Protein-induced conformational change in glycans decreases the resolution of glycoproteins in hydrophilic interaction liquid chromatography.

An understanding of why hydrophilic interaction liquid chromatography gives a higher resolution for glycans than for glycoproteins would facilitate column improvements. Separations of the glycoforms of ribonuclease B compared to its released glycans were studied using a commercial hydrophilic interaction liquid chromatography column. The findings were used to devise a new hydrophilic interaction liquid chromatography column. For the commercial column, chromatograms and van Deemter plots showed that selectivity and efficiency are comparable factors in the higher resolution of the released glycans. The higher selectivity for the released glycans was associated with more water molecules displaced per added mannose. To investigate why, three-dimensional structures of the glycoprotein and the glycan were computed under chromatographic conditions. These showed that hydrogen bonding within the free glycan makes its topology more planar, which would increase contact with the bonded phase. The protein sterically blocks the hydrogen bonding. The more globular-shaped glycan of the glycoprotein suggests that a thicker bonded phase might improve selectivity. This was tested by making a column with a copolymer bonded phase. The results confirmed that selectivity is increased. The findings are possibly broadly relevant to glycoprotein analysis since the structural motif involved in internal hydrogen bonding is common to N-linked glycans of human glycoproteins.

Fluorescent Probes for Monitoring Serine Ubiquitination.

In a radical departure from the classical E1-E2-E3 three-enzyme mediated ubiquitination of eukaryotes, the recently described bacterial enzymes of the SidE family of Legionella pneumophila effectors utilize NAD+ to ligate ubiquitin onto target substrate proteins. This outcome is achieved via a two-step mechanism involving (1) ADP ribosylation of ubiquitin followed by (2) phosphotransfer to a target serine residue. Here, using fluorescent NAD+ analogues as well as synthetic substrate mimics, we have developed continuous assays enabling real-time monitoring of both steps of this mechanism. These assays are amenable to biochemical studies and high-throughput screening of inhibitors of these effectors, and the discovery and characterization of putative enzymes similar to members of the SidE family in other organisms. We also show their utility in studying enzymes that can reverse and inhibit this post-translational modification.

Native Reversed-Phase Liquid Chromatography: A Technique for LCMS of Intact Antibody-Drug Conjugates.

The synthesis of antibody-drug conjugates (ADCs) using the interchain cysteines of the antibody inherently gives a mixture of proteins with varying drug-to-antibody ratio. The drug distribution profiles of ADCs are routinely characterized by hydrophobic interaction chromatography (HIC). Because HIC is not in-line compatible with mass spectrometry (MS) due to the high salt levels, it is laborious to identify the constituents of HIC peaks. An MS-compatible alternative to HIC is reported here: native reversed phase liquid chromatography (nRPLC). This novel technique employs a mobile phase 50 mM ammonium acetate for high sensitivity in MS and elution with a gradient of water/isopropanol. The key to the enhancement is a bonded phase giving weaker drug-surface interactions compared to the noncovalent interactions holding the antibody-drug conjugates together. The hydrophobicity of the bonded phase is varied, and the least hydrophobic bonded phase in the series, poly(methyl methacrylate), is found to resolve the intact constituents of a model ADC (Ab095-PZ) and a commercial ADC (brentuximab vedotin) under the MS-compatible conditions. The nRPLC-MS data show that all species, ranging from drug-to-antibody ratios of 1 to 8, remained intact in the column. Another desired advantage of the nRPLC is the ability of resolving multiple positional isomers of ADC that are not well-resolved in other chromatographic modes. This supports the premise that lower hydrophobicity of the bonded phase is the key to enabling online nRPLC-MS analysis of antibody-drug conjugates.

Electrophoresis of megaDalton proteins inside colloidal silica.

