Electrophoretic mobility modeling of proteins in free zone capillary electrophoresis and its application to monoclonal antibody microheterogeneity analysis

Determination of physicochemical parameters of (bio)molecules and (bio)particles by capillary electromigration methods

The review provides an overview of recent developments and applications of capillary electromigration (CE) methods for the determination of important physicochemical parameters of various (bio)molecules and (bio)particles. These parameters include actual and limiting (absolute) ionic mobilities, effective electrophoretic mobilities, effective charges, isoelectric points, electrokinetic potentials, hydrodynamic radii, diffusion coefficients, relative molecular masses, acidity (ionization) constants, binding constants and stoichiometry of (bio)molecular complexes, changes of Gibbs free energy, enthalpy and entropy and rate constants of chemical reactions and interactions, retention factors and partition and distribution coefficients. For the determination of these parameters, the following CE methods are employed: zone electrophoresis in a free solution or in sieving media, isotachophoresis, isoelectric focusing, affinity electrophoresis, electrokinetic chromatography, and electrochromatography. In the individual sections, the procedures for the determination of the above parameters by the particular CE methods are described.

Combining Scattering Experiments and Colloid Theory to Characterize Charge Effects in Concentrated Antibody Solutions

Charges and their contribution to protein-protein interactions are essential for the key structural and dynamic properties of monoclonal antibody (mAb) solutions. In fact, they influence the apparent molecular weight, the static structure factor, the collective diffusion coefficient, or the relative viscosity, and their concentration dependence. Further, charges play an important role in the colloidal stability of mAbs. There exist standard experimental tools to characterize mAb net charges, such as the measurement of the electrophoretic mobility, the second virial coefficient, or the diffusion interaction parameter. However, the resulting values are difficult to directly relate to the actual overall net charge of the antibody and to theoretical predictions based on its known molecular structure. Here, we report the results of a systematic investigation of the solution properties of a charged IgG1 mAb as a function of concentration and ionic strength using a combination of electrophoretic measurements, static and dynamic light scattering, small-angle X-ray scattering, and tracer particle-based microrheology. We analyze and interpret the experimental results using established colloid theory and coarse-grained computer simulations. We discuss the potential and limits of colloidal models for the description of the interaction effects of charged mAbs, in particular pointing out the importance of incorporating shape and charge anisotropy when attempting to predict structural and dynamic solution properties at high concentrations.

Modeling charge separation in charged nanochannels for single-molecule electrometry

We model the transport of electrically charged solute molecules by a laminar flow within a nanoslit microfluidic channel with electrostatic surface potential. We derive the governing convection–diffusion equation, solve it numerically, and compare it with a Taylor–Aris-like approximation, which gives excellent results for small Péclet numbers. We discuss our results in light of designing an assay that can measure simultaneously the hydrodynamic size and electric charge of single molecules by tracking their motion in such nanoslit channels with electrostatic surface potential.

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.

Predicting Electrophoretic Mobility of Proteoforms for Large-Scale Top-Down Proteomics

Large-scale top-down proteomics characterizes proteoforms in cells globally with high confidence and high throughput using reversed-phase liquid chromatography (RPLC)-tandem mass spectrometry (MS/MS) or capillary zone electrophoresis (CZE)-MS/MS. The false discovery rate (FDR) from the target-decoy database search is typically deployed to filter identified proteoforms to ensure high-confidence identifications (IDs). It has been demonstrated that the FDRs in top-down proteomics can be drastically underestimated. An alternative approach to the FDR can be useful for further evaluating the confidence of proteoform IDs after the database search. We argue that predicting retention/migration time of proteoforms from the RPLC/CZE separation accurately and comparing their predicted and experimental separation time could be a useful and practical approach. Based on our knowledge, there is still no report in the literature about predicting separation time of proteoforms using large top-down proteomics data sets. In this pilot study, for the first time, we evaluated various semiempirical models for predicting proteoforms' electrophoretic mobility (μef) using large-scale top-down proteomics data sets from CZE-MS/MS. We achieved a linear correlation between experimental and predicted μef of E. coli proteoforms (R2 = 0.98) with a simple semiempirical model, which utilizes the number of charges and molecular mass of each proteoform as the parameters. Our modeling data suggest that the complete unfolding of proteoforms during CZE separation benefits the prediction of their μef. Our results also indicate that N-terminal acetylation and phosphorylation both decrease the proteoforms' charge by roughly one charge unit.

