Modelling migration behavior of peptide hormones in capillary electrophoresis-electrospray mass spectrometry

CE-ESI-MS for bottom-up proteomics: Advances in separation, interfacing and applications

With the development of more sensitive hyphenation strategies for capillary electrophoresis-electrospray-mass spectrometry the technique has reemerged as technique with high separation power combined with high sensitivity in the analysis of peptides and protein digests. This review will discuss the newly developed hyphenation strategies for CE-ESI-MS and their application in bottom-up proteomics as well as the applications in the same time span, 2009 to present, using co-axial sheathliquid. Subsequently all separate aspects in the development of a CE-ESI-MS method for bottom-up proteomics shall be discussed, highlighting certain applications and discussing pros and cons of the various choices. The separation of peptides in a capillary electrophoresis system is discussed including the great potential for modeling of this migration of peptides due to the simple electrophoretic separation process. Furthermore, the technical aspects of method development are discussed, namely; background electrolyte choice, coating of the separation capillary and chosen loading method. Finally, conclusions and an outlook on future developments in the field of bottom-up proteomics by CE-ESI-MS will be provided.

Modelling the electrophoretic migration behaviour of peptides and glycopeptides from glycoprotein digests in capillary electrophoresis-mass spectrometry

In this study, the classical semiempirical relationships between the electrophoretic mobility and the charge-to-mass ratio (me vs. q/M(α)) were used to model the migration behaviour of peptides and glycopeptides originated from the digestion of recombinant human erythropoietin (rhEPO), a biologically and therapeutically relevant glycoprotein. The Stoke's law (α=1/3), the classical polymer model (α=1/2) and the Offord's surface law (α=2/3) were evaluated to predict migration of peptides and glycopeptides, with and without sialic acids (SiA), in rhEPO digested with trypsin and trypsin-neuraminidase. The Stoke's law resulted in better correlations for the set of peptides used to evaluate the models, while glycopeptides fitted better with the classical polymer model. Once predicted migration times with both models, it was easy to simulate their separation electropherogram. Results were later validated predicting migration and simulating separation of a different set of rhEPO glycopeptides and also human transferrin (Tf) peptides and glycopeptides. The excellent agreement between the experimental and the simulated electropherograms with rhEPO and Tf digests confirmed the potential applicability of this simple strategy to predict, in general, the peptide-glycopeptide electrophoretic map of any digested glycoprotein.

Identification of bioactive peptides in hypoallergenic infant milk formulas by CE-TOF-MS assisted by semiempirical model of electromigration behavior

Biologically active peptides derived from complex bovine milk protein hydrolysates are of particular interest in food science and nutrition because they have been shown to play different physiological roles, providing benefits in human health. In this study, we used CE-TOF-MS for separation and identification of bioactive peptides in three hypoallergenic infant milk formulas. An appropriate sample cleanup using a citrate buffer with DTT and urea followed by SPE with Sep-Pack® C18 and StrataX™ cartridges allowed the detection of a large number of low molecular mass bioactive peptides. This preliminary identification was solely based on the measured experimental monoisotopic molecular mass values (M(exp)). Later, we evaluated the classical semiempirical relationships between electrophoretic mobility and charge-to-mass ratio (m(e) vs. q/M(α), α = 1/2 for the classical polymer model) to describe their migration behavior. The assistance of migration prediction proved to be useful to improve reliability of the identification, avoiding misinterpretations and solving some identity conflicts. After revision, the identity of 24, 30, and 38 bioactive peptides was confirmed in each of the three infant milk formulas. A significant number of these peptides were reported as inhibitors of angiotensin-converting enzyme, however, the presence of sequences with other biological activities such as antihypertensive, antithrombotic, hypocholesterolemic, immunomodulation, cytotoxicity, antioxidant, antimicrobial, antigenic, or opioid was also confirmed.