With the growth of the biopharmaceutical industry, there is a need for rapid size-analysis of proteins on the megaDalton scale. The large pore sizes needed for such separations cannot be easily reached by pushing the current limits of size-exclusion chromatography or gel electrophoresis. The concept detailed here is the formation of arbitrarily wide pores by packing nonporous colloidal silica in capillaries. This method can be called packed-capillary electrophoresis, or "pCE". Electrophoresis of protein standards (11-155 kDa) by pCE, using 345 nm diameter particles in 100 µm diameter capillaries, gives 2x higher resolution than a typical PAGE gel in 1/6 of the time. The electropherograms show that pCE is highly efficient, with half-micrometer plate heights for all seven standards, giving 105 plates for a 50 mm length. The large pore radius of 65 nm enables baseline resolution of proteins of 0.72, 1.048 and 1.236 MDa in less than 15 min. The short separation time of pCE is attributed to the absence of small pores that restrict protein migration in gels. The pCE separation is applied to the analysis of a stressed pharmaceutical-grade IgG4 sample, giving unprecedented baseline resolution of monomer, dimer, trimer and tetramer in less than 10 min.

Ubiquitin Chains Modified by the Bacterial Ligase SdeA Are Protected from Deubiquitinase Hydrolysis.

The SidE family of Legionella pneumophila effectors is a unique group of ubiquitin-modifying enzymes. Along with catalyzing NAD+-dependent ubiquitination of certain host proteins independent of the canonical E1/E2/E3 pathway, they have also been shown to produce phosphoribosylated free ubiquitin. This modified ubiquitin product is incompatible with conventional E1/E2/E3 ubiquitination processes, with the potential to lock down various cellular functions that are dependent on ubiquitin signaling. Here, we show that in addition to free ubiquitin, Lys63-, Lys48-, Lys11-, and Met1-linked diubiquitin chains are also modified by SdeA in a similar fashion. Both the proximal and distal ubiquitin moieties are targeted in the phosphoribosylation reaction. Furthermore, this renders the ubiquitin chains unable to be processed by a variety of deubiquitinating enzymes. These observations broaden the scope of SdeA's modulatory functions during Legionella infection.

In-column bonded phase polymerization for improved packing uniformity.

It is difficult to pack chromatographic particles having polymeric-bonded phases because solvents used for making a stable slurry cause the polymer layer to swell. Growth of the polymer inside the column (in situ) after packing was investigated and compared with conventional, ex situ polymer growth. The method of activators generated by electron transfer, along with atom-transfer radical polymerization, enabled polymerization under ambient conditions. Nonporous, 0.62 µm silica particles with silane initiators were used. Polyacrylamide films with a hydrated thickness of 23 nm in 75:25 water/isopropanol grew in 55 min for both in situ and ex situ preparations, and the same carbon coverage was observed. Higher chromatographic resolution and better column-to-column reproducibility were observed for in situ polymer growth, as evaluated by hydrophilic interaction liquid chromatography for the model glycoprotein, ribonuclease B. In situ polymer growth was also found to give lower eddy diffusion, as shown by a narrower peak width for injected acetonitrile in 50:50 acetonitrile/water. When columns were packed more loosely, bed collapse occurred quickly for ex situ, but not for in situ, polymer growth. The higher resolution and stability for in situ polymer growth is explained by packing with hard, rather than soft, contacts between particles.

Submicrometer particles and slip flow in liquid chromatography.

Smaller particles have progressively led to higher efficiency in liquid chromatography, particularly for proteins, due to smaller diffusion distances. Particle diameter has recently entered the submicrometer region, with the back-pressure requirements alleviated by slip flow.

Efficient separations of intact proteins using slip-flow with nano-liquid chromatography-mass spectrometry.

A capillary with a pulled tip, densely packed with silica particles of 0.47 µm in diameter, is shown to provide higher peak capacity and sensitivity in the separation of intact proteins by reversed-phase liquid chromatography-mass spectrometry (LC-MS). For a C18 bonded phase, slip flow gave a 10-fold flow enhancement to allow for stable nanospray with a 4-cm column length. Model proteins were studied: ribonuclease A, trypsin inhibitor, and carbonic anhydrase, where the latter had impurities of superoxide dismutase and ubiquitin. The proteins were well separated at room temperature with negligible peak tailing. The peak capacity for ubiquitin was 195 for a 10-min gradient and 315 for a 40-min gradient based on Gaussian fitting of the entire peak, rather than extrapolating the full-width at half-maximum. Separation of a cell lysate with a 60 min gradient showed extremely high peak capacities of 750 and above for a peptide and relatively homogeneous proteins. Clean, low noise mass spectra for each model protein were obtained. The physical widths of the peaks were an order of magnitude narrower than those of conventional columns, giving increased sensitivity. All proteins except ubiquitin exhibited significant heterogeneity apparently due to multiple proteoforms, as indicated by both peak shapes and mass spectra. The chromatograms exhibited excellent reproducibility in retention time, with relative standard deviations of 0.09 to 0.34%. The results indicate that submicrometer particles are promising for improving the separation dimension of LC in top-down proteomics.

