Separation of proteins by gradient elution from ion-exchange columns

Design of experiments applications in bioprocessing: Chromatography process development using split design of experiments

Development of a chromatographic step in a time and resource efficient manner remains a serious bottleneck in protein purification. Chromatographic performance typically depends on raw material attributes, feed material attributes, process factors, and their interactions. Design of experiments (DOE) based process development is often chosen for this purpose. A challenge is, however, in performing a DOE with such a large number of process factors. A split DOE approach based on process knowledge in order to reduce the number of experiments is proposed. The first DOE targets optimizing factors that are likely to significantly impact the process and their effect on process performance is unknown. The second DOE aims to fine-tune another set of interacting process factors, impact of whom on process performance is known from process understanding. Furthermore, modeling of a large set of output response variables has been achieved by fitting the output responses to an empirical equation and then using the parametric constants of the equation as output response variables for regression modeling. Two case studies involving hydrophobic interaction chromatography for removal of aggregates and cation exchange chromatography for separation of charge variants and aggregates have been utilized to illustrate the proposed approach. Proposed methodology reduced total number of experiments by 25% and 72% compared to a single DOE based on central composite design and full factorial design, respectively. The proposed approach is likely to result in a significant reduction in resources required as well as time taken during process development. © 2018 American Institute of Chemical Engineers Biotechnol. Prog., 35: e2730, 2019.

Ion-exchange chromatography for the characterization of biopharmaceuticals

Ion-exchange chromatography (IEX) is a historical technique widely used for the detailed characterization of therapeutic proteins and can be considered as a reference and powerful technique for the qualitative and quantitative evaluation of charge heterogeneity. The goal of this review is to provide an overview of theoretical and practical aspects of modern IEX applied for the characterization of therapeutic proteins including monoclonal antibodies (Mabs) and antibody drug conjugates (ADCs). The section on method development describes how to select a suitable stationary phase chemistry and dimensions, the mobile phase conditions (pH, nature and concentration of salt), as well as the temperature and flow rate, considering proteins isoelectric point (pI). In addition, both salt-gradient and pH-gradient approaches were critically reviewed and benefits as well as limitations of these two strategies were provided. Finally, several applications, mostly from pharmaceutical industries, illustrate the potential of IEX for the characterization of charge variants of various types of biopharmaceutical products.

Can the theory of gradient liquid chromatography be useful in solving practical problems?

Advances in the theory of gradient liquid chromatography and their practical impacts are reviewed. Theoretical models describing retention in reversed-phase, normal-phase and ion-exchange modes are compared. Main attention is focused on practically useful models described by two- or three-parameter equations fitting the experimental data in the range of mobile phase composition utilized for sample migration during gradient elution. The applications of theory for gradient method development, optimization and transfer are addressed. The origins and possibilities for overcoming possible pitfalls are discussed, including the effects of the instrumental dwell volume, uptake of mobile phase components on the column and size of the sample molecules. Special attention is focused on gradient separations of large molecules.

Theory of peak capacity in gradient elution

Peak capacity is the best measure of the performance of a gradient separation. In this paper, the theory of peak capacity for the standard operating conditions of reversed-phase and ion-exchange chromatography is outlined. The influence of the operating conditions on the peak capacity of a separation are discussed. Finally, bandspreading phenomena in gradient chromatography are analyzed.

Prediction of protein retention at gradient elution conditions in ion-exchange chromatography

This work presents a prediction procedure for protein retention in ion-exchange chromatography, where two linear gradient experiments of different length give the protein retention time at other linear gradients. The procedure predicts the retention time of early and late eluting proteins with similar precision and predictions by extrapolation deviate approximately 3% or less from the experimental retention times. By using the ionic strength, this procedure predicts protein retention times obtained with divalent ions in the eluent more accurately than a well-established procedure that uses the protein co-ion concentration.

Influence of temperature on the retention behaviour of proteins in cation-exchange chromatography

