associates move very rapidly to the anode. Owing to the low pH, endoosmotic flow is negligible, and therefore the system has to be operated at reversed polarity.

Capillary electrophoresis in presence of sodium dodecyl sulfate and a sieving medium

For analysis of proteins in presence of sodium dodecyl sulfate (SDS), basic knowledge about their behavior in presence and absence of sieving medium is required. Capillary electrophoresis (CE) is particularly suitable for this purpose. Although various polymers are used as sieves in CE analysis in presence of SDS, attention must be paid to possible interaction between SDS and the polymer. For obtaining new insights polymers with defined molecular weight and distribution should be utilized.

Behavior of peptides in capillary electrophoresis: effect of peptide charge, mass and structure

In the last decade the large potential of capillary electrophoresis as a technique for separation and characterization of peptides has been demonstrated extensively. In this field, a large number of chemical structures has to be taken into consideration, for which very often no data or even standards are available. As a result, there has been a strong desire to relate electrophoretic behavior to molecular properties and structure of the compounds. The activities in that direction, in the area of capillary zone electrophoresis, are critically reviewed. Special attention is paid to peptide charge, mass, hydrophobicity and structure, and their influence on the selectivity of the separation. Also, some complexation phenomena are discussed.

The free solution mobility of DNA

The free solution mobility of DNA has been measured by capillary electrophoresis in the two buffers most commonly used for DNA gel electrophoresis, Tris-borate-EDTA (TBE) and Tris-acetate-EDTA (TAE). The capillaries were coated with polymers of either of two novel acrylamide monomers, N-acryloylaminoethoxyethanol or N-acryloylaminopropanol, both of which are stable at basic pH and effectively eliminate the electroendosmotic mobility due to the capillary walls. The free solution mobility of DNA in TAE buffer was found to be (3.75 +/- 0.04) x 10(-4) cm2 V-1 s-1 at 25 degrees C, independent of DNA concentration, sample size, electric field strength, and capillary coating, and in good agreement with other values in the literature. The free solution mobility was independent of DNA molecular weight from approximately 400 base pairs to 48.5 kilobase pairs, but decreased monotonically with decreasing molecular weight for smaller fragments. Surprisingly, the free solution mobility of DNA in TBE buffer was found to be (4.5 +/- 0.1) x 10(-4) cm2 V-1 s-1, about 20% larger than observed in TAE buffer, presumably because of the formation of nonspecific borate-deoxyribose complexes.

Quantitative micellar chromatographic analysis of interaction between peptides and sodium dodecyl sulfate micelles

Interaction between short, water-soluble peptides and sodium dodecyl sulfate (SDS) micelles was analyzed by chromatography of the peptides on a Sephadex G-50 M el bed in an eluent containing the micelles. Peptides that interacted with the micelles showed intermediate elution volumes. In the absence of micelles in the eluent, the peptides (7-16 amino acid residues) were eluted near the total volume, whereas micelles applied as a sample appeared at the void volume. The association constants, KA, were calculated from the capacity factors k' obtained at different SDS micelle concentrations. The range of the KA values was (0.5-11) x 10(4) M-1. As a rule, the longest peptides and the positively charged ones interacted most strongly with the micelles, but the amino acid sequences also affected the interaction. Partially hydrophobic peptides that may correspond to interfacial segments of a transmembrane protein, the glucose transporter Glutl, showed relatively weak interactions with SDS micelles. The peptide interaction with sodium 1-decane sulfonate micelles supplemented with SDS was similar to that with SDS micelles, whereas none of several peptides tested interacted significantly with micelles of the non-ionic detergent n-dodecyl octaoxyethylene. The peptide-micelle interaction was proposed to occur mostly at the micelle surface as in the protein-decorated micelle structure for SDS-protein complexes.

A new mode of size-dependent separation of proteins by capillary electrophoresis in presence of sodium dodecyl sulfate and concentrated oligomeric dextran

Capillary electrophoresis in presence of sodium dodecyl sulfate (SDS) and 10% w/v of oligomeric dextran with molecular weight of just above 1000 was found to be an efficient medium for size-dependent separation of proteins (Karim, M. R., Janson, J.-C., Takagi, T., Electrophoresis 1994, 15, 1531-1534). The separations were found to be highly reproducible, both with respect to the position and relative amounts of the peaks. No evidence for a network structure in the medium could be found on examination of the effect of dextran concentration and viscosity measurements. The separation mechanism appears to be different from sieving by a polymer network structure. In free solution, SDS-protein complexes with molecular weights > 10,000 migrate with almost the same mobilities, irrespective of their molecular weight (this behavior is called free draining). Such SDS-complexes of proteins or peptides with molecular weights < 10,000, however, tend to increase their negatively signed mobilities with decreasing molecular weight (Karim, M. R., Shinagawa, S., Takagi, T., Electrophoresis 1994, 15, 1141-1146). Our working hypothesis is that in presence of the oligomeric dextran, the range of deviation from the free-draining electrophoretic behavior extends to higher molecular weights, resulting in a novel separation made.

Size-dependent separation of proteins in the presence of sodium dodecyl sulfate and dextran in capillary electrophoresis: effect of molecular weight of dextran

Dextran solutions are widely used as sieving medium in protein analysis by capillary electrophoresis in the presence of sodium dodecyl sulfate. We studied the effect of dextran molecular weight on the separation efficiency using different dextran preparations with wide and very narrow molecular weight distributions, in the range between 1270 and 2,000,000. Migration times and band broadening of proteins were significantly affected by the molecular weight of dextran. Migration times of proteins decreased as molecular weights of the dextrans decreased. Satisfactory separation of all the proteins was possible with all dextrans except those with molecular weights of 70,000 and 23,800 where bovine serum albumin and phosphorylase b failed to be separated. Unexpectedly rapid separation of all the proteins with enhanced resolution could be observed using two dextrans with a narrow molecular weight distribution, with molecular weights of 1270 and 5220. Clearly the use of dextran with higher molecular weight is not the only way to achieve efficient separation of proteins. The separation mechanism in the presence of the low molecular weight dextrans remains to be made clear in a future study.