Determination of acidity constants and ionic mobilities of polyprotic peptide hormones by CZE

CZE has been applied to determination of thermodynamic acidity constants (pKa ) of ionogenic groups and actual ionic mobilities of polyprotic peptides-synthetic human and salmon gonadotropin-releasing hormones and their derivatives and fragments. First, the mixed acidity constants, pKa,imix, of ionogenic groups, and actual ionic mobilities, mi , of gonadotropin-releasing hormone peptides were determined by nonlinear regression analysis of pH dependence of their effective electrophoretic mobilities. The effective mobilities were measured by CZE in a series of BGEs within a broad pH range (1.80-12.10), at constant ionic strength (25 mM) and reference temperature (25°C). Second, the pKa,imix values were recalculated to thermodynamic pKa s using the Debye-Hückel theory. Thermodynamic pKa of carboxyl groups was estimated to be in the range of 2.5-3.3 for C-terminal amino acids of the above peptides, and 5.2 for glutamic acid in the middle of peptide chain; pKa of imidazolyl group of histidine residues was in the range of 5.7-6.8, pKa of N-terminal amino group of the peptide with free N-terminus was equal to 6.2, pKa of phenol group of tyrosine residues was in the range of 9.8-10.8, and pKa of guanidinyl group or arginine residues reached values 11.1-11.3, depending on the position of the residues in the peptide and on the amino acid sequence of the peptide. Absolute values of actual ionic mobilities of peptides with charge number ±2 were in the range (14.6-18.6) × 10(-9) m(2) V(-1) s(-1) , and ionic mobilities of peptides with charge number ±1 reached values (6.5-12.9) × 10(-9) m(2) V(-1) s(-1) .

Modeling the electrophoresis of oligolysines

CE is used to measure the electrophoretic mobility of low molecular mass oligo-L-lysines (n=1-8) in aqueous LiH₂PO₄ buffer, BGE, at pH 2.5 over a range of temperatures (25-50 °C) and ionic strengths (10-100 mM). Mobilities are corrected for Joule heating and under the conditions of the experiment, interaction of the peptides with the capillary walls can be ignored. A "coarse grained" bead modeling methodology (BMM) (H. Pei et al., J. Chromatogr. A 2009, 1216, 1908-1916) is used to model the mobilities. This model partially accounts for peptide conformation as well as the assumed form of its secondary structure. For highly charged oligolysines, it is necessary to properly account for the relaxation effect. In the present study, the BMM approach tends to overestimate oligolysine mobility and that effect tends to increase with increasing ionic strength and peptide length. It is proposed that association between the oligolysines and buffer components (H₂PO₄⁻ in this case) that go beyond classical electrostatic interactions are responsible for this discrepancy. A simple binding model is introduced that illustrates how this association can reconcile model and experiment.

Prediction of metabolite identity from accurate mass, migration time prediction and isotopic pattern information in CE-TOFMS data

CE-TOFMS is a powerful method for profiling charged metabolites. However, the limited availability of metabolite standards hinders the process of identifying compounds from detected features in CE-TOFMS data sets. To overcome this problem, we developed a method to identify unknown peaks based on the predicted migration time (t(m)) and accurate m/z values. We developed a predictive model using 375 standard cationic metabolites and support vector regression. The model yielded good correlations between the predicted and measured t(m) (R=0.952 and 0.905 using complete and cross-validation data sets, respectively). Using the trained model, we subsequently predicted the t(m) for 2938 metabolites available from the public databases and assigned tentative identities to noise-filtered features in human urine samples. While 38.9% of the peaks were assigned metabolite names by matching with the standard library alone, the proportion increased to 52.2%. The proposed methodology increases the value of metabolomic data sets obtained from CE-TOFMS profiling.

The design and synthesis of alanine-based indolicidin derivatives with identical physicochemical properties and their separation using capillary electrophoresis

Four novel alanine-based indolicidin peptide derivatives were designed containing one WPW motif and two alanine residues, resulting in peptides of similar sequence. The separation of these peptides with identical physicochemical properties including molar mass, charge, and secondary structure as characterized by circular dichroism spectroscopy is very difficult; and the separation of peptides with differing physicochemical properties has only previously been reported. Capillary electrophoresis parameters such as separation buffer concentration, separation buffer pH, capillary length, and separation voltage were investigated to optimize the analysis. Using optimized conditions of a background electrolyte containing 5 mM formic acid of pH 2.0, total capillary length of 51 cm and a voltage of 10 kV enabled a baseline separation of the four peptides. The relative standard deviation of the peak areas and migration times for method repeatability (n = 3) were found to be lower than 8% and 3%, respectively. In addition, reasoning for the separation of these peptides is proposed based on the acidity of the formic acid buffer and the hydrophobic grouping of the tryptophan residues in the peptide primary sequence.

