Rapid capillary coating by epoxy-poly-(dimethylacrylamide): performance in capillary zone electrophoresis of protein and polystyrene carboxylate

Neutral polymers as coatings for high resolution electrophoretic separation of Aβ peptides on glass microchips

This study reports a comparison of the performances of two neutral polymers, poly ethylene-oxide (PEO) and poly(dimethylacrylamide-co-allyl glycidyl ether) (EpDMA), in glass microchips to achieve zone electrophoresis separation of several truncated forms of beta amyloid (Aβ) peptides, sharing very similar structures. The peptides were derivatized by FluoProbes 488 NHS to allow their fluorescence detection. Two protocols based either on PEO or EpDMA led to good pH stabilities in addition to a significant reduction of the electroosmotic flow. These two polymer coatings allowed repeatable analyses and high resolution for the simultaneous analysis of three Aβ peptides, Aβ 1-38, Aβ 1-40 and Aβ 1-42, considered as potential biomarkers of Alzheimer's disease. A recovery study showed that EpDMA was superior in reducing the adsorption of the Aβ peptides on the coated inner wall. Finally, the separation method relying on the EpDMA coated microchips was validated as linear using a calibration curve and the LOD was estimated to be close to 200 nM. Despite very short migration distances, different N-terminal or C-terminal truncated Aβ peptides, corresponding to promising biomarker combinations for the future diagnostic, were fully resolved. The method was successfully applied to detect these peptides in spiked cerebrospinal fluid and has provided a first achievement towards the development of a microsystem that would integrate preconcentration and separation steps.

Highly charged polyelectrolyte coatings to prevent adsorption during protein and peptide analysis in capillary electrophoresis

Capillary electrophoresis (CE) is an interesting technique for protein and peptide analysis. However, one of the major problems concerns sample adsorption on the internal capillary wall. The use of non-covalent coatings using highly charged polyelectrolytes is an efficient, simple, and fast approach to reduce peptide and protein adsorption phenomena. We have studied in a systematic manner the effect of coating conditions on the stability and efficiency of multilayer coatings using poly(diallyldimethylammonium) chloride (PDADMAC) as polycation and polystyrene sulfonate (PSS) as polyanion. When optimal conditions defined in the protocols are used, very stable coatings are obtained and adsorption phenomena are eliminated. The coatings are stable over a large range of pH buffer (2-10) and in the presence of organic solvent. Hundreds of analyses can be performed without coating regeneration. Coated capillaries can be easily stored and reused.

Temperature dependence of acidity constants, a tool to affect separation selectivity in capillary electrophoresis

The mathematical models of migration and dispersion in capillary zone electrophoresis of small molecules form a sound basis for separation strategies of complex mixtures. It turned out that the key property is the effective mobility of the sample ions. To tune resolution parameters such as pH, complexation constants and ionic strength are widely used; temperature however is not although mobilities and pK(a) values depend in a more or less degree on temperature. From the temperature dependences of pK(a) values of a number of compounds listed in the literature a general rule can be derived: for carboxylic and inorganic acids dpK(a)/dT values are very small and the pK(a) values change less than +/-0.05 units/10K. Thermodynamically speaking, these compounds exhibit dissociation enthalpies close to zero. Phenols and amines, on the other hand, have systematically larger dpK(a)/dT values of about -0.1 to -0.2 units per 10K (the results of dissociation enthalpies of 20-70 kJ/mole). Based on this classification, a distinction can be made between different situations in capillary electrophoresis: (i) selectivity changes with temperature are largely due to the temperature dependence of the pK(a) of the buffering compound in the background electrolyte, (ii) selectivity changes mainly result from the temperature dependence of the pK(a) of the sample ions, and (iii) temperature effects on the pK(a) values of both, sample and buffer play a role. This work demonstrates such effects on selectivity in capillary electrophoresis highlighting the fact that in some instances temperature can be used to fine-tune separations.

Poly(N,N-dimethylacrylamide)-grafted polyacrylamide: a self-coating copolymer for sieving separation of native proteins by CE

The potential of a series of newly synthesized poly(N,N-dimethylacrylamide) (PDMA) grafted polyacrylamide (PAM) copolymers (P(AM-PDMA)) as a replaceable separation medium for protein analysis was studied. A comparative study with and without copolymers was performed; the separation efficiency, analysis reproducibility and protein recovery proved that the P(AM-PDMA) copolymers were efficient in suppressing the adsorption of basic proteins onto the silica capillary wall. Furthermore, the size-dependent retardation of native proteins in a representative P(AM-PDMA) copolymer was demonstrated by Ferguson analysis. The results showed that the P(AM-PDMA) copolymers combine the good coating property of PDMA and the sieving property of PAM and could be applied as a sieving matrix for the analysis of native proteins.