A Semiempirical Approach for a Rapid Comprehensive Evaluation of the Electrophoretic Behaviors of Small Molecules in Free Zone Electrophoresis

A phenomenological model is proposed for the evaluation of relative electrophoretic migration of charged substances present in mixtures and for the rapid pH optimization prior CZE method development. The simple and robust model is based on the Offord model that takes account of the chemical structure. The effective charge and the molecular mass of the molecule are needed; the charge can easily be calculated from pK a obtained from known sources or simulated with existing pK-calculation programs. A first example was chosen with the separation of hydroxy-s-triazines to illustrate the applicability of this simple approach for determination of the first buffer-pH conditions prior experimental method optimization when separation of different ions is needed. In a second example, the confirmation of aminoalcohols in the CZE method development of unsaturated hexahydro-triazines and oxasolidines.

A semi-empirical approach for a rapid comprehensive evaluation of the electrophoretic behaviors of small molecules in free-zone electrophoresis

A phenomenological model is proposed for the evaluation of relative electrophoretic migration of charged substances present in mixtures and for the rapid pH optimization prior to capillary zone electrophoresis method development. The simple and robust model is based on the Offord model, which takes account of the chemical structure. The effective charge and the molecular mass of the molecule are needed; the charge can easily be calculated from pKa obtained from known sources or simulated with existing pK-calculation programs. A first example was chosen with the separation of hydroxy-s-triazines to illustrate the applicability of this simple approach for determination of the first buffer-pH conditions prior experimental method optimization when separation of different ions is needed. In a second example, the confirmation of aminialcohols in the CZE method development of unsaturated hexahydro-triazines and oxasolidines.

Neural networks in analytical chemistry

This chapter covers a part of the spectrum of neural-network uses in analytical chemistry. Different architectures of neural networks are described briefly. The chapter focuses on the development of three-layer artificial neural network for modeling the anti-HIV activity of the HETP derivatives and activity parameters (pIC50) of heparanase inhibitors. The use of a genetic algorithm-kernel partial least squares algorithm combined with an artificial neural network (GA-KPLS-ANN) is described for predicting the activities of a series of aromatic sulfonamides. The retention behavior of terpenes and volatile organic compounds and predicting the response surface of different detection systems are presented as typical applications of ANNs in chromatographic area. The use of ANNs is explored in electrophoresis with emphasizes on its application on peptide mapping. Simulation of the electropherogram of glucagons and horse cytochrome C is described as peptide models. This chapter also focuses on discussing the role of ANNs in the simulation of mass and 13C-NMR spectra for noncyclic alkenes and alkanes and lignin and xanthones, respectively.

Modeling the electrophoresis of peptides and proteins: improvements in the "bead method" to include ion relaxation and "finite size effects"

A bead model methodology developed in our lab (Xin et al. J. Phys. Chem. B 2006, 110, 1038) and applicable to modeling the free solution electrophoretic mobility of peptides and proteins is generalized in two significant ways. First, an approximate account is taken of the relaxation effect, which makes the methodology applicable to more highly charged peptides and proteins than was previously possible. Second, a more accurate account is taken of the finite size of the beads making up the model structure. This improvement makes the method applicable at higher salt concentrations and/or to models consisting of larger sized subunits. The relaxation effect is accounted for by correcting "unrelaxed" mobilities on the basis of model size and average electrostatic surface, or zeta potential. Correction factors are estimated using those of spheres with the same hydrodynamic radius and zeta potential as the model structure. The correction factors of spheres are readily determined. The more general methodology is first applied to two sets of peptides (74 different peptides total) varying in size from 2 to 42 amino acids. The sets also cover a wide range of net charges. It is shown that accounting for finite bead size results in a small change in model mobilities under the conditions of the experiments (35 mM monovalent salt). The correction for ion relaxation, however, can be significant for highly charged peptides and improves agreement between model and experimental mobilities. Our correction procedure is also tested by examining the electrophoretic mobility of a particular protein "charge ladder" (Carbeck et al. J. Am. Chem. Soc. 1999, 121, 10,671), where the protein charge is varied over a wide range yet the conformation remains essentially constant. In summary, the effects of ion relaxation can be significant if the absolute electrophoretic mobility of a peptide exceeds approximately 0.20 cm2/(kV s).