The chromatographic behaviour of several amino acid derivatives, peptides and proteins has been investigated with the so called "tentacle-type" LiChrospher-100 SO3- adsorbent and an analogous poly(2-sulphoethylaspartamide) cation-exchange adsorbent, PolySulphoethyl A. In particular, the dependences of the retention properties of a range of biosolutes on temperature and the chromatographic residence time were evaluated with the objective of gaining further insight into the influence of ligand type and flexibility and the role of solute conformation on the chromatographic behaviour of proteins with these two strong cation-exchange chromatographic adsorbents. The results indicate that significant differences in the chromatographic retention behavior between proteins and low-molecular-mass solutes occur as a function of temperature and the type of co- and counter ions present in the mobile phase with both adsorbents. Moreover, the dependences of the Zc and log Kc values on temperature for most of the proteins examined exhibited significant changes in magnitude between 4 and 75 degrees C, whilst no equivalent changes were evident for low-molecular-mass solutes. With the "tentacle-type" LiChrospher-1000 SO3- adsorbent at higher temperatures, e.g., at 75 degrees C, most of the proteins could still be eluted although several exhibited very large increases in their retention parameters. With the PolySulphoethyl A adsorbent, on the other hand, none of the proteins examined were eluted at 75 degrees C. The results moreover indicate that hydrophobic interactions play an increasingly important role in protein retention with both types of ion-exchange adsorbents at higher temperatures, but are more dominant with the PolySulphoethyl A ligand. In general, the Zc values for the proteins with the "tentacle-type" LiChrospher-1000 SO3- adsorbent were greater than those obtained with the PolySulphoethyl A adsorbent, suggesting that the "tentacular" ligands present on this strong cation-exchange adsorbent interact with protein molecules through larger contact areas. Collectively, these investigations provide further support for the concept that the adsorption behaviour of proteins with the "tentacle-type" LiChrospher-1000 SO3- adsorbent and similar types of "tentacular" ligand systems involves a multilayer dissolution mechanism with the protein interacting with a more diffuse or extended Donnan double layer in the ion-exchange environment, resulting in multi-site binding processes.

Separation and purification of oligonucleotides using a new bonded-phase packing material

We describe a new bonded-phase packing material, based upon surface-stabilised microparticulate silica, suitable for the rapid separation and purification of oligonucleotides. Columns packed with this material were demonstrated to give rapid separations of individual oligonucleotide species of up to 44 base units with high purity; agarose gel electrophoresis showed that the products were essentially single bands, with only trace quantities of the (n-1)-mer present. Baseline resolution of the desired oligomer from (n +/- 1)-mer was achieved under preparative loading conditions, where up to 200-300 micrograms of oligonucleotide could be separated. The separation was essentially independent of structure or sequence of the oligonucleotides. The retention mechanism of the oligonucleotides was investigated, and the results used to determine the optimum column configuration and separation conditions.

High-performance liquid chromatography of amino acids, peptides and proteins. XCVIII. The influence of different displacer salts on the bandwidth properties of proteins separated by gradient elution anion-exchange chromatography

The influence of eight different displacer salts on the bandwidth properties of four globular proteins separated by a high-performance anion-exchange chromatography has been investigated. Proteins were eluted under gradient conditions with a range of alkali metal halide salts, in which the anion and cation were varied in the series F-, Cl- and Br- and Li+, Na+ and K+, respectively. The experimentally observed bandwidths (sigma v,exp) were found to deviate significantly from peak widths (sigma v,calc) predicted on the basis of plate theory for small molecules. For data accumulated under conditions of varied gradient time and constant flow-rate the solute bandwidth ratios (sigma v,exp/sigma v,calc) increased in the order Br- less than Cl- less than F- at low values of the gradient steepness parameter, b, or increasing column residence times. In addition, systematic changes in the cation influenced the bandwidth ratios (sigma v,exp/sigma v,calc) in the order K+ less than Na+ less than Li+. Significant deviations between predicted and observed bandwidth values were also observed under elution conditions of constant gradient time and varied flow-rate. The results of the present study further demonstrate the complex nature of the interaction between protein solutes and coulombic chromatographic surfaces.

High-performance liquid chromatography of amino acids, peptides and proteins. LXXXIX. The influence of different displacer salts on the retention properties of proteins separated by gradient anion-exchange chromatography

The influence of eight different displacer salts on the retention properties of four globular proteins, ranging in molecular weight from 14,000 to 43,000, was investigated by using the Mono-Q strong-anion-exchange resin as the stationary phase. Proteins were eluted under gradient conditions with a range of alkali metal halides to vary systematically the anion and cation species in the series F-, Cl-, and Br- and Li+, Na+ and K+. Protein Zc values (i.e. slopes of the ion-exchange retention plots, derived from the dependency of the logarithmic capacity factor log k' on the concentration of the ionic displacer) generally increased when both the anion and cation were either chaotropic, e.g. KBr, or kosmotropic, e.g. NaF, in nature. Conversely, Zc values decreased when the displacer salt contained an anion-cation combination of a chaotropic and a kosmotropic ion, e.g. KF. These results indicate that the lyotropic properties of salts are additive in their effect on the interactive properties of proteins in anion-exchange chromatography. The Zc values were also found to depend on the manner in which the ionic strength was manipulated to affect elution, i.e. isocratic or gradient change in concentration of the displacing salt. Thus, isocratic experiments and gradient experiments with varied gradient time or varied flow-rate were observed to result in log k' versus log l/c dependencies with non-coincident Zc values. The relationship between protein Zc values, the electrostatic contact area or ionotope, Ac, and the electrostatic potential of the protein surface psis, is discussed.