Determination of pKa values of diastereomers of phosphinic pseudopeptides by CZE

A CE method was used for the determination of acidity constants (pK(a)) of a series of ten phosphinic pseudopeptides, which varied in number and type of ionogenic groups. Effective electrophoretic mobilities were measured in the 1.8-12.0 pH range in the BGEs of constant ionic strength of 25 mM. Effective electrophoretic mobilities, corrected to standard temperature of 25 degrees C, were subjected to non-linear regression analysis and the obtained apparent pK(a) values were recalculated to thermodynamic pK(a)'s by extrapolation to zero ionic strength according to the extended Debye-Hückel model. The pK(a) values of the phosphinic acid group fell typically in the 1.5-2.25 interval, C-terminal carboxylic groups in the 2.94-3.50 interval, carboxylic groups of the lateral chain of glutamate and aspartate in the 4.68-4.97 interval, imidazolyl moiety of histidine in the 6.55-8.32 interval, N-terminal amino groups in the 7.65-8.28 interval and epsilon-amino group of the lateral chain of lysine in the 10.46-10.61 interval. Further, separation of diastereomers of the phosphinic pseudopeptides was investigated in achiral BGEs. Evaluation of the resolution of the diastereomers as a function of pH of the BGE revealed that most suitable pH region for separation of the diastereomers is around the pK(a) values of the central phosphinic acid group of the pseudopeptides. Successful separation of some diastereomers was, however, achieved in the neutral and alkaline BGEs as well.

CE as orthogonal technique to HPLC for alprazolam degradation product identification

The control of degradation products is currently a critical issue to the pharmaceutical industry. A degradation product that appeared in alprazolam tablets during their stability assay, 7-chloro-1-methyl-5-phenyl-[1,2,4]triazolo[4,3-a]quinolin-4-amine, also named triazolaminoquinoline, was tested as possible candidate in the HPLC method employed for the study. The impurity showed the same retention time and spectra as the degradation product; but as all these compounds are very closely related, a confirmation with an independent technique was necessary, and CE was chosen for that purpose. Problems related to the adsorption of the analytes to the negatively charged silica surface were solved by employing a new polymeric capillary coating consisting of poly(3-aminopropylmethylsiloxane). The polymer provided EOF towards the anode, and the two compounds were separated in less than 8 min in a 60 cm total-length capillary, 75 microm id capillary with a BGE containing 50 mM phosphate buffer at pH 2.0 with 20% ACN. When the sample containing the degradation product was injected, the presence of triazolaminoquinoline was confirmed.

Determination of limiting mobilities and dissociation constants of 21 amino acids by capillary zone electrophoresis at very low pH

The dependence of the effective electrophoretic mobility on pH of the background electrolyte was experimentally determined by capillary zone electrophoresis (CZE) for cationic forms of amino acids. The pH of the background electrolytes was in the highly acidic range, 1.6-2.6 pH units, to ensure a high degree of protonation of the amino acids. Poly(vinyl alcohol) was added to the background electrolytes to avoid possible adsorption of the analytes at the inner capillary wall. Non-linear regression of the experimental data was applied to obtain the parameters of the relevant regression functions--the actual mobilities and mixed dissociation constants corresponding to the actual ionic strength. The extended Onsager and Debye-Hückel law was used to calculate the limiting mobilities and thermodynamic dissociation constants. The comparison of the experimental electropherogram with the computer prediction by PeakMaster using the determined data is presented for the selected sample of amino acids.

Grafting epoxy-modified hydrophilic polymers onto poly(dimethylsiloxane) microfluidic chip to resist nonspecific protein adsorption

In order to achieve a simple covalent hydrophilic polymer coating on poly(dimethylsiloxane) (PDMS) microfluidic chip, epoxy modified hydrophilic polymers were synthesized in aqueous solution with a persulfate radical initiation system, and crosslinked onto PDMS pretreated by oxygen plasma and silanized with 3-aminopropyl-triethoxysilanes (APTES). Glycidyl methacrylate (GMA) was copolymerized with acrylamide (poly(AAM-co-GMA)) or dimethylacrylamide (poly(DAM-co-GMA)), and graft polymerized with polyvinylpyrrolidone (PVP-g-GMA) or polyvinylalcohol (PVA-g-GMA). The epoxy groups in the polymers were determined by UV spectra after derivation with benzylamine. Reflection absorption infrared spectroscopy (RAIRS) confirmed covalent grafting of GMA-modified polymers onto PDMS surface. Electroosmotic flow (EOF) in the polymer grafted microchannel was strongly suppressed within the range pH 3-11. Surface adsorption of lysozyme and bovine serum albumin (BSA) was reduced to less than 10% relative to that on the native PDMS surface. On the GMA-modified polymer coated PDMS microchip, basic proteins, peptides, and sodium dodecyl sulfate (SDS) denatured proteins were separated successfully.