Modeling the electrophoretic mobility and diffusion of weakly charged peptides

A bead model to determine the electrophoretic mobilities and translational diffusion constants of weakly charged peptides is developed that is based on a approximate structural model of peptides and is also grounded in electrohydrodynamic theory. A peptide made up of X amino acids is modeled as N=2X beads with 2 beads representing each amino acid in the chain. For the two beads representing a particular amino acid in a peptide, the radius of one bead is set to one-half the nearest neighbor Calpha-Calpha distance, and the radius of the other bead is chosen on the basis of the diffusion constant of the free amino acid. Peptide conformations, which are defined by a set of psi-phi dihedral angles, are randomly generated by using the transformation matrix approach of Flory (Flory, P. Statistical Mechanics of Chain Molecules; John Wiley: New York, 1969) and rejecting conformations which result in bead overlap. The mobility and diffusion constants are computed for each conformation and at least 100 independent conformations are examined for each peptide. In general, the mobility is found to depend only weakly on peptide conformation. Model and experimental mobilities are compared by examining the data of Janini and co-workers (Janini, G.; et al. J. Chromatogr. 1999, 848, 417-433). A total of 58 peptides consisting of from 2 to 39 amino acids are considered. The average relative error between experimental and model mobilities is found to be 1.0% and the rms relative error 7.7%. In specific cases, the discrepancy can be substantial and possible reasons for this are discussed. It should be emphasized that the input parameters of the peptide model are totally independent of experimental mobilities. It is hoped that the peptide model developed here will be useful in the prediction of peptide mobility as well as in using peptide mobilities to extract information about peptide structure, conformation, and charge. Finally, we show how simultaneous measurements of translational diffusion and mobility can be used to estimate peptide charge.

Artificial neural network modeling of peptide mobility and peptide mapping in capillary zone electrophoresis

Recently, we have developed an artificial neural network model, which was able to predict accurately the electrophoretic mobilities of relatively small peptides. To examine the robustness of this methodology, a 3-3-1 back-propagation artificial neural network (BP-ANN) model was developed using the same inputs as the previous model, which were the Offord's charge over mass term (Q/M(2/3)), corrected steric substituent constant (E(s,c)) and molar refractivity (MR). The data set consisted of 102 peptides with a larger range of size than that of our earlier report - up to 42 amino acid residues as compared to 13 amino acids in the initial study - that also included highly charged and hydrophobic peptides. The entire data set was obtained from the published result by Janini and co-workers. The results of this model are compared with those obtained using multiple linear regressions (MLR) model developed in this work and the multi-variable model released by Janini et al. Better predictive ability of the BP-ANN model over the MLR indicates the non-linear characteristics of the electrophoretic mobility of peptides. The present model exhibits better robustness than the MLR models in predicting CZE mobilities of a diverse data set at different experimental conditions. To explore the utility of the ANN model in simulation of the CZE peptide maps, the profiles for the endoproteinase digests of melittin, glucagon and horse cytochrome C is studied in the present work.

Determination of Homopolypeptide Conformational Changes by the Modeling of Electrophoretic Mobilities

This work deals with the modeling of electrophoretic mobilities of end-charged homopolypeptides. The ionic mobilities of six families of homopolypeptides (polyglycines, poly-L-alanines, poly-L-valines, poly-L-leucines, poly-L-isoleucines, and poly-L-phenylalanines), with polymerization degrees up to 11, have been carefully determined. The electrophoretic frictional coefficients derived from the ionic mobility values were modeled by the hydrodynamic frictional coefficient of an equivalent cylinder. The hydrodynamic modeling allowed the determination of the molecular dimensions of the homopolypeptides in the electrolyte. The results were in good accordance with the expected geometry of the molecules. This approach allows monitoring the change in peptide conformations as a function of the experimental conditions (temperature, nature of the solvent) through the determination of geometrical molecular parameters (total peptide length, lateral radius of the equivalent cylinder, and folding angle). The influence of the bulkiness of the homopolypeptide lateral chain on the conformation is also discussed.