RPLC of intact proteins using sub-0.5 µm particles and commercial instrumentation.

This paper addresses whether one can gain an improvement in speed or resolution with a silica colloidal crystal (SCC) of nonporous 470 nm particles when using a commercial nano-UHPLC. Compared to a capillary packed with nonporous 1.3 µm particles and the same C4 bonded phase, the peak width for BSA is decreased by a factor of 6.8 for the SCC. Some of this improvement is attributable to slip flow since the ratio of particle diameters is only 2.8. Resolution in protein separations was compared for a 2-cm capillary of SCC vs a 5-cm column of porous 1.7 µm particles. Both used a C4 bonded phase, and on-column fluorescence detection was used for the SCC. Split flow (5:1) before the SCC decreased the gradient delay time to 0.4 min and the injected volume to 0.4 nL. For variants from the labeling of BSA, the SCC had a 5-fold higher speed and 2-fold higher resolution than did the commercial column. For a monoclonal antibody and its aggregates, the SCC had a 3-fold higher speed and a 3-fold higher resolution compared to the commercial column. The SCC gave baseline resolution of the monomer, dimer and trimer in 5 min. The results show that a significant advantage can be gained using a commercial instrument with the SCC, despite the instrument not being designed for use with such small particles.

Modeling of protein electrophoresis in silica colloidal crystals having brush layers of polyacrylamide.

Sieving of proteins in silica colloidal crystals of millimeter dimensions is characterized for particle diameters of nominally 350 and 500 nm, where the colloidal crystals are chemically modified with a brush layer of polyacrylamide. A model is developed that relates the reduced electrophoretic mobility to the experimentally measurable porosity. The model fits the data with no adjustable parameters for the case of silica colloidal crystals packed in capillaries, for which independent measurements of the pore radii were made from flow data. The model also fits the data for electrophoresis in a highly ordered colloidal crystal formed in a channel, where the unknown pore radius was used as a fitting parameter. Plate heights as small as 0.4 µm point to the potential for miniaturized separations. Band broadening increases as the pore radius approaches the protein radius, indicating that the main contribution to broadening is the spatial heterogeneity of the pore radius. The results quantitatively support the notion that sieving occurs for proteins in silica colloidal crystals, and facilitate design of new separations that would benefit from miniaturization.

Obstructed diffusion in silica colloidal crystals.

The hindered diffusion in silica colloidal crystals was studied experimentally, both by fluorescence recovery after photobleaching and by measurement of ionic conductivity. Particle size was varied to include 120, 220, 470, and 1300 nm, and the porosities were determined by flow measurements. For fluorescein, the results showed that the obstruction factor, which is the ratio of the diffusion coefficients inside the media and in open solution, is equal to the porosity within experimental error. For proteins, the same conclusion is made after correction for size exclusion of the pores. The obstruction factors for these media are 2-fold lower than those measured for chromatographic media, 60% higher than theoretical predictions, and equal to what is assumed for electrophoretic sieving in random fibers.

Insights from theory and experiments on slip flow in chromatography.

