Effects of electroosmotic flow on zone mobilization in capillary isoelectric focusing

Mobilization in two-step capillary isoelectric focusing: Concepts assessed by computer simulation

The mobilization step in a two-step capillary isoelectric focusing protocol is discussed by means of dynamic computer simulation data for systems without and with spacer compounds that establish their zones at the beginning and end of the focusing column. After focusing in an electroosmosis-free environment (first step), mobilization (second step) can be induced electrophoretically, by the application of a hydrodynamic flow, or by a combination of both means. Dynamic simulations provide insight into the complexity of the various modes of electrophoretic mobilization and dispersion associated with hydrodynamic mobilization. The data are discussed together with the relevant literature.

Surface isoelectric focusing (sIEF) with carrier ampholyte pH gradient

Isoelectric focusing (IEF) is a powerful tool for amphoteric protein separations because of high sensitivity, bio-compatibility, and reduced complexity compared to chromatography or mechanical separation techniques. IEF miniaturization is attractive because it enables rapid analysis, easier adaptation to point of care applications, and smaller sample demands. However, existing small-scale IEF tools have not yet been able to analyze single protein spots from array libraries, which are ubiquitous in many pharmaceutical discovery and screening protocols. Thus, we introduce an in situ, novel, miniaturized protein analysis approach that we have termed surface isoelectric focusing (sIEF). Low volume printed sIEF gels can be run at length scales of ∼300 μm, utilize ∼0.9 ng of protein with voltages below 10 V. Further, the sIEF device platform is so simple that it can be integrated with protein library arrays to reduce cost; devices demonstrate reusability above 50 uses. An acrylamide monomer solution containing broad-range carrier ampholytes was microprinted with a Nano eNabler^TM between micropatterned gold electrodes spaced 300 μm apart on a glass slide. The acrylamide gel was polymerized in situ followed by protein loading via printed diffusional exchange. A pH gradient formed via carrier ampholyte stacking when electrodes were energized; the gradient was verified using ratiometric pH-sensitive FITC/TRITC dyes. Green fluorescent protein (GFP) and R-phycoerythrin (R-PE) were utilized both as pI markers and to test sIEF performance as a function of electric field strength and ampholyte concentration. Factors hampering sIEF included cathodic drift and pH gradient compression, but were reduced by co-printing non-ionic Synperonic® F-108 surfactant to reduce protein-gel interactions. sIEF gels achieved protein separations in <10 min yielding bands < 50 μm wide with peak capacities of ∼8 and minimum pI differences from 0.12 to 0.14. This new sIEF technique demonstrated comparable focusing at ∼100 times smaller dimensions than any previous IEF. Further, sample volumes required were reduced four orders of magnitude from 20 μL for slab gel IEF to 0.002 μL for sIEF. In summary, sIEF advantages include smaller volumes, reduced power consumption, and microchip surface accessibility to focused bands along with equivalent separation resolutions to prior IEF tools. These attributes position this new technology for rapid, in situ protein library analysis in clinical and pharmaceutical settings.

Whole column fluorescence imaging on a microchip by using a programmed organic light emitting diode array as a spatial-scanning light source and a single photomultiplier tube as detector

A novel miniaturized, integrated whole-column imaging detection (WCID) system on a microchip is presented. In this system, a program controlled organic light emitting diode (OLED) array was used as a spatial-scanning light source, to achieve imaging by the time sequence of the excited fluorescence. By this mechanism, a photomultiplier tube (PMT) instead of a charge coupled detector (CCD) can be applied to the imaging. Unlike conventional systems, no lenses, fibers or any mechanical components are required either. The novel flat light source provides uniform excitation light without size limitations and outputs a stronger power by pulse driving. The scanning mode greatly reduced the power consumption of the light source, which is valuable for a portable system. Meanwhile, this novel simplified system has a broader linear range, higher sensitivity and higher efficiency in data collection. Isoelectric focusing of R-phycoerythrin (PE) and monitoring of the overall process with WCID were performed on this system. The limit of detection (LOD) was 38 ng mL(-1) or 3.2 pg at 85 nL per column injection of PE. The system provides a technique for WCID capillary isoelectric focusing (cIEF) on chip and can be used for throughput analysis.