Prediction of electrophoretic mobilities of peptides in capillary zone electrophoresis by quantitative structure-mobility relationships using the offord model and artificial neural networks

The aim of this work was to explore the usefulness of empirical models and multivariate analysis techniques in predicting electrophoretic mobilities of small peptides in capillary zone electrophoresis (CZE). The data set consists of electrophoretic mobilities, measured at pH 2.5, for 125 peptides ranging in size between 2 and 14 amino acids. Among the existing empirical models, the Offord model (i.e., mu identical with Q/M(2/3)) gave the best correlation for the data set. A quantitative structure-mobility relationship (QSMR) was developed using the Offord's charge-over-mass term (Q/M(2/3)) as one descriptor combined with the corrected steric substituent constant (E(s, c)) and molar refractivity (MR) descriptors to account for the steric effects and bulkiness of the amino acid side chains. The multilinear regression (MLR) of the data set showed an improvement in the predictive ability of the model over the simple Offord's relationship. A 3-4-1 back propagation artificial neural networks (BP-ANN) model resulted in a significant improvement in the predictive ability of the QSMR over the MLR treatment, especially for peptides of higher charges that contain basic amino acids arginine, histidine, and lysine. The improved correlations by the BP-ANN analysis suggest the existence of nonlinear characteristic in the mobility-charge relationships.

Physicochemical Characterization of the Strychnos Alkaloids by Capillary Zone Electrophoresis

A capillary zone electrophoresis (CZE) method has been developed for investigating the physicochemical characteristics of five Strychnos alkaloids in Strychnos nux-vomica L. Firstly, the dissociation constants of the five Strychnos alkaloids were determined, based on the relation between the effective mobility of the solutes and the buffer pH. The mathematical relationship was strictly deduced from the fundamental electrophoretic theory and the dissociation equilibrium. Secondly, an equation describing the relation between the migration time of alkaloids of similar structure and their molecular weights was developed and used to predict the migration order and to calculate the electrosomotic velocity. The results predicted by the theory agreed with those from experiments.

Analysis of calcitonin and its analogues by capillary zone electrophoresis and matrix-assisted laser-desorption ionization time-of-flight mass spectrometry

Capillary zone electrophoretic (CZE) separations and mass spectrometric analysis of salmon calcitonin and related analogues were performed to generate electrophoresis and mass fingerprints for quality control of the recombinant polypeptide pharmaceutical salmon calcitonin. The calcitonins and their corresponding tryptic digests were successfully separated by CZE at low pH in fused silica capillaries dynamically modified with poly-cationic polymers. The poly-cationic modified inner surface of the fused silica capillaries generated a strong anionic electroosmotic flow (EOF). Analytes of negative, neutral, and positive charge were all swept through the capillary toward the positive electrode. Compared to Polybrene-coated capillaries, capillaries coated with PEI showed a markedly slower but much more stable electroosmotic flow. The migration order of the analytes was predicted by comparing approximate values of the charge to (molecular mass)2/3 ratios. The predicted migration order was confirmed by off-line analysis of CZE fractions with matrix-assisted laser-desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS).

Prediction of the electrophoretic mobilities of some carboxylic acids from theoretically derived descriptors

A 4-4-1 artificial neural network was constructed and trained for the prediction of the electrophoretic mobilities of some aliphatic and aromatic carboxylic acids based on quantitative structure-property relationships. The inputs of this network are theoretically derived descriptors that were chosen by the stepwise variables selection techniques. These descriptors are: shape factor, molecular surface area, the maximum value of electron density on atom in molecule, and the sum of atomic polarizability. In order to assess the accuracy and predictability of the proposed model, the cross-validation test was employed. The results obtained showed the ability of developed artificial neural network to prediction of electrophoretic mobilities of aliphatic and carboxylic acids. Also result reveals the superiority of the artificial neural network over the multiple linear regression models.