Evaluation of advanced silica packings for the separation of biopolymers by high-performance liquid chromatography. VI. Design, chromatographic performance and application of non-porous silica-based anion exchangers

The linear solvent strength model of Snyder was applied to describe fast protein separations on 2.1-micron non-porous, silica-based strong anion exchangers. It was demonstrated on short columns packed with these anion exchangers that (i) a substantially higher resolution of proteins and nucleotides was obtained at gradient times of less than 5 min than on porous anion exchangers; (ii) the low external surface area of the non-porous anion exchanger is not a critical parameter in analytical separations and (iii) microgram-amounts of enzymes of high purity and full biological activity were isolated.

High-performance liquid chromatography of amino acids, peptides and proteins. LXXXVII. Comparison of retention and bandwidth properties of proteins eluted by gradient and isocratic anion-exchange chromatography

The high-performance ion-exchange gradient-elution behaviour of a range of globular proteins has been investigated, using a strong anion exchanger as the stationary phase and sodium chloride as the displacer salt. Deviations were observed between the Zc values obtained from isocratic experiments and from gradient experiments with varied gradient time and varied flow-rate. These results indicate that theoretical treatments which relate gradient and isocratic elution processes do not adequately describe the retention behaviour of protein solutes separated by ion-exchange methods. Furthermore, the experimentally observed bandwidths deviated significantly from values predicted on the basis of plate theory for low-molecular-weight molecules. The significance of these results is discussed in terms of the influence of experimental parameters on the ability of particular electrostatically interactive areas on the surface of protein solutes to control the thermodynamic and kinetic properties of these polyelectrolyte molecules during ion-exchange chromatographic processes.

High-performance liquid chromatography of amino acids, peptides and proteins. LXXXVIII. Calculation of the average distance between protein solutes and the stationary phase during isocratic anion-exchange chromatography

This investigation deals with protein retention behaviour in high-performance anion-exchange chromatography in terms of the average distance of approach between the protein solute and the positively charged anion-exchange stationary-phase surface. The theoretical treatment is based on a modified Debye-Hückel theory for spherical impenetrable ions, where the electrostatic potential energy has been related to the chromatographic capacity factor, k'. Results are presented for three globular proteins, eluted isocratically from a Mono-Q strong anion-exchange resin with sodium chloride as the displacer salt by a mobile phase with pH in the range 5.50-9.60. Analysis of experimental retention data indicates that topographically predefined, charged regions on the protein surface, called ionotopes, control the orientation and approach distance of the protein solute.

Preparative separation of peptide and protein samples by high-performance liquid chromatography with gradient elution. II. Experimental examples compared with theory

Craig simulations of mass-overloaded gradient elution reported in the preceding paper have been extended to the case of non-Langmuir isotherms. Isotherms were selected that appear to be characteristic of peptide and protein samples in reversed-phase high-performance liquid chromatography. The dependence of bandwidth on sample size and gradient conditions was examined by Craig simulation and compared with experimental data for 13 different experimental systems involving four different proteins. There is a good correspondence between simulations and experimental data, and it seems possible to quantitatively predict bandwidth and resolution as a function of small-sample retention data, experimental conditions, and sample size. A systematic approach for designing the preparative or process-scale separation of protein mixtures by reversed-phase gradient elution is proposed.

Resolution of proteins in linear gradient elution ion-exchange and hydrophobic interaction chromatography

The following equation was derived for the resolution, Rs, of proteins in linear gradient elution ion-exchange and hydrophobic interaction chromatography: Rs alpha [(column length.molecular diffusivity)/(slope of the gradient.column gel volume.linear mobile phase velocity.particle diameter2)]1/2. Experimental results obtained under a wide range of experimental conditions (column length, 1-30 cm; particle diameter, 37-92 micron; linear mobile phase velocity, 0.2-3.5 cm/min; temperature, 15-35 degrees C) have shown that this relationship is valid except for very short columns with a shallow slope of the gradient and for low flow-rates. The relationship is very useful for scaling-up the linear gradient elution of proteins from data obtained with small columns.

General strategies in the separation of proteins by high-performance liquid chromatographic methods

General fractionation strategies for the high-resolution purification of proteins are described. The impact of different separation parameters and resolution optimisation approaches with tandem-based systems on retention and recovery behaviour is reviewed. Procedures for the successful linkage of different chromatographic steps into a preferred sequence of operations are discussed in terms of the underlying principles and modus operandi of high-performance liquid chromatographic purification of proteins and related biomacromolecules.

