Predictability of chromatographic protein separations

Mechanism modeling and application of Salvia miltiorrhiza percolation process

Percolation is a common extraction method of food processing industry. In this work, taking the percolation extraction of salvianolic acid B from Salvia miltiorrhiza (Salviae Miltiorrhizae Radix et Rhizoma) as an example, the percolation mechanism model was derived. The volume partition coefficient was calculated according to the impregnation. experiment. The bed layer voidage was measured by single-factor percolation experiment and the internal mass transfer coefficient was calculated by the parameters obtained by fitting the impregnation kinetic model. After screening, the Wilson and Geankoplis, and Koch and Brady formulas were used to calculate the external mass transfer coefficient and the axial diffusion coefficient, respectively. After substituting each parameter into the model, the process of percolation of Salvia miltiorrhiza was predicted, and the coefficient of determination R² was all greater than 0.94. Sensitivity analysis was used to show that all the parameters studied had a significant impact on the prediction effect. Based on the model, the design space including the range of raw material properties and process parameters was established and successfully verified. At the same time, the model was applied to the quantitative extraction and endpoint prediction of the percolation process.

Model-based development of an on-column PEGylation process

On-column PEGylation appears as an interesting alternative to classical solution reaction for more selective synthesis of the targeted mono-PEGylated protein. Indeed, it has the potential to inhibit the formation of the multi-PEGylated species and provide site selectivity by restricting the coupling reaction to fewer reaction sites.

Relation between the particle size distribution and the kinetic performance of packed columns

To study the influence of the particle size distribution (PSD), we measured the chromatographic performance of a series of sub-2 microm particle high performance liquid chromatography (HPLC) columns packed with four different particle mixtures having a purposely imposed different size distribution. Using the reduced kinetic plot representation by plotting the separation impedance (E(0)) versus the plate number ratio (N(opt)/N), the different columns could be classified according to their chromatographic performance without the need to specify a mean particle diameter or a molecular diffusion coefficient, as is needed in the classical reduced plate height and flow resistance analysis. The present analysis shows that it is not so much the width or span of the particle size distribution, but rather the presence of fines that greatly determines the chromatographic performance of particulate columns.

Pore size distributions of ion exchangers and relation to protein binding capacity

The pore structure of chromatographic media directly influences macromolecular transport and adsorption, and consequently separation resolution and loading capacity in chromatographic separations. The pore size distribution (PSD) is therefore a central structural characteristic of chromatographic materials and a critical determinant of chromatographic behavior. In this work the PSDs of a set of commercial anion exchangers were determined by inverse size-exclusion chromatography (ISEC). The PSDs were further utilized to develop relations to functional properties of adsorbents, such as intraparticle diffusivity, and static and dynamic binding capacities. We find that the detailed PSD is useful in semi-quantitative understanding of chromatographic behavior. However, more accurate prediction of column behavior requires more thorough knowledge of the pore structure, specifically the connectivity of the pore network, as well as improved understanding of the function of grafted resins.

Effects of pore structure and molecular size on diffusion in chromatographic adsorbents

Two computational approaches, namely Brownian dynamics and network modeling, are presented for predicting effective diffusion coefficients of probes of different sizes in three chromatographic adsorbents, the structural properties of which were determined previously using electron tomography. Three-dimensional reconstructions of the adsorbents provide detailed, explicit characteristics of the pore network, so that no assumptions have to be made regarding pore properties such as connectivity, pore radius and pore length. The diffusivity predictions obtained from the two modeling approaches were compared to experimental diffusivities measured for dextran and protein probes. Both computational methods captured the same qualitative results, while their predictive capabilities varied among adsorbents.

Evaluating and monitoring the packing behavior of process-scale chromatography columns

The packing characteristics of process-scale chromatography columns were evaluated using the responses to conductivity-based pulse and step inputs derived from tracer experiments and in-process transitions (i.e. column equilibration and regeneration steps). Characteristics of the measured residence time distributions (RTDs) were quantified by statistical moments and using the equations derived from the Gaussian model. The first and second moments calculated from in-process step transitions for multiple runs were in good agreement with those moments calculated from the pulse-input experiments conducted immediately after column packing. This indicates that most of the time the bed behavior at the time of packing is consistent with that at the time of operation. Due to the significant resistance to protein mass transfer inside the particles, estimated plate heights for protein solutes are expected to be much greater than those observed from the experiments using saltbased tracers. Thus, the column efficiency derived from salt-based experiments can be a useful measure of packing consistency rather than a significant parameter influencing the outcome of protein separations.