Applicability of predictive models to the peptide mobility analysis by capillary electrophoresis–electrospray mass spectrometry

The prediction of peptide mobility by capillary electrophoresis (CE) coupled to electrospray mass spectrometry (MS) is studied in order to verify the validity of the semi-empirical models developed in classical CE. This work relies on the experimental determination of the electrophoretic mobilities of 68 peptides, different in charge and in size. The results indicate that the prediction is possible in CE-MS experiments, in spite of the restraints inherent in the coupling conditions. The best fit of experimental data was obtained with the Offord's model. The efficiency of the model was confirmed by the analysis of a peptide mixture in CE-MS.

Classical approach to interpretation of the charge-dependence of peptide mobilities obtained by capillary zone electrophoresis

Published mobility measurements obtained by capillary zone electrophoresis of human growth hormone peptides are described reasonably well by the classical theoretical relationships for electrophoretic migration. This conformity between theory and experiment has rendered possible a more critical assessment of a commonly employed empirical relationship between mobility (u), net charge (z) and molecular mass (M) of peptides in capillary electrophoresis. The assumed linear dependence between u and z/M(2/3) is shown to be an approximate description of a shallow curvilinear dependence convex to the abscissa. An improved procedure for the calculation of peptide charge (valence) is also described.

Investigation of the chemico-physical characteristics of the active components in the Chinese herb Gastrodia elata Bl. by capillary zone electrophoresis

A CE method for investigating the chemico-physical characteristics of the active components of low molecular weight in Gastrodia elata Bl. is described. First, the dissociation constants of five active components were determined based on the relation between the effective mobility of the solutes and the buffer pH value. Second, an equation that describes the relation of the migration time and the molecular weight was developed and used to predict the migration order and to calculate the electroosmotic velocity. The results predicted by theory agreed well with that from experiments.

Capillary electrophoretic analysis of proteins and peptides of biomedical and pharmacological interest

Capillary electrophoresis (CE) is an automated approach to electrokinetic separations that has had a deep impact in all fields of life sciences, including biomedical and biotechnological research and clinical and forensic practice. The present review highlights aspects of peptides and proteins separations, with particular emphasis on macromolecular analytes of biomedical interest. Among the various CE techniques available, a novel methodology is here illustrated consisting in separations in acidic, isoelectric buffers, which have the advantage of protonating the silica wall, thus minimizing interactions of proteinaceous material with the siliceous surface, while allowing delivery of high voltage gradients, due to their low conductivities. The review ends with applications of CE to the analysis of folding/unfolding/refolding/misfolding of proteins, a field which has deep implications in the biomedical arena, since it is connected to a host of disorders, such as prion protein diseases.

Peptide mapping by capillary zone electrophoresis: how close is theoretical simulation to experimental determination

A multi-variable computer model is presented for the prediction of the electrophoretic mobilities of peptides at pH 2.5 from known physico-chemical constants of their amino acid residues. The model is empirical and does not claim any theoretical dependencies; however, the results suggest that, at least at this pH, peptides may be theoretically represented as classical polymers of freely joined amino acid residues of unequal sizes. The model assumes that the electrophoretic mobility can be represented by a product of three functions that return the contributions of peptide charge, length and width, respectively to the mobility. The model relies on accurate experimental determination of the electrophoretic mobilities of a diverse set of peptides, by capillary zone electrophoresis (CZE), at 22 degrees C, with a 50 mM phosphate buffer, at pH 2.5. The electrophoretic mobilities of a basis set of 102 peptides that varied in charge from 0.65 to 16 and in size from two to 42 amino acid residues were accurately measured at these fixed experimental conditions using a stable 10% linear polyacrylamide-coated column. Data from this basis set was used to derive the peptide charge, length, and width functions respectively. The main purpose of this endeavor is to use the model for the prediction of peptide mobilities at pH 2.5, and for simulation of CZE peptide maps of protein digests. Excellent agreement was obtained between predicted and experimental electrophoretic mobilities for all categories of peptides, including the highly charged and the hydrophobic. To illustrate the utility of this model in protein studies it was used to simulate theoretical peptide maps of the digests of glucagon and horse cytochrome c. The resulting maps were compared and contrasted with their experimental counterparts. The potential of this approach and its limitations are discussed.