Use of high-performance size-exclusion, ion-exchange, and hydrophobic interaction chromatography for the measurement of protein conformational change and stability

Size-exclusion, ion-exchange, and hydrophobic interaction chromatographic techniques able to detect conformational changes induced by urea were developed for three globular proteins: bovine serum albumin, lysozyme, and trypsin. Alterations in tertiary structure were manifest chromatographically by highly reproducible changes in peak height, retention, and appearance of multiple peaks. Denaturation equilibria and kinetics obtained by classical physical methods, such as fluorescence intensity measurements for bovine serum albumin and enzyme activity for trypsin, could be correlated to particular changes in chromatographic behavior. The chromatographic methods utilized were selective toward specific structural changes and monitored independent denaturation steps via multiphasic kinetics. The correlation of chromatographic behavior to both independent measures of conformational changes and to assays which measure loss in biological activity, in the case of select proteins indicates that non-denaturing high-performance liquid chromatography can be a useful tool to detect and quantitate perturbations of protein three dimensional structure which result in a loss in biological activity.

Evaluation of advanced silica packings for the separation of biopolymers by high-performance liquid chromatography. V. Performance of non-porous monodisperse 1.5-microns bonded silicas in the separation of proteins by hydrophobic-interaction chromatography

Non-porous monodisperse 1.5-microns silicas were allowed to react with (A) and (B) N-acetylaminopropyltriethoxysilane to generate bonded phases useful in high-performance hydrophobic-interaction chromatography (HIC). Differences in the selectivity were observed between the amide and the ether phase. Peak capacities between 10 and 30 were achieved for several proteins with the amide and ether phase packed into columns of 36 X 8 mm I.D. and elution of the proteins under chromatographic conditions in which the gradient volume, VG, was held constant by varying the gradient time between 20 and 2.5 min and the flow-rate between 0.5 and 4.0 ml/min. The S values derived from the dependences of log k' on the volume fraction of the low ionic strength buffer, phi b, were of the same magnitude as reported for porous HIC silicas and showed a dependence on the molecular weight of the protein. Using these HIC stationary phases based on non-porous 1.5-microns supports, fast separations (less than 5 min) could be carried out with high biological recoveries.

Predicting bandwidth in the high-performance liquid chromatographic separation of large biomolecules. I. Size-exclusion studies and the role of solute stokes diameter versus particle pore diameter

Column plate numbers, N, were measured for 12 different proteins as a function of mobile phase flow-rate in two gel filtration systems (either denaturing or non-denaturing conditions). These data were used to extend a previous model that predicts bandwidths in reversed-phase and ion-exchange chromatography. Restriction of diffusion of large molecules within column packing pores is now defined more precisely, with a single relationship describing this effect for both reversed-phase and size-exclusion chromatography (SEC) (and presumably other high-performance liquid chromatography systems). Separations by gel filtration (SEC) are now included in our general model. A total of 17 flow-rate studies were carried out, involving different proteins, columns and/or mobile phase conditions (denaturing or non-denaturing). Comparisons of plate numbers predicted by the model with experimental values were satisfactory in 15 out of 17 cases. The remaining two cases appear to represent "non-well-behaved" systems, where experimental bandwidths were higher than predicted values by more than 20%. Initial attempts at understanding the origin of these non-ideal effects are described.

Predicting bandwidth in the high-performance liquid chromatographic separation of large biomolecules. II. A general model for the four common high-performance liquid chromatography methods

A general model for describing gradient elution separations of peptides and proteins by reversed-phase high-performance liquid chromatography (HPLC) has been presented previously. This model has now been modified so that it can be applied to any of the four HPLC methods used for separating biological macromolecules: reversed-phase, ion-exchange, hydrophobic-interaction and size-exclusion chromatography, carried out in either an isocratic or gradient elution mode. The role of sample molecule structure and the particular column used has been further studied, so that previous empirical parameters for different column/sample choices can now be estimated from three physical properties of the sample and the column: sample molecular weight, native vs. denatured sample, column packing pore diameter. This eliminates much of the empiricism of our preceding model, and minimizes the number of experimental runs now required in order to apply the model in practice. The final model has been tested for several hundred runs involving peptides and proteins in the molecular weight range 600-162,000, all four of these HPLC methods, in both isocratic and gradient elution modes, and using data from several different laboratories (including our own). The model is able to predict bandwidth in HPLC separations of proteins and peptides with an accuracy of +/- 17% (1 standard deviation), for the case of "well-behaved" separations. Separations that are not "well-behaved" will give wider bands than predicted by the model.