Bioseparations

Here we review key applications of separation technology in applied biology. We first sketch out the field as a whole, but then narrow our scope to the processing of fermentation products, particularly to high-value biologicals such as proteins and nucleotides. We go on to provide a qualitative overview describing the importance and general nature of this large field, major trends, and the strategies that have proven most fruitful in evolving effective separation and purification processes. We then give a detailed description of individual separations equipment and the principles governing their operation. We concentrate throughout on making the available literature accessible to the reader; we provide what is hoped to be a representative set of basic references. However, these references, in turn, include some that suggest promising new developments as well as a number of more specialized reviews. We hope that our overall result provides the reader with access to the most relevant literature.

Entropic interaction chromatography: Separating proteins on the basis of size using end-grafted polymer brushes

Partitioning of a macromolecule into the interfacial volume occupied by a grafted polymer brush decreases the configurational entropy (DeltaSbrush(c)) of the terminally attached linear polymer chains due to a loss of free volume. Self-consistent field theory (SCF) calculations are used to show that DeltaSbrush(c) is a strong function of both the size (MWp) of the partitioning macromolecule and the depth of penetration into the brush volume. We further demonstrate that the strong dependence of DeltaSbrush(c) on MWp provides a novel and powerful platform, which we call entropic interaction chromatography (EIC), for efficiently separating mixtures of proteins on the basis of size. Two EIC columns, differing primarily in polymer grafting density, were prepared by growing a brush of poly(methoxyethyl acrylamide) chains on the surface of a wide-pore (1,000-A pores, 64-microm diameter rigid beads) resin (Toyopearl AF-650M) bearing surface aldehyde groups. Semipreparative 0.1-L columns packed with either EIC resin provide reduced-plate heights of 2 or less for efficient separation of globular protein mixtures over at least three molecular-weight decades. Protein partitioning within these wide-pore EIC columns is shown to be effectively modeled as a thermodynamically controlled process, allowing partition coefficients (K(P)) and elution chromatograms to be accurately predicted using a column model that combines SCF calculation of K(P) values with an equilibrium-dispersion type model of solute transport through the column. This model is used to explore the dependence of column separation efficiency on brush properties, predicting that optimal separation of proteins over a broad MWp range is achieved at low to moderate grafting densities and intermediate chain lengths.

Modeling ion-exchange adsorption of proteins in a spherical particle

This paper presents a simple model of single- and multicomponent protein adsorption in a spherical particle. The model includes radial diffusion of salt and protein in the liquid phase coupled to adsorption by an ion-exchange mechanism described by the steric mass action isotherm. The molecular diffusivities of the protein and salt are reduced in the model by a factor which accounts for the tortuous nature of the pores and pore constrictions. The model parameters are selected from published values in the chromatographic literature. Of particular interest are the observations of induced salt gradients during protein adsorption and of multicomponent displacement when more that one protein is adsorbed simultaneously. These results cannot be predicted on the basis of the traditional Langmuir isotherm or other currently available descriptions of adsorption. The use of such a model during stationary phase design is discussed.

Adsorptive membrane chromatography for purification of plasmid DNA

Adsorptive membranes were investigated for the downstream processing of plasmid DNA by quantifying both separation efficiencies and adsorption uptake with the anion-exchange membranes. Separation efficiencies of the 10-ml Mustang-Q were measured using pulses of 6.1-kilo base pair plasmid DNA and lysozyme tracers, and comparing the responses for both conventional and reverse-flow operation. The plasmid exhibited nearly 200 plates/cm, almost as high efficiency as the protein despite the large difference in size. This behavior contrasts strongly with typical behavior for spherical porous particle packings, which predicted large decreases in efficiency with increases in tracer size. Batch adsorption isotherms for the 6.1-kilo base pair plasmid on small sheets of anion-exchange membranes at various ionic strengths showed high capacities for very large biomolecules. The maximum binding capacity for the membrane unit was calculated as 10 mg plasmid/ml, an order of magnitude greater than typical values reported for porous beads.

Evaluation of protein-A chromatography media

In process-scale antibody purification, protein-A affinity chromatography is commonly used as the initial purification step. In this paper, two different protein-A media were evaluated. These adsorbents have a porous glass backbone with different pore sizes: 700 A and 1000 A. Adsorption equilibrium data of human immunoglobulins on these media were measured via a batch technique and correlated using the Langmuir isotherm model. A larger static capacity was found for the smaller pore size material, which is probably a result of the larger specific surface area and associated higher ligand concentration. The protein uptake kinetics were also obtained via a stirred tank experiment using different initial protein concentrations. A surface layer model was used to represent the protein uptake by the media and to estimate values of a concentration-independent effective diffusivity within the particle. Experimental breakthrough curves were also obtained from packed beds operated under different conditions. Calculated breakthrough profiles were found to be in good agreement with the experimental results. Experimental breakthrough data were used to determine the dependence of the dynamic capacity of the media as a function of the fluid residence time. A larger dynamic capacity was also found for the smaller pore size media. The permeability of large scale packed beds was also reported and used in conjunction with the dynamic capacity to calculate the process production rate.