From small charged molecules to oligomers: a semiempirical approach to the modeling of actual mobility in free solution

According to Stokes' treatment, the ionic mobility of particles, which are small with respect to Debye length, is usually considered to be proportional to the nominal charge and inversely proportional to the hydrodynamic radius. Experimentally, it is well known, however, that the ionic mobility of a small multicharged molecule does not depend linearly on its nominal charge in a wide range. This behavior can be accounted for by a condensation of the charge or a modification of the friction coefficient with the charge. This paper presents a semiempirical modeling of the actual mobility based on the assumption of additivity of frictional contributions pertaining to the uncharged molecular backbone and to each charged or uncharged moiety. Condensation of the charge was not considered. The model first appeared to be suitable for multicharged analytes having a characteristic dimension smaller than the Debye length, such as benzene polycarboxylic acids and polysulfated disaccharides. This approach was then adapted to account for the actual mobilities of singly and evenly charged oligomers (N-mers) having a dimension smaller than or similar to the Debye length. Rather good experimental agreement was obtained for polyalanines and polyglycines (N < or = 6), fatty acid homologs, fully sulfonated polystyrene oligomers (N < or = 13), and polycytidines (N < or = 10). Especially the influence of the polymerization degree on the mobility of oligomers having identical charge densities was clarified. It is also shown that the electrophoretic contribution to the overall friction coefficient increases linearly with the nominal charge but hardly depends on the chemical nature of the charged moieties. This model should be of interest to evaluate the role of various physicochemical phenomena (hydrodynamic and electrophoretic frictions, hydrodynamic coupling, charge condensation) involved in the migration of charged oligomers.

Capillary electrophoresis of peptides

This article gives a review of the recent developments in capillary electrophoresis (CE) of peptides. New approaches to the theoretical description of electromigration behavior of peptides are described, and methodological aspects of CE separations of peptides such as selection of separation conditions, sample treatment, suppression of peptide adsorption to the capillary wall and specificities of CE separation modes are discussed. Progress in application of high performance detection schemes, namely laser-induced fluorescence and mass spectrometry, in peptide separations by CE is presented. Applications of different CE techniques, zone electrophoresis, isotachophoresis, isoelectric focusing, affinity electrophoresis, electrokinetic chromatography and electrochromatography to peptide analysis, preparation and physicochemical characterization are demonstrated.

Peptide mobility and peptide mapping in capillary zone electrophoresis. Experimental determination and theoretical simulation

The electrophoretic mobilities of 58 peptides that varied in size from 2 to 39 amino acids and varied in charge from 0.65 to 7.82 are presented. The measurements were conducted at 22 degrees C using a 10% linear polyacrylamide-coated column and a 50 mM phosphate buffer at pH 2.5. Excellent separation of peptides and highly reliable peptide maps of protein digests are routinely obtained using these experimental conditions. The electrophoretic data were used to test existing theoretical models that correlate electrophoretic mobility with physical parameters. The results indicate that the Offord model that correlates electrophoretic mobility with the charge-to-size parameter q/M2/3 offers the best fit of our reliable experimental data. Furthermore, we also obtained the capillary zone electrophoretic profile of the endoproteinase Lys-C digests of a peptide sequencing standard, melittin, and horse myoglobin under the same experimental conditions as described above. The resulting peptide maps were compared with corresponding theoretical simulation.

Characterization of latex allergenic components by capillary zone electrophoresis and N-terminal sequence analysis

In a previous study, protein components purified from latex gloves that elicited allergenic reactions were characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and yielded apparent molecular weights of 14, 22, 30, 34, 46, and 58 kD. These allergenic components were isolated for further characterization by capillary zone electrophoresis and N-terminal amino acid sequence analysis. These components all migrated at approximately 25 and 35 min on capillary zone electrophoresis. Diode array spectral analysis detected indistinguishable characteristics between these two protein peaks. In addition, complex formation of these components with patients' immunoglobulin was demonstrated by capillary zone electrophoresis. Analysis of components separated by SDS-PAGE on a polyvinylidene difluoride membrane showed that the first 13 residues were identical to the sequence of hevein. Based on the criteria of charge-to-mass ratio and N-terminal amino acid sequence, our results suggest that these components of latex proteins are similar in the primary structure.

