Purification of a Large Protein Using Ion-Exchange Membranes

Pore-blocking steric mass-action model for adsorption of bioparticles

The steric mass-action (SMA) model has been widely reported to describe the adsorption of proteins in different types of chromatographic adsorbents. Here in the present work, a pore-blocking steric mass-action model (PB-SMA) was developed for the adsorption of large-size bioparticles, which usually exhibit the unique pore-blocking characteristic on the adsorbent and thus lead to a fraction of ligands in the deep channels physically inaccessible to bioparticles adsorption, instead of being shielded due to steric hindrance by adsorbed bioparticles. This unique phenomenon was taken into account by introducing an additional parameter, Lin, which is defined as the inaccessible ligand densities in the physically blocked pore area, into the PB-SMA model. This fraction of ligand densities (Lin) will be deducted from the total ligand (Lt) for model development, thus the steric factor (σ) in the proposed PB-SMA will reflect the steric shielding effect on binding sites by adsorbed bioparticles more accurately than the conventional SMA model, which assumes that all ligands on the adsorbent have the same accessibility to the bioparticles. Based on a series of model assumptions, a PB-SMA model was firstly developed for inactivated foot-and-mouth disease virus (iFMDV) adsorption on immobilized metal affinity chromatography (IMAC) adsorbents. Model parameters for static adsorption including equilibrium constant (K), characteristic number of binding sites (n), and steric factor (σ) were determined. Compared with those derived from the conventional SMA model, the σ values derived from the PB-SMA model were dozens of times smaller and much closer to the theoretical maximum number of ligands shielded by a single adsorbed iFMDV, indicating the modified model was more accurate for bioparticles adsorption. The applicability of the PB-SMA model was further validated by the adsorption of hepatitis B surface antigen virus-like particles (HBsAg VLPs) on an ion exchange adsorbent with reasonably improved accuracy. Thus, it is considered that the PB-SMA model would be more accurate in describing the adsorption of bioparticles on different types of chromatographic adsorbents.

Design and characterization of an electrochemically-modulated membrane chromatography device

Membrane separations offer a compelling alternative to traditional chromatographic methods by overcoming mass transport limitations. We introduce an additional degree of freedom in modulating membrane chromatography by using metalized membranes in a potential-driven process. Investigating the impact of a gold coating on membrane characteristics, the sputtered gold layer enhances the surface conductivity with stable electrochemical behavior. However, this comes at the expense of reduced permeability, wettability, and static binding capacity (∼ 474 µg g-1 of maleic acid). The designed device displayed a homogenous flow distribution, and the membrane electrodes exhibit predominantly capacitive behavior during potential application. Modulating the electrical potential during the adsorption and desorption phase strongly influenced the binding and elution behavior of anion-exchange membranes. Switching potentials between ±1.0 V vs. Ag/AgCl induces desorption, confirming the process principle. Elution efficiency reaches up to 58 % at -1.0 V vs. Ag/AgCl in the desorption phase without any alteration of the mobile phase. Increasing the potential perturbation ranging from +1.0 V to -1.0 V vs. Ag/AgCl resulted in reduced peak width and improved elution behavior, demonstrating the feasibility of electrochemically-modulated membrane chromatography. The developed process has great potential as a gentle and sustainable separation step in the biotechnological and chemical industry.

Adsorption Performance of a Multimodal Anion-Exchange Chromatography Membrane: Effect of Liquid Phase Composition and Separation Mode

Membrane chromatography is a modern, high-throughput separation method that finds important applications in therapeutic protein purification. Multimodal, salt-tolerant membranes are the most recent innovation in chromatographic membrane adsorbents. Due to the complex structure of their ligands and the bimodal texture of their carriers, their adsorption properties have not been sufficiently investigated. This work deals with the equilibrium and kinetic properties of a multimodal anion-exchange chromatography membrane, Sartobind STIC. Single- and two-component adsorption experiments were carried out with bovine serum albumin (BSA) and salmon DNA as model target and impurity components. The effect of the Hofmeister series ions and ionic strength on the BSA/DNA adsorption was investigated in micromembrane flow experiments. A significant difference was observed between the effects of monovalent and polyvalent ions when strong kosmotropic salts with polyvalent anions acted as strong displacers of BSA. On the contrary, DNA binding was rather high at elevated ionic strength, independent of the salt type. Two-component micromembrane experiments confirmed very high selectivity of DNA binding at a rather low sodium sulfate feed content and at pH 8. The strength of binding was examined in more than a dozen different desorption experiments. While BSA was desorbed relatively easily using high salt concentrations independent of buffer type and pH, while DNA was desorbed only in a very limited measure under any conditions. Separation experiments in a laboratory membrane module were carried out for the feed containing 1 g/L of BSA, 0.3 g/L of DNA, and 0.15 M of sodium sulfate. The negative flow-through mode was found to be more advantageous than the bind-elute mode, as BSA was obtained with 99% purity and a 97% yield. Membrane reuse was investigated in three adsorption-desorption-regeneration cycles.