Enhancement of electroosmotic flow using zwitterionic additives

Zwitterionic additives provide a means of altering the EOF without increasing conductivity. The magnitude of the EOF in a bare silica capillary increased by as much as 69% upon addition of 500 mM of zwitterion to the running buffer. The EOF enhancement increases linearly with the zwitterion concentration. With zwitterionic additives of the form +NH3-(CH2)n-COO-, the magnitude of the EOF increase is directly related to the number of methylene groups, (n), which ranges from n = 1 to 7. The endgroups on the zwitterions also affect the EOF enhancement. The effect of Z1-methyl (+N(CH3)3CH2CH2CH2SO3-) on EOF was not a function of either the buffer cation or pH. The EOF enhancement is a function of the dielectric increment of the additive and the nature of the amine functionality.

A rejuvenation method for poly(N,N-dimethylacrylamide)-coated glass microfluidic chips

As microfluidic chips come to integrate the higher levels of functionality required for the implementation of advanced bioanalytical protocols, a crucial factor is that of cost. Although glass chips provide advantages in multilayer integrations, their cost is far higher than that of polymer chips. However, a simple and effective rejuvenation protocol for glass microchips may enable higher levels of integration and functionality on glass microchips. Here we present a method to rejuvenate glass microchips that had been used for capillary electrophoresis to the extent that their performance was degraded. This degradation was due to one of the two mechanisms: (i) a deterioration of the polymer coating on the inner surface of the microchannel or (ii) an aging of the glass substrate. Using the method presented here, we have rejuvenated more than 50 such "aged" microchips. The performance of these microchips was fully restored after the rejuvenation and lasted for hundreds of DNA separation runs. Our experiments indicate that the loss of resolution in microchip separations was not associated with glass aging, but was due to the degradation of the polymer coating on the inner surface of microchannels. This suggests that it is possible to extend the microchip lifetime "forever" using the rejuvenation protocol and that the exploration of higher levels of integration and functionality on glass microchips (or of hybrid structures involving materials capable of withstanding the reagents and elevated temperatures used) is feasible.

Functional materials for microscale genomic and proteomic analyses

The design of functional materials for genomic and proteomic analyses in microscale systems has begun to mature, from materials designed for capillary-based electrophoresis systems to those tailored for microfluidic-based or 'chip-based' platforms. In particular, recent research has focused on evaluating different polymer chemistries for microchannel surface passivation and improved DNA separation matrix performance. Additionally, novel bioconjugate materials designed specifically for electrophoretic separations in microscale channels are facilitating new separation modalities.

Modification of capillary electrophoresis capillaries by poly(hydroxyethyl methacrylate), poly(diethylene glycol monomethacrylate) and poly(triethylene glycol monomethacrylate)

Modification of capillary electrophoresis (CE) capillaries by poly(hydroxyethyl methacrylate) (poly(HEMA), poly(diethylene glycol monomethacrylate) (poly(DEGMA) and poly(triethylene glycol monomethacrylate) (poly(TEGMA), was studied. Methods based on physical adsorption of the modifier and on its chemical binding were compared on the basis of the electroosmotic flow (EOF) reproducibility, the EOF dependence on the pH, the symmetry of the peak of positively charged tyramine, the stability of the coating and the separation of standard and milk proteins in the modified capillaries. Reproducible coatings were obtained by chemical binding of the polymers to the capillary walls and by coating with a solution of a polymer, as also demonstrated by the atomic force microscopy.

Recent advances in capillary electrophoresis of peptides

The article gives a comprehensive review on the recent developments in the applications of high-performance capillary electromigration methods, including zone electrophoresis, isotachophoresis, isoelectric focusing, affinity electrophoresis, electrokinetic chromatography and electrochromatography, to analysis, preparation and physicochemical characterization of peptides. The article presents new approaches to the theoretical description and experimental verification of electromigration behavior of peptides, and covers the methodological aspects of capillary electroseparations of peptides, such as strategy and rules for the rational selection of separation mode and experimental conditions, sample treatment, suppression of peptide adsorption to the inner capillary wall, new developments in individual separation modes and new designs of detection systems. Several types of applications of capillary electromigration methods to peptide analysis are presented: conventional qualitative and quantitative analysis for determination of purity, determination in biomatrices, monitoring of physical and chemical changes and enzymatic conversions, amino acid and sequence analysis and peptide mapping of proteins. Some examples of micropreparative peptide separations are given and capabilities of capillary electromigration techniques to provide important physicochemical characteristics of peptides are demonstrated.

Capillary electrophoresis of proteins 1999-2001

This review article with 223 references describes recent developments in capillary electrophoresis (CE) of proteins and covers papers published during last two years, from the previous review (V. Dolnik, Electrophoresis 1999, 20, 3106-3115) through Spring 2001. It describes the topics related to CE of proteins including modeling of the electrophoretic properties of proteins, sample pretreatment, wall coatings, improving selectivity, detection, special electrophoretic techniques, and applications.