Capillary Electrophoresis Separation of Small Peptides: Effect Of pH, Buffer Additives, and Temperature

Mean net charge of intrinsically disordered proteins: experimental determination of protein valence by electrophoretic mobility measurements

Under physiological conditions, intrinsically disordered proteins (IDPs) are unfolded, mainly because of their low hydrophobicity and the strong electrostatic repulsion between charged residues of the same sign within the protein. Softwares have been designed to facilitate the computation of the mean net charge of proteins (formally protein valence) from their amino acid sequences. Nevertheless, discrepancies between experimental and computed valence values for several proteins have been reported in the literature. Hence, experimental approaches are required to obtain accurate estimation of protein valence in solution. Moreover, ligand-induced disorder-to-order transition is involved in the folding of numerous IDPs. Some of the ligands are cations or anions, which, upon protein binding, decrease the mean net charge of the protein, favoring its folding via a charge reduction effect. An accurate determination of the mean net charge of protein in both its ligand-free intrinsically disordered state and in its folded, ligand-bound state allows one to estimate the number of ligands bound to the protein in the holo-state. Here, we describe an experimental protocol to determine the mean net charge of protein, from its electrophoretic mobility, its molecular mass and its hydrodynamic radius.

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.

Characterization and analysis of biphalin: an opioid peptide with a palindromic sequence

Among the many opioid peptides developed to date as nonaddictive analgesics, biphalin has exhibited extraordinary high potency and many other desirable characteristics. Biphalin is an octapeptide consisting of two monomers of a modified enkephalin, attached via a hydrazine bridge, and with the amino acids assembled in a palindromic sequence. Its structure is (Tyr-D-Ala-Gly-Phe-NH-)-2. However, this unique peptide, like any other synthetic peptide, needs strict quality control because of certain drawbacks associated with peptide synthesis. This paper discusses our approaches to characterizing and analyzing biphalin. Many techniques were used, including elemental analysis, amino acid analysis, amino acid sequence analysis (AASA), mass spectrometry (MS), 1H-NMR, 1H-correlated spectroscopy (COSY)-NMR, high-performance liquid chromatography (HPLC) and capillary electrophoresis (CE). Electrospray ionization (ESI) mass spectrometry, which included both ESI-MS and ESI-MS/MS, was performed to confirm the full sequence because AASA results alone verified only the monomer sequence, and not the full sequence. Although the 1H-NMR results led to a preliminary assignment of many protons, the 1H COSY-NMR results allowed for unequivocal assignment of almost all protons. Peptide purity was determined using two techniques, reversed-phase HPLC and CE. The counter-ion of the peptide, trifluoroacetic acid, was determined by CE, using an indirect detection method developed previously in our laboratory. This paper illustrates successful application of nonconventional techniques to characterize and analyze a structurally modified peptide, biphalin, when standard techniques for peptide analysis are inadequate.

Proceedings of the 10th Annual Frederick Conference on Capillary Electrophoresis. Frederick, Maryland, USA. October 18-20, 1999

Since the introduction of the first commercial capillary electrophoresis (CE) instrument a decade ago, CE applications have become widespread. Today, CE is a versatile analytical technique which is successfully used for the separation of small ions, neutral molecules, and large biomolecules and for the study of physicochemical parameters. It is being utilized in widely different fields, such as analytical chemistry, forensic chemistry, clinical chemistry, organic chemistry, natural products, pharmaceutical industry, chiral separations, molecular biology, and others. It is not only used as a separation technique but to answer physicochemical questions. In this review, we will discuss different modes of CE such as capillary zone electrophoresis, micellar electrokinetic chromatography, capillary gel electrophoresis, capillary isoelectric focusing, and capillary electrochromatography, and will comment on the future direction of CE, including array capillary electrophoresis and array microchip separations.

Separation of closely related peptide substrates of human proteinases by micellar electrokinetic chromatography with anionic and nonionic surfactants

In order to use micellar electrokinetic chromatography to determine the proteolytic activity of different proteinases simultaneously present in physiological fluids, the technique must be able to separate mixtures of substrates with closely related structures. In an attempt to determine the best electrophoretic conditions for resolving six p-nitroanilide peptides used as synthetic substrates of the elastolytic enzymes (human neutrophil elastase, cathepsin G, Pseudomonas aeruginosa elastase) most commonly involved in pulmonary diseases, we investigated the efficiency of ionic and nonionic surfactants in achieving the separation of this complex mixture. The results presented here show that, of all the electrophoretic systems tested, 30 mM sodium tetraborate, pH 9.3, containing 25 mM Brij 35 as micellar agent offered the best performance; the separation efficiency of peptides is greater than that obtained with other reagents and all peaks are baseline resolved and unambiguously identifiable. Analysis of the micelle-solute interaction with the surfactants investigated allowed better definition of the mechanism involved in the distribution of these peptides to the micelles and identification of some structural features that determined the magnitude of the micelle peptide complex formation.

