Chromatographic media for bioseparation

Ring-opening metathesis polymerization-derived large-volume monolithic supports for reversed-phase and anion-exchange chromatography of biomolecules

Preparative-scale monolithic columns up to 433.5 mL in volume were prepared via transition metal-catalyzed ring-opening metathesis polymerization (ROMP) from norborn-2-ene (NBE) and trimethylolpropane-tris(5-norbornene-2-carboxylate) (CL) using the 1(st)-generation Grubbs initiator RuCl(2)(PCy(3))(2)(CHPh) (Cy = cyclohexyl) (1) in the presence of a macro- and microporogen, i.e. of 2-propanol and toluene. To prepare large-volume monoliths, bulk polymerizations were completed within borosilicate or PEEK column formats with diameters in the range of 3 to 49 mm. The pore structure of the large-volume monoliths was investigated by electron microscopy and inverse-size exclusion chromatography (ISEC), respectively. Monolithic columns with inner diameters (I.D.s) in the range of 10-49 mm were tested for the separation of a mixture of five proteins, i.e., insulin, cytochrome C, lysozyme, conalbumin, and β-lactoglobulin. Preparative separation of these proteins was achieved within less than 12 min in a 433.5 mL monolithic column by applying gradient elution in the RP-HPLC mode. Furthermore, weak and strong anion exchangers were prepared via post-synthesis grafting of bicyclo[2.2.1]hept-5-en-2-yl-methyl-N,N-dimethylammonium hydrochloride (4) and bicyclo[2.2.1]hept-5-en-2-ylmethyl-N,N,N-trimethylammonium iodide (5), respectively. The weak and strong anion exchangers were used for the preparative-scale separation of 5'-phosphorylated oligodeoxythymidylic acid fragments of d[pT](12-18) at pH values ranging from 5 to 9.

Sample displacement chromatography as a method for purification of proteins and peptides from complex mixtures

Sample displacement chromatography (SDC) in reversed-phase and ion-exchange modes was introduced approximately twenty years ago. This method takes advantage of relative binding affinities of components in a sample mixture. During loading, there is a competition among different sample components for the sorption on the surface of the stationary phase. SDC was first used for the preparative purification of proteins. Later, it was demonstrated that this kind of chromatography can also be performed in ion-exchange, affinity and hydrophobic-interaction mode. It has also been shown that SDC can be performed on monoliths and membrane-based supports in both analytical and preparative scale. Recently, SDC in ion-exchange and hydrophobic interaction mode was also employed successfully for the removal of trace proteins from monoclonal antibody preparations and for the enrichment of low abundance proteins from human plasma. In this review, the principals of SDC are introduced, and the potential for separation of proteins and peptides in micro-analytical, analytical and preparative scale is discussed.

Modeling of dispersion in a polymeric chromatographic monolith

Dispersion in a commercial polymeric monolith was simulated on a sample geometry obtained by direct imaging using high-resolution electron microscopy. A parallelized random walk algorithm, implemented using a velocity field obtained previously by the lattice-Boltzmann method, was used to model mass transfer. Both point particles and probes of finite size were studied. Dispersion simulations with point particles using periodic boundaries resulted in plate heights that varied almost linearly with flow rate, at odds with the weaker dependence suggested by experimental observations and predicted by theory. This discrepancy resulted from the combined effect of the artificial symmetry in the velocity field and the periodic boundaries implemented to emulate macroscopic column lengths. Eliminating periodicity and simulating a single block length instead resulted in a functional dependence of plate heights on flow rate more in accord with experimental trends and theoretical predictions for random media. The lower values of the simulated plate heights than experimental ones are attributed in part to the presence of walls in real systems, an effect not modeled by the algorithm. On the other hand, analysis of transient dispersion coefficients and comparison of lateral particle positions at the entry and exit hinted at non-asymptotic behavior and a strong degree of correlation that was presumably a consequence of preferential high-velocity pathways in the raw sample block. Simulations with finite-sized probes resulted in particle trajectories that frequently terminated at narrow constrictions of the geometry. The amount of entrapment was predicted to increase monotonically with flow rate, evidently due to the relative contributions to transport by convection that carries particles to choke-points and diffusion that dislodges these entrapped particles. The overall effect is very similar to a flow-dependent entrapment phenomenon previously observed experimentally for adenovirus.