Enhanced neuropeptide profiling via capillary electrophoresis off-line coupled with MALDI FTMS

An off-line interface incorporating sheathless flow and counter-flow balance is developed to couple capillary electrophoresis (CE) to matrix-assisted laser desorption ionization Fourier transform mass spectrometry (MALDI FTMS) for neuropeptide analysis of complex tissue samples. The new interface provides excellent performance due to the integration of three aspects: (1) A porous polymer joint constructed near the capillary outlet for the electrical circuit completion has simplified the CE interface by eliminating a coaxial sheath liquid and enables independent optimization of separation and deposition. (2) The electroosmotic flow at reversed polarity (negative) mode CE is balanced and reversed by a pressure-initiated capillary siphoning (PICS) phenomenon, which offers improved CE resolution and simultaneously generates a low flow (<100 nL/min) for fraction collection. (3) The predeposited nanoliter volume 2,5-dihydroxybenzoic acid (DHB) spots on a Parafilm-coated MALDI sample plate offers an improved substrate for effective effluent enrichment. Compared with direct MALDI MS analysis, CE separation followed by MALDI MS detection consumes nearly 10-fold less sample (50 nL) while exhibiting 5-10-fold enhancement in S/N ratio that yields the limit of detection down to 1.5 nM, or 75 attomoles. This improvement in sensitivity allows 230 peaks detected in crude extracts from only a few pooled neuronal tissues and increases the number of identified peptides from 19 to 43 (Cancer borealis pericardial organs (n = 4)) in a single analysis. In addition, via the characteristic migration behaviors in CE, some specific structural and chemical information of the neuropeptides such as post-translational modifications and family variations has been visualized, making the off-line CE-MALDI MS a promising strategy for enhanced neuropeptidomic profiling.

Hydration, charge, size, and shape characteristics of peptides from their CZE analyses

A CZE model is presented for peptide characterization on the basis of well-established physicochemical equations. The effective mobility is used as basic data in the model to estimate relevant peptide properties such as, for instance, hydration, net and total electrical charge numbers, hydrodynamic size and shape, particle average orientation, and pH-microenvironment from the charge regulation phenomenon. Therefore 102 experimental effective mobilities of different peptides are studied and discussed in relation to previous work. An equation for the estimation of peptide hydration as a function of ionizing, polar, and non-polar amino acid residues is included in the model. It is also shown that the shape-orientation factor of peptides may be either lower or higher than one, and its value depends on a complex interplay among total charge number, molar mass, hydration, and amino acid sequence.

Recent developments in CE and CEC of peptides

The article brings a comprehensive survey of recent developments and applications of high-performance capillary electromigration methods, zone electrophoresis, ITP, IEF, affinity electrophoresis, EKC, and electrochromatography, to analysis, preparation, and physicochemical characterization of peptides. New approaches to the theoretical description and experimental verification of electromigration behavior of peptides and to methodology of their separations, such as sample preparation, adsorption suppression, and detection, are presented. Novel developments in individual CE and CEC modes are shown and several types of their applications to peptide analysis are presented: conventional qualitative and quantitative analysis, purity control, determination in biomatrices, monitoring of chemical and enzymatical reactions and physical changes, amino acid and sequence analysis, and peptide mapping of proteins. Some examples of micropreparative peptide separations are given and capabilities of CE and CEC techniques to provide important physicochemical characteristics of peptides are demonstrated.

Separation and investigation of structure–mobility relationship of gonadotropin-releasing hormones by capillary zone electrophoresis in conventional and isoelectric acidic background electrolytes

Capillary zone electrophoresis (CZE) has been applied to qualitative and quantitative analysis, separation and physicochemical characterization of synthetic gonadotropin-releasing hormones (GnRHs) and their analogs and fragments. Structurally related peptides were separated in conventional and isoelectric acidic background electrolytes (BGEs), pH 2.18-2.50. Best separation was achieved in isoelectric BGE composed of 200 mM iminodiacetic acid, pH 2.32. The effective electrophoretic mobilities, m(ep), of GnRHs in five BGEs were determined and four semiempirical models correlating effective mobility with charge, q, and relative molecular mass, M(r), (m(ep) versus q/M(r)(k), where k is related to the molecular shape) were tested to describe the migration behavior of GnRHs in CZE. None of the models was found to be quite definitively applicable for the whole set of 10 GnRHs differing in size (tetrapeptide-decapeptide) and positive charge (0.91-3.00 elementary charges). Nevertheless, for the dependence of m(ep) on q/M(r)(k), the highest coefficient of correlation, R=0.995-0.999, was obtained for k close to the value 0.5 in all five acidic BGEs. This indicates that the most probable structure of GnRHs in these BGEs can be predicted as a random coil.