Strategies for capillary electrophoresis: method development and validation for pharmaceutical and biological applications

This review is in support of the development of selective, reproducible and validated capillary electrophoretis (CE) methods. Focusing on pharmaceutical and biological applications, the successful use of CE is demonstrated by more than 800 references, mainly from 1994 until 1998. Approximately 80 recent reviews have been catalogued. These articles sum up the existing strategies for method development in CE, especially in the search for generally accepted concepts, but also looking for new, promising reagents and ideas. General strategies for method development were derived not only with regard to selectivity and efficiency, but also with regard to precision, short analysis time, limit of detection, sample pretreatment requirements and validation. Standard buffer recipes, surfactants used in micellar electrokinetic capillary chromatography (MEKC), chiral selectors, useful buffer additives, polymeric separation media, electroosmotic flow (EOF) modifiers, dynamic and permanent coatings, actions to deal with complex matrices and aspects of validation are collected in 20 tables. Detailed schemes for the development of MEKC methods and chiral separations, for optimizing separation efficiency, means of troubleshooting, and other important information for key decisions during method development are given in 19 diagrams. Method development for peptide and protein separations, possibilities to influence the EOF and how to stabilize it, as well as indirect detection are considered in special sections.

Capillary electrophoresis separation of an asparagine containing hexapeptide and its deamidation products

A capillary electrophoresis (CE) method was developed for the separation of a model hexapeptide (L-Val-L-Tyr-L-Pro-L-Asn-Gly-L-Ala) and its degradation products. Separations using CE had much shorter analysis times than the RP-HPLC assay used previously. CE was also evaluated for possible use in analyzing peptide samples containing polymers. With a thorough rinsing procedure between runs, the presence of a model polymer did not affect the separation of the peptides. The CE assay can be applied to the study of peptide stability in polymers.

Separation and quantitation of monoclonal antibodies in cell growth medium using capillary zone electrophoresis

IgG1 is separated from its impurities in cell growth medium under simple CZE conditions without specific sample pretreatment. Linearity, limit of quantitation, limit of detection, precision and accuracy for the method are demonstrated. The quantitation for IgG1 in the cell growth medium is obtained by generating a calibration curve and by using standard additions. This CE method can offer a good alternative to conventional HPLC methods. Attempts are also made to separate the heterogeneous species in monoclonal antibodies using both CZE and MECC.

HPCE methods for the identification and quantitation of antibodies, their conjugates and complexes

We review here much of the existing literature that deals with analysis, resolution, characterization, and (at times) quantitation of antibodies in capillary electrophoresis modes. Each major mode of CE shown applicable to antibody analysis is described, along with the major applications of that mode for antibodies. Discussions are presented as to the mechanisms of antibody resolution in CE, interactions of various buffer components with the proteins leading to resolution, and methods of quantitation for antibodies. The literature is critically reviewed with regard to true application of CE for antibody analysis, limitations, information possible, information implied, and which samples have actually been assayed by CE modes. The literature is critically reviewed up to and including 1996, both for the scientific and commercial literature, especially vendor applications and real world applications possible.

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.

Capillary electrophoresis of biotechnology-derived proteins

Applications of capillary electrophoretic techniques for the analysis of biotechnology-derived proteins continue at a steady pace. This review summarizes the analyses of these proteins achieved using capillary isoelectric focusing, micellar electrokinetic capillary chromatography, capillary electrophoresis/mass spectrometry and capillary gel electrophoresis. A very brief summary of each technique is included. Application of capillary isoelectric focusing to the analysis of recombinant tissue plasminogen activator in quality control type applications is examined in some detail. Analyses of dosage forms of biotechnology derived proteins of pharmaceutical importance as well as the identification and examination of the peptides, obtained after enzymatic cleavage of proteins, by capillary electrophoresis have been included. Examples of capillary gel electrophoresis as a substitute sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) of biotechnology-derived proteins are also reviewed.

Capillary zone electrophoresis of proteins

This review article with 237 references is focused on capillary zone electrophoresis (CZE) of proteins. It includes discussion of modeling electrophoretic migration of proteins, sample pretreatment before the analysis, methods reducing the sorptions of proteins on the capillary wall, and techniques for increasing selectivity by using electrolyte additives including the sieving matrices. Significant progress in detection techniques, namely in laser-induced fluorescence and mass spectrometry, is emphasized. Modifications of CZE using specific interactions, such as affinity capillary electrophoresis or capillary immunoelectrophoresis, are debated as well as combination of CZE with other separation methods such as high performance liquid chromatography (HPLC). A number of practical applications of CZE of proteins are described.