Protein adsorption and transport in polymer-functionalized ion-exchangers

A wide variety of stationary phases is available for use in preparative chromatography of proteins, covering different base matrices, pore structures and modes of chromatography. There has recently been significant growth in the number of such materials in which the base matrix is derivatized to add a covalently attached or grafted polymer layer or, in some cases, a hydrogel that fills the pore space. This review summarizes the main structural and functional features of ion exchangers of this kind, which represent the largest class of such materials. Although the adsorption and transport properties may generally be used operationally and modeled phenomenologically using the same methods as are used for proteins in conventional media, there are noteworthy mechanistic differences in protein behavior in these adsorbents. A fundamental difference in protein retention is that it may be portrayed as partitioning into a three-dimensional polymer phase rather than adsorption at an extended two-dimensional surface, as applies in more conventional media. Beyond this partitioning behavior, however, the polymer-functionalized media often display rapid intraparticle transport that, while qualitatively comparable to that in conventional media, is sufficiently rapid quantitatively under certain conditions that it can lead to clear benefits in key measures of performance such as the dynamic binding capacity. Although possible mechanistic bases for the retention and transport properties are discussed, appreciable areas of uncertainty make detailed mechanistic modeling very challenging, and more detailed experimental characterization is likely to be more productive.

One-pot synthesis and characterization of cross-linked quaternized chitosan microspheres as protein adsorbent

The one-pot synthesis and characterization of cross-linked quaternized chitosan microspheres (CQCM) as a protein adsorbent are presented. First of all, chitosan particles were prepared by spray drying method, and then they were quaternized and cross-linked in turn with glycidyltrimethylammonium (GTMAC) chloride and glutaraldehyde in isopropanol containing 10% water in one-pot. The effect of the reaction temperature, reaction time and the amounts of added GTMAC and glutaraldehyde on the protein adsorption ability of CQCM was investigated. The adsorption behavior of the CQCM prepared in the optimum synthetic conditions was well described by the Langmuir isotherm with maximum adsorption capacity equal to 1424 mg BSA/g dry weight. The particle size ranged from 7.6 to 48.9 μm. The mechanism of adsorption-desorption of BSA to the CQCM was ion-exchange. Finally, the extraction of soybean peroxidase from crude soybean peroxidase solution using the CQCM was performed.

Downstream processing of cell culture-derived virus particles

Manufacturing of cell culture-derived virus particles for vaccination and gene therapy is a rapidly growing field in the biopharmaceutical industry. The process involves a number of complex tasks and unit operations ranging from selection of host cells and virus strains for the cultivation in bioreactors to the purification and formulation of the final product. For the majority of cell culture-derived products, efforts focused on maximization of bioreactor yields, whereas design and optimization of downstream processes were often neglected. Owing to this biased focus, downstream procedures today often constitute a bottleneck in various manufacturing processes and account for the majority of the overall production costs. For efficient production methods, particularly in sight of constantly increasing economic pressure within human healthcare systems, highly productive downstream schemes have to be developed. Here, we discuss unit operations and downstream trains to purify virus particles for use as vaccines and vectors for gene therapy.

Modeling of flow in a polymeric chromatographic monolith

The flow behavior of a commercial polymeric monolith was investigated by direct numerical simulations employing the lattice-Boltzmann (LB) methodology. An explicit structural representation of the monolith was obtained by serial sectioning of a portion of the monolith and imaging by scanning electron microscopy. After image processing, the three-dimensional structure of a sample block with dimensions of 17.8 μm × 17.8 μm × 14.1 μm was obtained, with uniform 18.5 nm voxel size. Flow was simulated on this reconstructed block using the LB method to obtain the velocity distribution, and in turn macroscopic flow properties such as the permeability and the average velocity. The computed axial velocity distribution exhibits a sharp peak with an exponentially decaying tail. Analysis of the local components of the flow field suggests that flow is not evenly distributed throughout the sample geometry, as is also seen in geometries that exhibit preferential flow paths, such as sphere pack arrays with defects. A significant fraction of negative axial velocities are observed; the largest of these are due to flow along horizontal pores that are also slightly oriented in the negative axial direction. Possible implications for mass transfer are discussed.

Chromatographic and traditional albumin isotherms on cellulose: a model for wound protein adsorption on modified cotton

Albumin is the most abundant protein found in healing wounds. Traditional and chromatographic protein isotherms of albumin binding on modified cotton fibers are useful in understanding albumin binding to cellulose wound dressings. An important consideration in the design of cellulosic wound dressings is adsorption and accumulation of proteins like albumin at the solid-liquid interface of the biological fluid and wound dressing fiber. To better understand the effect of fiber charge and molecular modifications in cellulose-containing fibers on the binding of serum albumin as observed in protease sequestrant dressings, albumin binding to modified cotton fibers was compared with traditional and chromatographic isotherms. Modified cotton including carboxymethylated, citrate-crosslinked, dialdehyde and phosphorylated cotton, which sequester elastase and collagenase, were compared for their albumin binding isotherms. Albumin isotherms on citrate-cellulose, cross-linked cotton demonstrated a two-fold increased binding affinity over untreated cotton. A comparison of albumin binding between traditional, solution isotherms and chromatographic isotherms on modified cellulose yielded similar equilibrium constants. Application of the binding affinity of albumin obtained in the in vitro protein isotherm to the in vivo wound dressing uptake of the protein is discussed. The chromatographic approach to assessment of albumin isotherms on modified cellulose offers a more rapid approach to evaluating protein binding on modified cellulose over traditional solution approaches.