The use of predictive models to develop chromatography-based purification processes

Chromatography is the workhorse of biopharmaceutical downstream processing because it can selectively enrich a target product while removing impurities from complex feed streams. This is achieved by exploiting differences in molecular properties, such as size, charge and hydrophobicity (alone or in different combinations). Accordingly, many parameters must be tested during process development in order to maximize product purity and recovery, including resin and ligand types, conductivity, pH, gradient profiles, and the sequence of separation operations. The number of possible experimental conditions quickly becomes unmanageable. Although the range of suitable conditions can be narrowed based on experience, the time and cost of the work remain high even when using high-throughput laboratory automation. In contrast, chromatography modeling using inexpensive, parallelized computer hardware can provide expert knowledge, predicting conditions that achieve high purity and efficient recovery. The prediction of suitable conditions in silico reduces the number of empirical tests required and provides in-depth process understanding, which is recommended by regulatory authorities. In this article, we discuss the benefits and specific challenges of chromatography modeling. We describe the experimental characterization of chromatography devices and settings prior to modeling, such as the determination of column porosity. We also consider the challenges that must be overcome when models are set up and calibrated, including the cross-validation and verification of data-driven and hybrid (combined data-driven and mechanistic) models. This review will therefore support researchers intending to establish a chromatography modeling workflow in their laboratory.

Identification of Inhibitors of Fungal Fatty Acid Biosynthesis

Fungal fatty acid (FA) synthase and desaturase enzymes are essential for the growth and virulence of human fungal pathogens. These enzymes are structurally distinct from their mammalian counterparts, making them attractive targets for antifungal development. However, there has been little progress in identifying chemotypes that target fungal FA biosynthesis. To accomplish this, we applied a whole-cell-based method known as Target Abundance-based FItness Screening using Candida albicans. Strains with varying levels of FA synthase or desaturase expression were grown in competition to screen a custom small-molecule library. Hit compounds were defined as preferentially inhibiting the growth of the low target-expressing strains. Dose-response experiments confirmed that 16 hits (11 with an acyl hydrazide core) differentially inhibited the growth of strains with an altered desaturase expression, indicating a specific chemical-target interaction. Exogenous unsaturated FAs restored C. albicans growth in the presence of inhibitory concentrations of the most potent acyl hydrazides, further supporting the primary mechanism being inhibition of FA desaturase. A systematic analysis of the structure-activity relationship confirmed the acyl hydrazide core as essential for inhibitory activity. This collection demonstrated broad-spectrum activity against Candida auris and mucormycetes and retained the activity against azole-resistant candida isolates. Finally, a preliminary analysis of toxicity to mammalian cells identified potential lead compounds with desirable selectivities. Collectively, these results establish a scaffold that targets fungal FA biosynthesis with a potential for development into novel therapeutics.

Electrosorptive removal of organic water constituents by positively charged electrically conductive UF membranes

Negatively charged electrically conductive ultrafiltration (UF) membranes have been intensively investigated for fouling mitigation and rejection enhancement in recent years. This study reports the novel approach of applying positive charge (+2.5 V cell potential) to a conductive membrane to induce electrosorption of negatively charged substances onto the membrane. Subsequently, desorption of negatively charged substances is achieved by changing the potential periodically (e.g., after 30 min) to negative charge (-2.5 V cell potential). For this purpose, sputter deposition of ultra-thin gold layers (40 nm) is used to generate electrically conductive gold-polymer-gold flat sheet membranes by coating the active and the support layer of two commercial polymer UF membranes (polyethersulfone UP150, polyamide M5). When M5 membrane was charged positively during filtration (+2.5 V), Suwannee River NOM, Hohloh lake NOM, humic acid and Brilliant Blue ionic dye showed removal rates of 70 %, 75% and 93% and 99%, respectively. Whereas, when no potential was applied (0 V) removal rates were only 1 - 5 %. When a positive potential was applied to the active membrane layer and a negative potential was applied to the support layer (cell potential 2.5 V), a significant increase of flux with 25 L/(m² h) was observed due to the induction of electro-osmosis. Electrosorption was only observed for M5 membrane (ζ: +13 mV, pH 7) and not with UP150 membrane (ζ: -29 mV, pH 7). Due to a low current density of 1.1 A/m² at a flux of 100 L/(m² h), the additional energy consumption of electrosorption and desorption process was low with 0.03 kWh per m³ of permeate. This study delivered the proof of concept for the novel process of electrosorptive UF with energy consumption between microfiltration and ultrafiltration but NOM removal rates of nanofiltration membranes.