Rules relating electrophoretic mobility, charge and molecular size of peptides and proteins

The absence of supporting media in free solution high-performance capillary electrophoresis (HPCE) makes it an ideal system for the study of the relationship between electrophoretic mobility (mu(em)) and the molecular size and charge of proteins and peptides. In this review, the theory of electrophoresis, developed for rigid, insulating, spherical particles, is modified to develop models for the electrophoretic behaviour of proteins and peptides. For a given set of experimental conditions, mu(em) of a protein/peptide is proportional to its charge (q) and is inversely proportional to its Stoke's radius (r). Furthermore, mu(em) is most sensitive to changes in q and, as a consequence, the reliability of equations relating mu(em) to protein/peptide q and r is dependent upon the accurate calculation or determination of q. For convenience, q and r of proteins and peptides are generally expressed in terms of calculated valence (Zc) and molecular mass (M), respectively, both of which can be determined from the amino acid sequence of the protein/peptide. However, the calculation of q using Zc is made more complex by the effects of electrostatic charge suppression, such that Zc is an overestimation of actual charge. Charge suppression becomes increasingly significant as the protein/peptide charge increases, such that, for peptides, the relationship between q and Zc can be approximated by a logarithmic function. The mu(em) for peptides, therefore, can be approximated by the equation: mu(em) = ln(Zc + 1)/K Ms where s varies between 1/3 and 2/3, and K is a constant that is valid for a particular set of experimental conditions. The rather simplistic compensation for charge suppression in this equation is inadequate for proteins where the magnitude of charge suppression is greater and the mechanisms are more complex. For proteins, the relationship suggested for the prediction of mu(em) from Zc and M is: mu(em) = Zc/KFzMs where s again varies between 1/3 and 2/3 and Fz is a pH-independent proportionality factor defined as the quotient, Zc/Za, with Za being actual protein valence. The factor Fz can be determined empirically, however, it is valid only for the particular set of experimental conditions under which it is determined. For peptides, the mass exponent, s, approaches 1/3 when the peptides have high charge densities and open structures. However, s approaches 1/2 for peptides with lower charge densities that are capable of more randomized motion during electrophoresis. Finally, s approaches 2/3 for proteins, suggesting that the frictional forces acting on a protein undergoing electrophoretic motion are proportional to the surface area of these larger, more rigid, structures. In conclusion, the development of relationships between mu(em), M and Zc for peptides and proteins offers a powerful tool, not only for predicting electrophoretic mobility, but also for optimising HPCE separations, studying structural modifications (e.g. phosphorylation, glycosylation, deamidation, etc.), and for the investigation of surface charge characteristics and conformation.

Peptide analysis by capillary (zone) electrophoresis

In this review various aspects concerning the application of capillary (zone) electrophoresis for peptide analysis will be critically examined. First, the basic instrumental requirements of CE apparatus and the strategies employed to enhance sensitivity in the analysis of underivatized sample are described. Multidimensional separative techniques of complex peptide mixtures that use CE as final step and the coupling of CE with mass spectrometry are subsequently discussed. A theoretical section describes the relationships existing between peptide mobility and the pH of the separation buffer. These relationships evidence that proton dissociation constants and Stokes radius at different protonation stages can be calculated by measuring the electrophoretic mobility at different pH values. Investigation of peptide mobility dependence on pH allows us to establish the optimum conditions, in terms of resolution, for peptide separation. Subsequently, a critical discussion about semiempirical models predicting peptide mobility as a function of chemico-physical peptide properties is presented. A section is devoted to the description of principles of peptide affinity capillary electrophoresis, underlining the similarity with peptide-proton interaction. CE separations performed in aquo-organic solvents are also critically discussed, showing the good performance obtained by using water-2,2,2-trifluoroethanol solutions. Finally, selected CE applications for the determination of peptide chemico-physical properties and conventional analysis, like peptide mapping, are reported.