Silicon, silica and its surface patterning/activation with alkoxy- and amino-silanes for nanomedical applications

Silica and silicates are widely used in nanomedicine with applications as diverse as medical device coatings to replacement materials in tissue engineering. Although much is known about silica and its synthesis, relatively few biomedical scientists fully appreciate the link that exists between its formulation and its resultant structure and function. This article attempts to provide insight into relevant issues in that context, as well as highlighting their importance in the material’s eventual surface patterning/activation with alkoxy- and organo-silanes. The use of aminosilanes in that context is discussed at some length to permit an understanding of the specific variables that are important in the reproducible and robust aminoactivation of surfaces using such molecules. Recent investigative work is cited to underline the fact that although aminosilanization is a historically accepted mechanism for surface activation, there is still much to be explained about how and why the process works in the way it does. In the last section of this article, there is a detailed discussion of two classical approaches for the use of aminosilanized materials in the covalent immobilization of bioligands, amino-aldehyde and amino-carboxyl coupling. In the former case, the use of the homobifunctional coupler glutaraldehyde is explored, and in the latter, carbodiimides. Although these chemistries have long been employed in bioconjugations, it is apparent that there are still variables to be explored in the processes (as witnessed by continuing investigations into the chemistries concerned). Aspects regarding optimization, standardization and reproducibility of the fabrication of amino functionalized surfaces are discussed in detail and illustrated with practical examples to aid the reader in their own studies, in terms of considerations to be taken into account when producing such materials. Finally, the article attempts to remind readers that although the chemistry and materials involved are ‘old hat’, there is still much to be learnt about the methods involved. The article also reminds readers that although many highly specific and costly conjugation chemistries now exist for bioligands, there still remains a place for these relatively simple and cost-effective approaches in bioligand conjugate fabrication.

Effect of chemical additives on biomass deposition onto beaded adsorbents

Common limitations encountered during the direct recovery of bioproducts from an unclarified feedstock are related to the presence of biomass in such processing systems. Biomass-related effects can be described as biomass-to-support deposition and cell-to-cell aggregation. In this work, a number of chemical additives were screened for their ability to inhibit either biomass deposition, cell aggregation, or a combination of both effects. Several interacting pairs were screened. These were composed of (i) a commercial chromatographic matrix harbouring a variety of ligand types and (ii) intact yeast cells - as a model biomass type. Studies were performed based on partitioning tests, colloid deposition experiments, and sorption performance studies in expanded beds. Results indicated that the coating of anion-exchanger beads with the synthetic polymer PVP 360 alleviated biomass deposition and consequently restored EBA process performance. This behaviour correlated well with calculations performed according to the XDLVO approach: the secondary (interaction) free energy pockets decreased from -230 kT to -100 kT in the absence and in the presence of PVP 360, respectively. Experiments performed in parallel demonstrated that total binding capacity for the model protein (BSA) - under dynamic conditions - remained almost constant (≈ 55.7 kg m(-3)). Other combinations of additives and adsorbents were tested. However, no solution chemistry was able to inhibit biomass deposition onto strong (composite) ion exchangers. Moreover, yeast cell deposition was only marginally decreased when hydrophobic interaction and pseudo-affinity supports were explored. The utilization of non-toxic polymers could help to avoid detrimental biomass deposition during expanded bed adsorption of bioproducts and other direct contact sequestration methods.

Flow-dependent entrapment of large bioparticles in porous process media

The need for purification of biomolecules extends to larger bioparticles as well. For example, virus purification is required for production of many vaccines and gene delivery vectors, and understanding virus removal in porous media is also important in downstream processing of therapeutic proteins and in purification of water in soils. A convective entrapment mechanism for retention of large bioparticles is discussed here based on retention of such bioparticles in pore constrictions at high enough flow rates, even under non-binding conditions. A simple equation to predict whether such entrapment is expected to occur in a given system is derived based on a Péclet number that is proportional to the flow rate and to the cube of the bioparticle diameter. To test the theory, adenovirus was spiked onto chromatographic beds. As expected from the theory, under non-interacting conditions a progressively larger amount of virus becomes trapped with increasing flow rate. The entrapment is reversible upon flow rate reduction, which, within the proposed model, is based on the possibility of diffusive escape from pore constrictions. This mechanism can be exploited for virus purification or removal, and the theory is also consistent with the anecdotal evidence that monoliths and membranes are more difficult to clean than conventional chromatographic beds, especially at high flow rates.