Analysis of nucleotides by pressure-assisted capillary electrophoresis-mass spectrometry using silanol mask technique

A method for the determination of nucleotides based on pressure-assisted capillary electrophoresis-electrospray ionization mass spectrometry (PACE-MS) is described. To prevent multi-phosphorylated species from adsorbing onto the fused-silica capillary, silanol groups were masked with phosphate ions by preconditioning the capillary with the background electrolyte containing phosphate. During preconditioning, nebulizer gas was turned off to avoid contamination of MS detector with phosphate ions. To detect nucleotides using the CE positive mode at a pH 7.5, it was necessary to apply air pressure to the inlet capillary during electrophoresis to supplement the electroosmotic flow (EOF) toward the cathode. Moreover, we exchanged the running electrolyte every analysis using the buffer replenishment system to obtain the required reproducibility. Under the optimized conditions, 14 phosphorylated species such as nucleotides, nicotinamide-adenine dinucleotides and coenzyme A (CoA) compounds were well determined in less than 20 min. The relative standard deviations (n=6) of the method were better than 0.9% for migration times and between 1.7% and 8.1% for peak areas. The detection limits for these species were between 0.5 and 1.7 micromol/L with pressure injection of 50 mbar for 30 s (30 nL) at a signal-to-noise ratio of 3. This approach is robust and quantitative compared to the previous method, and its utility is demonstrated by the analysis of intracellular nucleotides and CoA compounds extracted from Escherichia coli wild type, pfkA and pfkB knockout mutants. The methodology was used to suggest that pfkA is the main functional enzyme.

Metabolite profiling of human urine by CE-ESI-MS using separation electrolytes at low pH

We investigated the potential of CE coupled to electrospray MS (CE-ESI-MS) in metabolite profiling of human urine without any sample prefractionation step. A heterogeneous mixture of biologically relevant compounds covering a broad range of physicochemical properties was used to optimize separation conditions in fused-silica capillaries. A running electrolyte containing 50 mM of acetic acid and 50 mM of formic acid at pH 2.5 was used for the CE separations. A sheath-flow electrospray interface was employed for CE-ESI-MS analysis. Sheath liquids containing 80:20 v/v methanol/water with 0.1% v/v of acetic acid or 60:40 v/v isopropanol/water with 0.5% v/v of ammonia were selected for optimum detection in positive and negative ESI modes, respectively. Reproducibility and sensitivity were studied, and strategies for identification of the separated urinary compounds are suggested. We report major advantages and disadvantages of CE-ESI-MS for metabolite profiling of human body fluids. This work may be regarded as a first step in the use of CE-ESI-MS for reliable differential analysis of body fluids from healthy and diseased individuals.

Evaluation of the electrophoretic behaviour of opioid peptides

A general equation established in a previous study was used to model the electrophoretic mobility of a series of opioid peptides as a function of pH of the separation electrolyte. The concordance between the predicted and the experimental electrophoretic mobilities was excellent and the optimum pH for the separation of the modelled compounds could be predicted from a limited amount of experimental data. The equations were also useful for the accurate determination of the ionization constants of the polyprotic analytes. It was also demonstrated that if ionization constant values are known, the CE separations of the studied peptides can easily be predicted taking into account the classical semiempirical relationships between electrophoretic mobility and charge-to-mass ratio (m(e) versus q/M(alpha)). The separations simulated considering the accurate charge-to-mass ratios of each peptide at a certain pH value were in good agreement with the experimental results. Once an optimum separation pH value and a running buffer compatible with electrospray mass spectrometry (ESI) detection were selected, a method for the separation and characterization of this series of analytes by capillary electrophoresis-electrospray ionization mass spectrometry (CE-ESI-MS) was established using a commercial sheath-flow interface. Method validation was performed in order to prove the suitability of the proposed method for quantitative analysis. Thus, quality parameters, such as repeatability, reproducibility, limits of detection and linearity were determined.