Performance and scale-up of adsorptive membrane chromatography

Separation efficiency and scalability of Pall Corporation's new Mustang stacked membrane chromatographic devices were investigated, using both the 10-ml and l(-1) models and comparing the responses of tracer pulses obtained for conventional and reverse-flow operation. Tracers included AMP, lysozyme, and thyroglobulin, which vary in relative molecular mass from less than 1000 up to 650000. Both devices showed marked insensitivity to tracer size and flow-rate and gave sharper peaks than would have been expected from conventional 15-microm bead packings. However, reverse-flow peaks were always significantly sharper than those for conventional operation, and the differences were ascribed primarily to non-uniform header residence times. Numerical simulations of the macroscopic flow confirmed that this was indeed the case. This problem was much less pronounced for the l(-1) device so scale-up is conservative.

Characterizing the performance of industrial-scale columns

The performance of a large commercial chromatographic column was investigated using a short pulse of a tracer and an extension of the reverse-flow technique. This technique permits separate determination of the unavoidable irreversible microscopic processes and the reversible effects of flow maldistribution, and allows for the separation of flow maldistribution in the flow distributors from flow maldistribution inside the packed bed. This analysis was performed on a 0.44 m Millipore IsoPak column using Cellufine GC 700, cellulosic-based media with an average particle diameter of 75 microm, for the stationary phase. The column efficiency was quantified by analysis of the effluent curve from a short pulse of a 5% aqueous acetone tracer. The study examined behavior of beds of different lengths (10-24 cm) and beds packed from different slurry concentrations (10-75% v/v). The slurry-packed columns were very uniform, and no significant macroscopic flow maldistribution was observed inside the column. The observed bed plate heights conformed to the predictions of available one-dimensional continuum models. Dispersion in the flow distributors was significant, corresponding to 15-25% of the intracolumn dispersion when the full 24 cm available bed length was used and a proportionally larger increase for shorter bed lengths. Thus, the headers are shown to produce a significant increase in the observed plate height.

Performance comparison of suspended bed and batch contactor chromatography

In some applications, the purification and recovery of biomolecules is performed via a cascade of batch adsorption and desorption stages using agitated contactors and related filtration devices. Suspended bed chromatography is a recent process-scale innovation that is applicable to these separations. This hybrid technique exploits the benefits of combining batch adsorption in an agitated contactor with elution in an enclosed column system. To some extent, the process is similar to batch contactor chromatography but can be fully contained and significantly quicker. The process has two steps; first the fluid containing the sample is mixed with the adsorbent in a stirred tank. Second, the slurry suspension is transferred directly into a specialized column, such as an IsoPak column. The media with the adsorbed product is formed as a packed bed, whilst the suspension liquid is passed out of the column. The product is then eluted from the packed bed utilizing standard column-chromatography techniques. The performance of the suspended bed and the agitated contactor operations are demonstrated both by full-scale experimental results and process simulations. The purification of ovalbumin from a hen-egg white feedstock by anion-exchange chromatography was used as a case study in order to prove the concept. With the availability of both pump-packed systems and shear-resistant media, suspended bed chromatography is a better alternative for a range of applications than the traditional batch separations using agitated contactors.

Refining the scale-up of chromatographic separations

The use of heavily loaded columns and complex processing conditions makes scale-up of chromatographic separations a non-trivial process. The wide ranges of process conditions that must be investigated demands that a large number of preliminary experiments must usually be made in small columns and laboratory-scale work stations. These preliminary data can be biased by improper column packing, poor distributors and dispersion in auxiliary apparatus, and it is important to understand these disturbing factors in detail. Moreover, it is precisely at this macroscopic level that our understanding of the chromatographic process is weakest, for large columns as well as small. This paper addresses three of these factors: Efficient elimination of peripheral effects and characterization of both header flow distribution and packing non-uniformity. This will be done using a variety of experimental and analytical approaches including nuclear magnetic resonance imaging, computational fluid dynamics and mass transfer, and careful experimentation.

Comparing steady counterflow separation with differential chromatography

Separation of closely related solutes by steady solid-fluid counterflow is compared with differential separation in a fixed chromatographic bed. Analogous expressions for exit concentration and mean residence time in the two systems are presented. A counterpart to chromatographic resolution is derived for binary steady counterflow separations. Estimated counterflow savings in product-concentration dilution, solvent volume requirement and solid-phase volume requirement obtained with these expressions relative to comparable chromatographic operations are compared with experimental results from adsorptive, simulated moving beds. Analysis of a size-exclusion protein separation suggests counterflow substantially decreases solvent and resin usage relative to conventional, batch operation.