Probing hydrophobic interactions using trajectories of amphiphilic molecules at a hydrophobic/water interface

Individual molecules of fluorophore-labeled alkanoic acids with various chain lengths, BODIPY-(CH(2))(n)-COOH (abbreviated as fl-Cn), were observed to adsorb and move at the methylated fused silica-water interface as a function of temperature using total internal reflection fluorescence microscopy. The statistical analysis of squared-displacement distributions indicated that the molecular trajectories were consistent with a diffusive model involving two intertwined modes. The slower mode, typically responsible for <50% of the molecular diffusion time, had a diffusion coefficient of <0.005 mum(2)/s and could not be distinguished from the apparent motions of immobilized molecules because of the limitations of experimental resolution. The faster mode exhibited diffusion coefficients that increased with temperature for all chain lengths, permitting an Arrhenius analysis. Both the effective activation energies and kinetic prefactors associated with the fast-mode diffusion coefficients increased systematically with chain length for fl-C2 through fl-C10; however, fl-C15 did not follow this trend but instead exhibited anomalously small values of both parameters. These observations were considered in the context of hydrophobic interactions between the adsorbate molecules and the methylated surface in the presence of water. Specifically, it was hypothesized that fl-C2, fl-C4, and fl-C10 adopted primarily extended molecular conformations on the hydrophobic surface. The increases in activation energy and entropy with chain length for these molecules are consistent with a picture of the transition state in which the molecule partially detaches from the surface and exhibits greater conformational freedom. In contrast, the small activation energy and entropy for fl-C15 are consistent with a scenario in which the surface-bound molecule adopts a compact/globular conformation with limited surface contact and conformational flexibility.

Hydrophobic interaction microscopy: mapping the solid/ liquid interface using amphiphilic probe molecules

The adsorption, interfacial mobility, and desorption of amphiphilic molecules are extremely sensitive to the chemical properties of the interface at which they adsorb; we demonstrate here that this sensitivity can be used to map subtle spatial variations of surface hydrophobicity. We have used total internal reflection fluorescence microscopy to observe the dynamic behavior of individual fluorescently labeled fatty acid molecules at the interface between water and a hydrophobically modified fused silica surface. Patterned surfaces were prepared by photodegradation of trimethylsilane-modified surfaces using a contact photomask; the degree of hydrophobic contrast was varied by controlling the dose of ultraviolet radiation. Cumulative images of single-molecule fluorescence, integrated over various exposure times, exhibited structural features consistent with the photopattern, and the fluorescence contrast was systematically related to the hydrophobic contrast. Lateral force microscopy was also used to characterize the patterned surfaces and provided qualitative images when the hydrophobic contrast was relatively high. However, the fluorescent probes provided more sensitive, reproducible, and reliable images of the lateral hydrophobic variations.

A new approach on the purification of recombinant human soluble catechol-O-methyltransferase from an Escherichia coli extract using hydrophobic interaction chromatography

Catechol-O-methyltransferase (COMT) is a significant target in protein engineering due to its role not only in normal brain function but also to its possible involvement in some human disorders. In this work, a new approach was employed for the purification of recombinant human soluble COMT (hSCOMT) using hydrophobic interaction chromatography, as the main isolation method, from an Escherichia coli culture broth. A simplified overall process flow is proposed. Indeed, with an optimized heterologous expression system for recombinant hSCOMT production, such as E. coli, it was possible to produce and recover the active monomeric enzyme directly from the cell crude culture broth either by a freeze/thaw or ultrasonication lysis step. The recombinant enzyme present in the bacterial soluble fraction, exhibited similar affinity for epinephrine (K(m) 276 [215; 337] microM) and the methyl donor (S-adenosyl-L-methionine, SAMe) (K(m) 36 [30; 41]microM) as human SCOMT. After the precipitation step by 55% of ammonium sulphate, a HIC step on the butyl-sepharose resin was found to be highly effective in selectively eluting a range of contaminating key proteins present in the concentrate soluble extract. Consequently, the partially purified eluate from HIC could then be loaded and polished by gel filtration in order to increase the process efficiency. The final product appeared as a single band in sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE). The procedure resulted in a global 10.9-fold purification with a specific activity of 5500 nmol/h/mg of protein. The widespread applicability of the process, here described, to different COMT sources could make this protocol highly useful for all studies requiring purified and active COMT proteins.