Observable but Not Isolable: The RhAu24 (PET)181+ Nanocluster

The synthesis and characterization of RhAu₂₄ (PET)₁₈ (PET = 2-phenylethanethiol) is described. The cluster is cosynthesized with Au₂₅ (PET)₁₈ and rhodium thiolates in a coreduction of RhCl₃ , HAuCl₄ , and PET. Rapid decomposition of RhAu₂₄ (PET)₁₈ occurs when purified from the other reaction products, precluding the study of isolated cluster. Mixtures containing RhAu₂₄ (PET)₁₈ , Au₂₅ (PET)₁₈ , and rhodium thiolates are therefore characterized. Mass spectrometry, X-ray photoelectron spectroscopy, and chromatography methods suggest a combination of charge-charge and metallophilic interactions among Au₂₅ (PET)₁₈^1- , rhodium thiolates and RhAu₂₄ (PET)₁₈ resulting in stabilization of RhAu₂₄ (PET)₁₈ . The charge of RhAu₂₄ (PET)₁₈ is assigned as 1+ on the basis of its stoichiometric 1:1 presence with anionic Au₂₅ (PET)₁₈ , and its stability is contextualized within the superatom electron counting rules. This analysis concludes that the Rh atom absorbs one superatomic electron to close its d-shell, giving RhAu₂₄ (PET)₁₈¹⁺ a superatomic electron configuration of 1S² 1P⁴ . Overall, an updated framework for rationalizing open d-shell heterometal dopant electronics in thiolated gold nanoclusters emerges.

Affinity Membranes and Monoliths for Protein Purification

Affinity capture represents an important step in downstream processing of proteins and it is conventionally performed through a chromatographic process. The performance of this step highly depends on the type of matrix employed. In particular, resin beads and convective materials, such as membranes and monoliths, are the commonly available supports. The present work deals with non-competitive binding of bovine serum albumin (BSA) on different chromatographic media functionalized with Cibacron Blue F3GA (CB). The aim is to set up the development of the purification process starting from the lab-scale characterization of a commercially available CB resin, regenerated cellulose membranes and polymeric monoliths, functionalized with CB to identify the best option. The performance of the three different chromatographic media is evaluated in terms of BSA binding capacity and productivity. The experimental investigation shows promising results for regenerated cellulose membranes and monoliths, whose performance are comparable with those of the packed column tested. It was demonstrated that the capacity of convective stationary phases does not depend on flow rate, in the range investigated, and that the productivity that can be achieved with membranes is 10 to 20 times higher depending on the initial BSA concentration value, and with monoliths it is approximately twice that of beads, at the same superficial velocity.

Nanoenabled Bioseparations: Current Developments and Future Prospects

The use of nanomaterials in bioseparations has been recently introduced to overcome the drawbacks of the conventional methods. Different forms of nanomaterials, particularly magnetic nanoparticles (MNPs), carbon nanotubes (CNTs), casted nanoporous membranes, and electrospun nanofiber membranes were utilized in biological separation for the aim of production of different biomolecules such as proteins, amino acids, nucleic acids, and enzymes. This paper critically reviews the state-of-the-art efforts undertaken in this regard, with emphasis on the synthesis and performance evaluation of each nanoform. Challenges and future prospects in developing nanoenabled bioseparations are also discussed, for the purpose of highlighting potential advances in the synthesis and fabrication of novel nanomaterials as well as in the design of efficient nanoenabled processes for separating a wide spectrum of biomolecules.

