Long-term precision in capillary isoelectric focusing for protein analysis

Recent advances in capillary electrophoretic migration techniques for pharmaceutical analysis

Since the introduction about 30 years ago, CE techniques have gained a significant impact in pharmaceutical analysis. The present review covers recent advances and applications of CE for the analysis of pharmaceuticals. Both small molecules and biomolecules such as proteins are considered. The applications range from the determination of drug-related substances to the analysis of counterions and the determination of physicochemical parameters. Furthermore, general considerations of CE methods in pharmaceutical analysis are described.

Capillary isoelectric focusing with pH 9.7 cathode for the analysis of gastric biopsies

Capillary isoelectric focusing tends to suffer from poor reproducibility, particularly for the analysis of complex protein samples from cellular or tissue homogenates. This poor reproducibility appears to be associated with erratic variations in electroosmotic flow. One cause of electroosmotic flow variation is degradation of the capillary coating caused by the extremely basic solution commonly used during mobilization and focusing; this degradation of the capillary coating can be reduced by employing a CAPS mobilization buffer at pH 9. Another cause of variation is protein adsorption to the capillary wall, which causes an increase in electroosmotic flow. The effects of protein adsorption can be reduced by use of surfactants in the buffer and by employing an extremely low sample loading. We report the use of CAPS mobilization buffer in combination with an ultrasensitive laser-induced fluorescence detector for the reproducible analysis of ∼2 ng of protein from a Barrett's esophagus biopsy.

Quantitative gel electrophoresis: new records in precision by elaborated staining and detection protocols

Gel electrophoresis (GE) is a very common analytical technique for proteome research and protein analysis. Despite being developed decades ago, there is still a considerable need to improve its precision. Using the fluorescence of Colloidal Coomassie Blue -stained proteins in near-infrared (NIR), the major error source caused by the unpredictable background staining is strongly reduced. This result was generalized for various types of detectors. Since GE is a multi-step procedure, standardization of every single step is required. After detailed analysis of all steps, the staining and destaining were identified as the major source of the remaining variation. By employing standardized protocols, pooled percent relative standard deviations of 1.2-3.1% for band intensities were achieved for one-dimensional separations in repetitive experiments. The analysis of variance suggests that the same batch of staining solution should be used for gels of one experimental series to minimize day-to-day variation and to obtain high precision.

Femtomolar concentration detection limit and zeptomole mass detection limit for protein separation by capillary isoelectric focusing and laser-induced fluorescence detection

Fluorescence tends to produce the lowest detection limits for most forms of capillary electrophoresis. Two issues have discouraged its use in capillary isoelectric focusing. The first issue is fluorescent labeling of proteins. Most labeling reagents react with lysine residues and convert the cationic residue to a neutral or anionic product. At best, these reagents perturb the isoelectric point of the protein. At worse, they convert each protein into hundreds of different fluorescent products that confound analysis. The second issue is the large background signal generated by impurities within commercial ampholytes. This background signal is particularly strong when excited in the blue portion of the spectrum, which is required by many common fluorescent labeling reagents. This paper addresses these issues. For labeling, we employ Chromeo P540, which is a fluorogenic reagent that converts cationic lysine residues to cationic fluorescent products. The reaction products are excited in the green, which reduces the background signal generated by impurities present within the ampholytes. To further reduce the background signal, we photobleach ampholytes with high-power photodiodes. Photobleaching reduced the noise in the ampholyte blank by an order of magnitude. Isoelectric focusing performed with photobleached pH 3-10 ampholytes produced concentration detection limits of 270 +/- 25 fM and mass detection limits of 150 +/- 15 zmol for Chromeo P540 labeled beta-lactoglobulin. Concentration detection limits were 520 +/- 40 fM and mass detection limits were 310 +/- 30 zmol with pH 4-8 ampholytes. A homogenate was prepared from a Barrett's esophagus cell line and separated by capillary isoelectric focusing, reproducibly generating dozens of peaks. The sample taken for the separation was equal to the labeled protein homogenate from three cells.

Capillary isoelectric focusing coupled offline to matrix assisted laser desorption/ionization mass spectrometry

This work presents several critical details for making cIEF-MALDI-MS a robust technique which will allow for more routine application and aid in automation. This includes emphasis on the hardware necessary for syringe pump mobilization and proper protocol for preventing disruption from gas bubbles. Following these guidelines, excellent elution time reproducibility is demonstrated for six pI markers (RSD <5%). Additionally, the pI markers are used to calibrate the pH gradient and determine experimental pIs of proteins detected offline by mass spectrometry. This was demonstrated using a standard protein mixture of myoglobin and two forms of beta-lactoglobulin. Experimental determination of protein pIs and molecular weights were found to be in agreement with literature values. The technical details discussed provide a sound foundation for applying the offline coupling of MALDI-MS with cIEF.