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.

Micellar electrokinetic chromatography separations of dynorphin peptide analogs

Prodynorphin is a precursor that has multiple cleavage sites to release various dynorphin opioid peptides. The dynorphin analogs used in this study have 18 amino acid residues. A series of dynorphin-like peptides, differing by a single residue (alanine substitution) were assembled by Fmoc solid-phase procedures and purified by preparative high performance liquid chromatography (HPLC). Separation of the Ala-scan dynorphin analogs was investigated by micellar electrokinetic chromatography (MEKC) employing anionic, cationic and zwitterionic surfactants. The role of electrostatic and hydrophobic forces in analyte-surfactant interactions is discussed with respect to the observed elution patterns. Separation of all dynorphin analogs by MEKC using a zwitterionic surfactant shows this technique to be powerful for separating closely related peptide species. It also demonstrates the potential for using MEKC for the prescreening of peptide libraries to determine their biological activity toward specific receptors. Results from the separation of dynorphin analogs by free solution and ion-pairing capillary electrophoresis are also presented.

Development and validation of a capillary electrophoresis method for the characterization of protegrin IB-367

A capillary electrophoresis (CE) method was developed to characterize protegrin IB-367, an antimicrobial peptide being developed for the treatment of oral mucositis and for other topical applications. The electrophoretic purity and levels of potential impurities/degradation products of IB-367 drug substance are determined by CE using area normalization. Electrophoresis parameters were optimized to allow optimal resolution, reproducibility and minimal analysis time. The separation and resolution between this polycationic peptide and truncated analogs determined by the CE method was much greater than those by the HPLC methods. In addition, the CE methods separates the potential impurities/degradation products from each other while the HPLC methods failed to resolve them. The CE method was validated in the aspects of accuracy, precision, linearity, range, limit of detection, limit of quantitation, specificity, system suitability and robustness. An internal standard was used for the quantitation purpose. The selection criteria of the internal standard as well as the method validation results are presented. The truncated peptide analogs were used to demonstrate the specificity of the method. These analogs were also used to evaluate the limit of quantitation of potential impurities. The relative response factors of these analogs were assessed to determine area normalization feasibility. System suitability tests were established.

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.

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.

Influence of the inorganic counterion on the chiral micellar electrokinetic separation of basic drugs using the surfactant N-dodecoxycarbonylvaline

The chiral surfactant N-dodecoxycarbonylvaline (DDCV) has previously been used with a sodium counterion in micellar electrokinetic chromatography (MEKC) separations of beta-blockers (pindolol, atenolol, metoprolol, acebutolol, alprenolol, oxprenolol and propranolol), beta-agonists (N-methylpseudoephedrine, pseudoephedrine, ephedrine and norephedrine) and phenolic amino alcohols (norphenylephrine, synephrine, octopamine and salbutamol) in this laboratory. In the present study, we investigated the effects of three monovalent counterions--Li+, Na+ and K+--on the peak shape, efficiency, selectivity and retention of these 15 chiral pharmaceutical compounds with DDCV micelles. A much better (more symmetrical) peak shape was observed when Li+ was employed, due to a better if still imperfect match of analyte and counterion mobilities; average asymmetry factors in LiDDCV, NaDDCV, and KDDCV buffers were 1.9, 3.7, and 4.2, respectively. An increase in efficiency of 50-100% for hydrophobic solutes and over 100% for many hydrophilic solutes was also observed in LiDDCV compared to NaDDCV and KDDCV, probably due to enhanced mass transfer. The influence of an organic modifier (acetonitrile) in the separation buffer was also studied for the LiDDCV, NaDDCV and KDDCV systems. Although a concentration of either 0 or 5% acetonitrile resulted in the best enantiomeric resolution for most analytes, the most hydrophobic solutes (alprenolol and propranolol) required the addition of 20-25% ACN. Due to differences in the conductance of Li+, Na+ and K+, the advantage of using Li+ as the counterion for anionic DDCV micelles (better peak shape, higher efficiency and resolution, with shorter analysis times), although apparent for comparisons at a given applied voltage, is more significant when considered in the more appropriate context of equivalent Joule heating.