Measurement of electroosmotic flow in capillary and microchip electrophoresis

Microfluidics is the science and technology of systems that process or manipulate small amounts of fluids, using channels with dimensions of tens of micrometers. Electroosmotic flow (EOF) is an important characteristic of fluids in microchannels. In this paper, EOF generation, effects on separation and definition of EOF are introduced. And EOF measurement methods on capillary electrophoresis (CE) and microchip CE are systematically reviewed based on detection principle, hallmarks of EOF measurement methods are presented, the devices and signals are also schematically described. This paper offers researchers a guidance to obtain an estimate of EOF mobility in capillary and microchip electrophoresis.

Continuous mode of operation for large volume dosing in analytical carrier ampholyte-free isoelectric focusing of proteins applied to off-line detection of fractions

Mass spectrometry is being increasingly used for analysis of proteome complex samples. Sample preparation is often necessary to remove matrix interferences and to concentrate analytes prior to MS measurement. A useful method for this purpose is Carrier Ampholyte Free-Isoelectric Focusing (CAF-IEF). In this paper CAF-IEF of ampholytes was performed on a commercial apparatus EA101 (Villa Labeco, Slovakia) equipped with a specially made column for samples of large volume (up to 0.5 mL). A new continuous mode without voltage interruption or electrolyte replacement was developed. In this mode, a low molecular mass pI marker (PIM 7.4) and low concentrations of myoglobin and insulin (16 mg/L), respectively, were concentrated, and then 5-microL fractions collected for off-line analyses. The total time of focusing was 66 minutes. The concentration of PIM 7.4 in the fractions was increased up to 75 times (determined by UV-VIS spectrometry). The concentration in the fractions was increased up to 30 times for myoglobin and 10 times for insulin.

Comparison of different capillary isoelectric focusing methods--use of "narrow pH cuts" of carrier ampholytes as original tools to improve resolution

Two capillary isoelectric focusing (CIEF) systems have first been optimized: one uses a bare silica capillary and 30% (v/v) of glycerol in the separation medium while the other uses a coated capillary and an aqueous background electrolyte. To perform permanent capillary coating, two neutral polymers have been compared: hydroxypropylcellulose (HPC) and polyvinylalcohol (PVA). HPC coating gave best results for electroosmotic flow (EOF) limitation on a wide pH range: as compared to a bare silica capillary, it allowed to decrease EOF by 96% at pH 7.2 after acidic and basic treatments, whereas PVA coating lead only to a 76% decrease. The glycerol CIEF system was more satisfying for the separation of model proteins classically used as pI markers. Finally, the use of "narrow pH cuts" of carrier ampholytes added to commercial ampholyte mixtures allowed increasing resolution up to a factor 2.4 at a chosen pH for the separation of pI markers and milk proteins.

On-chip coupling of isoelectric focusing and free solution electrophoresis for multidimensional separations

We have developed an acrylic microfluidic device that sequentially couples liquid-phase isoelectric focusing (IEF) and free solution capillary electrophoresis (CE). Rapid separation (<1 min) and preconcentration (73x) of species were achieved in the initial IEF dimension. Using full-field fluorescence imaging, we observed nondispersive mobilization velocities on the order of 20 microm/s during characterization of the IEF step. This transport behavior allowed controlled electrokinetic mobilization of focused sample bands to a channel junction, where voltage switching was used to repeatedly inject effluent from the IEF dimension into an ampholyte-based CE separation. This second dimension was capable of analyzing all fluid volumes of interest from the IEF dimension, as IEF was 'parked' during each CE analysis and refocused prior to additional CE analyses. Investigation of each dimension of the integrated system showed time-dependent species displacement and band-broadening behavior consistent with IEF and CE, respectively. The peak capacity of the 2D system was approximately 1300. A comprehensive 2D analysis of a fluid volume spanning 15% of the total IEF channel length was completed in less than 5 min.