Slip flow has become a topic of interest in reversed-phase liquid chromatography because it gives a flow enhancement that facilitates the use of submicrometer particles, providing a large improvement in separation efficiency. Moreover, slip flow provides an additional improvement in efficiency by reducing the velocity distribution in the mobile phase. The phenomenon of slip flow in open tubes is described in chromatographically relative terms. A recent paper in this journal is discussed, as it provides the first theoretical study of slip flow in packed beds, in this case for face-centered cubic geometry. The theory paper reveals that the presence of the packed bed introduces a heterogeneity in fluid velocities that is absent in open tubes, reducing the additional improvement in efficiency from slip flow. The recent paper also suggests that there is yet another factor improving efficiency, which is size-exclusion of proteins from regions of stagnant flow. The latter is supported by recently published data on restricted protein diffusion in face-centered cubic silica colloidal crystals. Extremely low plate heights are enabled by use of submicrometer particles, and further improvement appears to be possible when the analyte size is on the order of 1% of the particle diameter or larger.

Trajectory of isoelectric focusing from gels to capillaries to immobilized gradients in capillaries.

This review presents the need for replacing gels in 2D separations for proteomics, where speed, high-throughput, and the ability to characterize trace level proteins or small samples are the current desires. The theme of the review is isoelectric focusing, which is a valuable tool because it pre-concentrates proteins in addition to separating with high peak capacity. The review traces the technological progress from gel IEF to CIEF to packed capillaries with immobilized gradients for CIEF. Multiple capillary techniques are progressing toward meeting the current desires, providing extremely high sensitivity with regard to concentration and to small samples, integrated automation, and high peak capacity from multiple dimensions of separation. Capillaries with immobilized pH gradients for CIEF are emerging, which will alleviate interference from ampholytes and improve reproducibility in separation times when this valuable technique can be used as one of the dimensions.

Slip flow in colloidal crystals for ultraefficient chromatography.

Slip flow occurs in colloidal crystals made of 470 nm silica spheres that are chemically modified with hydrocarbon, giving enhanced volume flow rates and a narrower distribution of fluid velocities. Bovine serum albumin separates by pressure-driven flow with a zone that is 15-fold narrower than the theoretical limit for Hagen-Poiseuille flow. The zone variance, normalized for separation length, is 15 nm, which is 500-fold smaller than previous reports for pressure-driven protein chromatography. A colloidal crystal is shown to separate a monoclonal antibody from its aggregates in only 40 s, representing a 10-fold increase in speed. Slip flow, thus, has profound implications for protein chromatography.

Plate heights below 50 nm for protein electrochromatography using silica colloidal crystals.

Silica colloidal crystals formed from 330 nm nonporous silica spheres inside of 75 µm i.d. fused silica capillaries were evaluated for the efficiency of capillary electrochromatography of proteins. Three proteins, ribonuclease A, cytochrome C, and lysozyme, each covalently labeled with fluorophor, were well separated over a distance of 1 cm by isocratic electromigration, using 40:60 acetonitrile/water with 0.1% formic acid. A van Deemter plot showed that the plate height for lysozyme, which was the purest of the three proteins, was diffusion-limited for electric fields ranging from 400 to 1400 V/cm. The plate height for lysozyme was below 50 nm at almost all of the migration velocities, and it approached 10 nm at the highest velocity. Eddy diffusion was negligible. Lysozyme migrated over a 12 mm separation length with more than 10(6) plates in 1.5 min. These results indicate that silica colloidal crystals are well suited for electrically driven separations of large, highly charged analytes such as proteins. The 10(6) plates observed for a separation length of barely more than a centimeter means they are potentially valuable for miniaturized separations in microchip and lab-on-a-chip devices.

Field-free remobilization of proteins after isoelectric focusing in packed capillaries.

Pressure-driven remobilization without an applied electric field is shown to be possible with capillary isoelectric focusing using packed capillaries. The capillary dimensions are 100 µm i.d. and 2 cm in length, and the packing is made of 0.9 µm nonporous silica particles that are chemically modified with a brush layer of polyacrylamide. Both reversible and irreversible adsorption are shown to be negligible. The packed capillaries eliminate the problem of unwanted hydrodynamic flow between reservoirs. Three proteins are focused: trypsin inhibitor, carbonic anhydrase II, and myoglobin. The time required for focusing in the packed capillaries is increased by only a factor of 2 compared to the open capillary, giving complete focusing in less than 15 min at 200 V/cm. The packed capillaries allow the use of higher electric fields, with resolution continually increasing up to at least 1500 V/cm. The packing obstructs diffusional broadening after the field is turned off: for trypsin inhibitor, D = 6.1(±0.3) × 10(-8) cm(2)/s for the packed capillary vs D = 28.8(±0.3) × 10(-8) cm(2)/s for the open capillary. The broadening contributed by the packing during remobilization is from eddy diffusion, and it is described by its plate height, H, which is the variance per unit length: H = σ(2)/L = 0.64 µm. This limits the resolution to 0.1 pH units for the 2 cm capillary having a pH range of 3-10, giving a theoretical peak capacity of 47.