Self-consistent framework for standardising mobilities in free solution capillary electrophoresis: applications to oligoglycines and oligoalanines

A theoretical analysis of deviations from ideality in ionic transport is presented to correct mobilities, mu, measured in free solution capillary electrophoresis (CE) to mobility at infinite dilution, mu degree (limiting mobility). Non-ideality is treated at the same level of approximation as in equilibrium, using a correction factor for the sum of the analyte and counter-ion radius originally suggested by Robinson and Stokes (Electrolyte Solutions, 1961). Unlike previous corrections using Debye-Hückel-Onsager theory, which are strictly applicable only at very low ionic strengths, this treatment is expected to be valid for univalent ions migrating in a uni-univalent background electrolyte for ionic strengths up to 0.075 mol kg-1, a range typical of CE experiments. The analysis is applied to the determination of mu degree in acidic and basic buffers for oligoalanines and oligoglycines with degree of polymerisation 2 to 6. Limiting mobilities for the fully protonated and deprotonated peptides are found to be numerically equal but opposite in sign, consistent with a change in charge from +1 to -1. In all uni-univalent buffers studied (borate, citrate, low pH lithium phosphate and sodium phosphate) mu degree values established using data over a range of pH and ionic strength are found to be identical and in excellent agreement with previous values from isotachophoresis. Values of mu degree in high pH sodium phosphate buffer are systematically 0.2.10(-8) m2 V-1 s-1 higher than those in other buffers; this may be attributed to limitations of the model for a buffer with 1+:2- and 1+:3- ions. This self-consistent framework for standardising mobilities in free solution CE is expected to be widely applicable to univalent analytes migrating in a 1:1 background electrolyte.

Analysis of macromolecular branched chain polypeptides by capillary electrophoresis and micellar electrokinetic chromatography

Amphoteric poly(Lys-[Glu1.0-DL-Ala4.1]), (EAK) and anionic poly(Lys-Ac-Glu0.98-DL-Ala3.98]), (AcEAK) branched chain polypeptides were analyzed by capillary electrophoresis (CE) and micellar elektrokinetic chromatography (MEKC) in the following buffers. A1: 0.25 N triethyl ammonium phosphate (TEAP) buffer (pH 2.25); A2: 100 mM sodium dodecyl sulfate (SDS) in buffer A1; B1: Na-borate buffer (pH 7.7); B2: 100 mM SDS in buffer B1; C1: Na-borate buffer (pH 11.0); C2: 100 mM SDS in buffer C1. Both EAK and AcEAK could be separated by a CE mechanism at pH 2.25 and by an MEKC mechanism at pH 11.0. Optimum results were achieved with CE in buffer A1 and with MEKC in buffer C2.

Comparative analysis of somatostatin analog peptides by capillary electrophoresis and micellar elektrokinetic chromatography

Capillary electrophoresis (CE) and micellar electrokinetic chromatography (MEKC) methods, utilizing uncoated silica capillary and triethyl ammonium phosphate or sodium borate buffers in the pH range of 2.25-11.0, containing sodium dodecyl sulfate (SDS) (0-100 mM) for analysis of somatostatin-analog peptides were developed. The method presented here was compared with the reversed-phase high performance liquid chromatographic (RP-HPLC) and CE methods developed for analysis of peptides. The peptides investigated in this work can be separated by CE on the basis of their electrophoretic mobility in aqueous buffer of low pH value (pH 2.25) or by MEKC on the basis of their hydrophobicity in SDS containing buffer of high pH value (pH 11.0). Optimal MEKC separation of the investigated peptides has been achieved at pH 11.0 in an Na-borate buffer containing 100 mM SDS. CE at pH 2.25 proved insensitive to the hydrophobicity of the peptides investigated. By contrast, results obtained with MEKC at pH 11.0 proved to be anologous to those obtained by RP-HPLC, with highly hydrophobic peptides-migrating slower than peptides without hydrophobic moieties.

Use of capillary zone electrophoresis in an investigation of peptide uptake by dairy starter bacteria

A capillary zone electrophoresis (CZE) method to separate the peptide series val-glyn, where n is 1 to 4, has been evaluated and compared to separation by reversed-phase high-performance liquid chromatography (RP-HPLC). The method was able to quantitate peptides present a very low concentrations (down to 0.05 mM) with high reproducibility and accuracy and was capable of separating peptides differing in size by only a single glycine residue. It could also separate the peptides val-gly and leu-gly which differed in only a single side-chain methylene group. The method was fast, required small sample volumes, and proved to be superior to RP-HPLC. The suitability of the CZE method to analyze peptide uptake by dairy starter bacteria is discussed.

Effect of pH and ionic strength of running buffer on peptide behavior in capillary electrophoresis: theoretical calculation and experimental evaluation

The effect of pH and ionic strength of running buffer on peptide behavior in capillary electrophoresis (CE) is studied. A system for predictions of peptide migration in CE (SPPMCE) developed in our laboratory has been tested in a wide range of pH and buffer concentrations. The SPPMCE consists of a computer program for calculating peptide pKa values, an equation which relates peptide structures to their electrophoretic mobilities and a coupled computer program for the prediction of electropherograms. More than 25 different buffers have been employed, covering a pH range of 2-11 and a concentration range of 5-100 mM. Results from experiments are compared with the theoretical predictions. Good agreement is observed, which confirms the utility of the SPPMCE and allows fast and easy optimization of peptide separations in CE, with nothing more than the amino acid sequence of the linear peptide as the input.