Room-temperature imprinting method for plastic microchannel fabrication

A new plastic imprinting method using a silicon template is demonstrated. This new approach obviates the necessity of heating the plastic substrate during the stamping process, thus improving the device yield from approximately 10 devices to above 100 devices per template. The dimensions of the imprinted microchannels were found to be very reproducible, with variations of less than 2%. The channel depths were dependent on the pressures applied and the materials used. Rather than bonding the open channels with another piece of plastic, a flexible and adhesive poly(dimethylsiloxane) film is used to seal the microchannels, which offers many advantages. As an application, isoelectric focusing of green fluorescence protein on these plastic microfluidic devices is illustrated.

Stepwise mobilization of focused proteins in capillary isoelectric focusing mass spectrometry

A stepwise mobilization strategy has been developed for the elution of complex protein mixtures, separated by capillary isoelectric focusing (CIEF) for detection using on-line electrospray ionization mass spectrometry (ESI-MS). Carrier polyampholytes are used to establish a pH gradient as well as to control the electroosmotic flow arising from the use of uncoated fused-silica capillaries. Elution of focused protein zones is achieved by controlling the mobilization pressure and voltage, leaving the remaining protein zones focused inside the capillary. Protein zones are stepwise eluted from the capillary by changing the mobilization conditions. Stepwise mobilization improves separation resolution and simplifies coupling with multistage MS (i.e., MSn) analysis since it allows more effective temporal control of protein elution from the CIEF capillary. We also describe a modified configuration for coupling CIEF with ESI-MS using a coaxial sheath flow interface that facilitate the automation of on-line CIEF-ESI-MS analyses. The stepwise mobilization strategy is demonstrated for the analysis of standard protein mixtures and soluble E. coli lysate proteins using CIEF-ESI-MS. These results indicate that inlet pressure or voltage programming to control the elution of the protein zones from the capillary (i.e., gradient mobilization) may allow for the optimization of the mobilization conditions and provide higher resolution for CIEF separation of complex mixtures with on-line MS.

One-step capillary isoelectric focusing for the separation of the recombinant human immunodeficiency virus envelope glycoprotein glycoforms

One-step capillary isoelectric focusing was investigated as a rapid method to resolve the glycoforms of the heterogeneous recombinant human immunodeficiency virus (HIV) envelope glycoprotein (rgp 160sMN/LAI). The separation was performed in a poly(vinyl alcohol) (PVA) coated capillary using a mixture of ampholyte of narrow and wide pH range. A combination of saccaharose and 3-(cyclohexylamino)-1-propanesulfonic acid was shown to be the most efficient additive to avoid protein precipitation which occurs at a pH close to its pI. Although the calibration curve [isoelectric point (pI) vs. migration times] showed a non-linear relationship, an adequate linearity could be yielded for short pI ranges permitting to exhibit the acidic character of the different glycoforms of the rgp 160s MN/LAI (pI from 4.00 to 4.95). Reproducibility evaluated by comparing the performance of a polyacrylamide and a PVA coated capillary showed that low RSD values were obtained for intra-day (0.5 to 1.9%) and inter-day (1.6 to 7.6%) measurements using the PVA capillary. Moreover, the long term stability of the PVA capillary was demonstrated by measuring the variation of migration times of the protein markers for a long period of use. Finally, this method was able to differentiate the glycoform pattern of two close glycoproteins such as the rgp 160 of two sub-populations of the virus HIV-1.

Electrophoretic methods for process monitoring and the quality assessment of recombinant glycoproteins

In many ways electrophoretic techniques appear ideal for quality monitoring of proteins and are thus well suited for the analysis of recombinant glycoproteins. The requirements of high throughput, comparative analysis and resolution of many variants are met by several electrophoretic techniques. A wide variety of such techniques are available to biotechnologists in the rapidly developing area of recombinant glycoproteins. It is the aim of this review to specifically cover recent work which has been applied to the analysis of DNA-derived glycoproteins, both from a process control standpoint and final product validation. All major areas of electrophoresis including sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), isoelectric focusing and techniques utilizing capillary electrophoresis are covered, with emphasis on analysis of glycoforms and oligosaccharide profiles of recombinant glycoproteins. As illustration, actual examples rather than standard glycoproteins are given to indicate the potential and limitations which may be encountered. It is anticipated that this review will prove a useful and practical guide to the latest developments by indicating the relevant merits of different methods.