Submicrometer plate heights for capillaries packed with silica colloidal crystals.

Extremely uniform packing of colloidal silica in capillaries is shown. Reversed-phase electrochromatograms of DiI-C(12) exhibit plate heights as low as 0.23 microm and a reduced plate height as low as 0.7, using 75 microm i.d. capillaries packed with 330 nm silica particles. The contribution from the A term is 0 +/- 20 nm in electrochromatography. The particles are shown to form colloidal crystals inside the capillaries. Optical images show Bragg diffraction, indicative of crystallinity. Scanning electron microscopy (SEM) images show face-centered cubic crystallinity, and the porosity is 0.25 +/- 0.01, which is in agreement with that for face-centered cubic crystals. The capillaries are fritless, and 100 microm i.d. capillaries packed with silica colloidal crystals withstand pressures of at least 12,400 psi.

Lipid bilayer blanketing versus penetrating silica colloidal crystals.

Fluorescence intensities and diffusion coefficients were measured for a labeled lipid in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayers deposited by vesicle fusion on two types of silica colloidal crystals, one made from 145 nm particles and the other from 490 nm particles. The bilayer is shown to blanket the top of the silica colloidal crystal for the 145 nm case and to coat the entire surface area for the 450 nm case. The diffusion coefficient of the labeled lipid is shown to be the same in both cases, showing that the diffusion coefficient is not an indicator of bilayer penetration.

Making Sharper Peaks for Reverse-Phase Liquid Chromatography of Proteins.

Protein separations have gained increasing interest over the past two decades owing to the dramatic growth of proteins as therapeutics and the completion of the Human Genome Project. About every decade, the field of protein high-performance liquid chromatography (HPLC) seems to mature, having reached what appears to be a theoretical limit. But then scientists well versed in the basic principles of HPLC invented a way around the limit, generating another decade of exciting progress. There is still the need for higher resolution and better compatibility with mass spectrometry because it is an essential tool for identification of proteins and their modifications. To make advances, the fundamental principles need to be understood. This review covers recent advances and current needs in the context of the principles that underlie the many contributions to peak broadening.

Evaluation of particle and bed integrity of aqueous size-exclusion columns packed with sub-2 µm particles operated at high pressure.

The performance of columns packed with 1.7 µm particles for aqueous size-exclusion chromatography was assessed at high-pressure conditions and linked to particle- and column-bed integrity. Decreasing the particle size from 3.5 µm to 1.7 µm increases the resolution due to the improved mass-transfer characteristics, allowing to significantly speed-up analysis without compromising the selectivity. A sub-minute separation of intact proteins was realized on a 4.6 mm i.d × 75 mm long column packed with 1.7 µm SEC particles applying a flow rate of 1.8 mL/min, corresponding to a column pressure of 530 bar. Ultra-high pressure operation (exceeding manufacturer's recommendations) resulted in peak deformation, a shift towards earlier retention times, and an alteration in selectivity. To gain insights in the mechanisms of column deterioration, short 30 mm long columns were operated at UHPLC conditions, maximizing the pressure drop over individual particles. This resulted in the presence of fractured particles situated at the column outlet, as verified by scanning electron micrographs. Mercury-intrusion porosimetry and argon-adsorption measurements did not reveal significant differences in intraparticle volume between particle batches sampled before and after pressure stress testing. As particles at the column outlet fracture (but not collapse) at high pressure operation, a void was formed at the column inlet. The degradation of the separation performance appeared to be the result of a decrease in interparticle pore volume.