Virus separation using membranes

Industrial manufacturing of cell culture-derived viruses or virus-like particles for gene therapy or vaccine production are complex multistep processes. In addition to the bioreactor, such processes require a multitude of downstream unit operations for product separation, concentration, or purification. Similarly, before a biopharmaceutical product can enter the market, removal or inactivation of potential viral contamination has to be demonstrated. Given the complexity of biological solutions and the high standards on composition and purity of biopharmaceuticals, downstream processing is the bottleneck in many biotechnological production trains. Membrane-based filtration can be an economically attractive and efficient technology for virus separation. Viral clearance, for instance, of up to seven orders of magnitude has been reported for state of the art polymeric membranes under best conditions.This chapter summarizes the fundamentals of virus ultrafiltration, diafiltration, or purification with adsorptive membranes. In lieu of an impractical universally applicable protocol for virus filtration, application of these principles is demonstrated with two examples. The chapter provides detailed methods for production, concentration, purification, and removal of a rod-shaped baculovirus (Autographa californica M nucleopolyhedrovirus, about 40 × 300 nm in size, a potential vector for gene therapy, and an industrially important protein expression system) or a spherical parvovirus (minute virus of mice, 22-26 nm in size, a model virus for virus clearance validation studies).

Enhanced Electrostatic Discrimination of Proteins on Nanoparticle-Coated Surfaces

Two β-lactoglobulin (BLG) isoforms, BLGA and BLGB, were used a test bed for the differentiation of proteins using electrostatics. In these studies, the BLGA and BLGB binding to a highly charged, cationic gold nanoparticle (GNP) modified surface was investigated by atomic force microscopy (AFM) and surface plasmon resonance (SPR) spectroscopy The binding affinity, and more importantly, the selectivity of this surface towards these two almost identical protein isoforms were both significantly increased on the cationic GNP surface array relative to the values measured with the same free cationic GNP in solution. While protein recognition is traditionally achieved almost exclusively via orientation dependent short-range interactions such as hydrogen bonds and hydrophobic interactions, our results show the potential of protein recognition platforms based on enhanced electrostatic interactions.

Effect of bioparticle size on dispersion and retention in monolithic and perfusive beds

Single-component pulse response studies were used to compare the retention and transport behavior of small molecules, proteins, and a virus on commercially available monolithic and perfusive ion-exchangers. Temporal distortion and extra-column effects were corrected for using a simple algorithm based on the method of moments. It was found that temporal distortion is inversely related to the number of theoretical plates. With increasing bioparticle size, retention increased and the transition from a non-eluting to a non-adsorbing state with increasing ionic strength became more abrupt. Both of these observations are qualitatively explained by calculations of particle-surface electrostatic attractive energy. Calculations also suggest that, for sufficiently large bioparticles, such as viruses or cells, hydrodynamic drag can promote elution. Under non-adsorbing conditions, plate height increased only weakly with flow rate and the skew remained unchanged. With increasing retention, plate height increased dramatically for proteins. Plate height was scaled by permeability rather than bead diameter to enable comparison among different stationary phases.

Relation of structure to performance characteristics of monolithic and perfusive stationary phases

Commercially available polymer-based monolithic and perfusive stationary phases were evaluated for their applicability in chromatography of biologics. Information on bed geometry, including that from electron microscopy (EM), was used to interpret and predict accessible volumes, binding capacities, and pressure drops. For preparative purification of biologics up to at least 7 nm in diameter, monoliths and perfusive resins are inferior to conventional stationary phases due to their low binding capacities (20-30 g/L for BSA). For larger biologics, up to several hundred nanometers in diameter, calculations from EM images predict a potential increase in binding capacity to nearly 100 g/L. The accessible volume for adenovirus calculated from the EM images matched the experimental value. While the pores of perfusive resins are essentially inaccessible to adenovirus under binding conditions, under non-adsorbing conditions the accessible intrabead porosity is almost as large as the interbead porosity. Modeling of breakthrough curves showed that the experimentally observed slow approach to full saturation can be explained by the distribution of pore sizes.

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.

Recovery of proteins from corn and soybean extracts by membrane adsorption

Efficient separation strategies for the recovery of high-value proteins (native or recombinant) from plant agriculture are an important aspect of many different processes, from biopharmaceuticals to byproduct recovery during biofuel production. Here we report the use of membrane adsorption for the recovery of proteins from soybean and corn extracts, and compare the results with packed bed adsorption. Two alternative operating modes were investigated, a flowthrough strategy and a bind and elute method. Overall, membrane adsorption provided faster throughput, and had equal or slightly higher dynamic binding capacities compared with resin beads, without compromising yield and purity of the chosen target. Soybean was found to be an ideal plant host when capturing native protein on an anion exchange medium. This provided an opportunity to capture a large percentage (>80%) of native protein as the product, and/or allowed for elevated enrichment factors (>20) during purification of a recombinant target with pI > 7.0, using a flowthrough approach. On the other hand, for corn, a single ion-exchange step was not able to capture more than 60% of the native protein. However, the bind and elute method with corn as the host for a recombinant product allowed for higher enrichment factors compared to soybean. In all cases, the concentration of a recombinant protein (as dictated by expression level) was found to play a significant role in the level of dynamic binding capacity, with higher concentration leading to elevated capacity. Likewise, a higher concentration of competing proteins was shown to decrease the overall capacity of a recombinant target.

Creation of functional membranes using polyelectrolyte multilayers and polymer brushes

Over the last 15 years, the layer-by-layer deposition of polyelectrolytes and the growth of polymer brushes from surfaces have become established techniques for the formation of a wide range of thin films. This article discusses the use of these techniques in creating the skin layer of nanofiltration or gas-separation membranes and in functionalizing the interior of membranes for protein adsorption or catalysis. In the case of separation membranes for nanofiltration, the minimal thickness of layer-by-layer films allows for high flux, and the wide range of available polyelectrolytes that can form these films permits the tailoring of membranes for separations such as water softening, the reduction of F (-) concentrations, and the removal of dyes from wastewater. For gas separation, polymers grown from surfaces are more attractive than layer-by-layer coatings because most polyelectrolyte films are not highly gas-selective. Cross-linked poly(ethylene glycol dimethacrylate) films grown from porous alumina exhibit CO(2)/CH(4) selectivities of around 20, and the careful selection of monomers should further improve the selectivity of similar membranes. Both layer-by-layer methods and polymer brushes can also be employed to modify the interior of membranes, and we have utilized these techniques to create catalysts, antibody arrays in membranes, and membrane absorbers for protein purification. Polymer brushes are particularly attractive because they allow the absorption of multilayers of protein to yield membranes with binding capacities as high as 150 mg protein/cm(3). Some challenges in the practical implementation of these systems, such as the economical formation of membranes using highly permeable polymeric supports, and future directions in research on membrane modification with multilayer films and polymer brushes are also discussed herein.

Membrane adsorbers as purification tools for monoclonal antibody purification

Downstream purification processes for monoclonal antibody production typically involve multiple steps; some of them are conventionally performed by bead-based column chromatography. Affinity chromatography with Protein A is the most selective method for protein purification and is conventionally used for the initial capturing step to facilitate rapid volume reduction as well as separation of the antibody. However, conventional affinity chromatography has some limitations that are inherent with the method, it exhibits slow intraparticle diffusion and high pressure drop within the column. Membrane-based separation processes can be used in order to overcome these mass transfer limitations. The ligand is immobilized in the membrane pores and the convective flow brings the solute molecules very close to the ligand and hence minimizes the diffusional limitations associated with the beads. Nonetheless, the adoption of this technology has been slow because membrane chromatography has been limited by a lower binding capacity than that of conventional columns, even though the high flux advantages provided by membrane adsorbers would lead to higher productivity. This review considers the use of membrane adsorbers as an alternative technology for capture and polishing steps for the purification of monoclonal antibodies. Promising industrial applications as well as new trends in research will be addressed.

Modelling and simulation of affinity membrane adsorption

A mathematical model for the adsorption of biomolecules on affinity membranes is presented. The model considers convection, diffusion and adsorption kinetics on the membrane module as well as the influence of dead end volumes and lag times; an analysis of flow distribution on the whole system is also included. The parameters used in the simulations were obtained from equilibrium and dynamic experimental data measured for the adsorption of human IgG on A2P-Sartoepoxy affinity membranes. The identification of a bi-Langmuir kinetic mechanisms for the experimental system investigated was paramount for a correct process description and the simulated breakthrough curves were in good agreement with the experimental data. The proposed model provides a new insight into the phenomena involved in the adsorption on affinity membranes and it is a valuable tool to assess the use of membrane adsorbers in large scale processes.

Sorption processes in ion-exchange chromatography of viruses

Purified viruses are used in gene therapy and vaccine production. Ion-exchange chromatography (IEC) is the most common method for large-scale downstream purification of viruses and proteins. Published IEC protocols provide details for specific separations but not general methods for selecting operating parameters. To make the selection more systematic, we study adenovirus type 5 (Ad5) as a model virus and develop batch uptake and light scattering methods for optimizing the ionic strength and pH of adsorption, as well as providing heuristics for resin geometry. The static capacity for Ad5 was found to go through a maximum with increasing ionic strength. Comparison to a protein-resin system shows that resin capacity for the virus is at least an order of magnitude lower, even on a wide-pore resin. Virus penetration into the wide-pore resin is only partial and the uptake rate is an order of magnitude slower than the uptake onto a narrow-pore resin.