High-performance liquid chromatography of amino acids, peptides and proteins. XCII Thermodynamic and kinetic investigations on rigid and soft affinity gels with varying particle and pore sizes

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.

Studies with an immobilized metal affinity chromatography cassette system involving binuclear triazacyclononane-derived ligands: automation of batch adsorption measurements with tagged recombinant proteins

This study describes the determination of the adsorption isotherms and binding kinetics of tagged recombinant proteins using a recently developed IMAC cassette system and employing automated robotic liquid handling procedures for IMAC resin screening. These results confirm that these new IMAC resins, generated from a variety of different metal-charged binuclear 1,4,7-triaza-cyclononane (tacn) ligands, interact with recombinant proteins containing a novel N-terminal metal binding tag, NT1A, with static binding capacities similar to those obtained with conventional hexa-His tagged proteins, but with significantly increased association constants. In addition, higher kinetic binding rates were observed with these new IMAC systems, an attribute that can be positively exploited to increase process productivity. The results from this investigation demonstrate that enhancements in binding capacities and affinities were achieved with these new IMAC resins and chosen NT1A tagged protein. Further, differences in the binding performances of the bis(tacn) xylenyl-bridged ligands were consistent with the distance between the metal binding centres of the two tacn moieties, the flexibility of the ligand and the potential contribution from the aromatic ring of the xylenyl group to undergo π/π stacking interactions with the tagged proteins.

A new affinity-HPLC packing for protein separation: Cibacron blue attached uniform porous poly(HEMA-co-EDM) beads

In this study, a new affinity high-performance liquid chromatography (HPLC) stationary phase suitable for protein separation was synthesized. In the first stage of the synthesis, uniform porous poly(2-hydroxyethyl methacrylate-co-ethylene dimethacrylate), poly(HEMA-co-EDM), beads 6.2 mum in size were obtained. Homogeneous distribution of hydroxyl groups in the bead interior was confirmed by confocal laser scanning microscopy. The plain poly(HEMA-co-EDM) particles gave very low non-specific protein adsorption with albumin. The selected dye ligand Cibacron blue F3G-A (CB F3G-A) was covalently linked onto the beads via hydroxyl groups. In the batch experiments, albumin adsorption up to 60 mg BSA/g particles was obtained with the CB F3G-A carrying poly(HEMA-co-EDM) beads. The affinity-HPLC of selected proteins (albumin and lysozyme) was investigated in a 25 mm x 4.0-mm inner diameter column packed with CB F3G-A carrying beads and both proteins were successfully resolved. By a single injection, 200 mug of protein was loaded and quantitatively eluted from the column. The protein recovery increased with increasing flow rate and salt concentration of the elution buffer and decreased with the increasing protein feed concentration. During the albumin elution, theoretical plate numbers up to 30,000 plates/m were achieved by increasing the salt concentration.

Fractionation of whey proteins with high-capacity superparamagnetic ion-exchangers

In this study we describe the design, preparation and testing of superparamagnetic anion-exchangers, and their use together with cation-exchangers in the fractionation of bovine whey proteins as a model study for high-gradient magnetic fishing. Adsorbents prepared by attachment of trimethyl amine to particles activated in sequential reactions with allyl bromide and N-bromosuccinimide yielded a maximum bovine serum albumin binding capacity of 156 mg g(-1) combined with a dissociation constant of 0.60 microM, whereas ion-exchangers created by linking polyethylene imine through superficial aldehydes bound up to 337 mg g(-1) with a dissociation constant of 0.042 microM. The latter anion-exchanger was selected for studies of whey protein fractionation. In these, crude bovine whey was treated with a superparamagnetic cation-exchanger to adsorb basic protein species, and the supernatant arising from this treatment was then contacted with the anion-exchanger. For both adsorbent classes of ion-exchanger, desorption selectivity was subsequently studied by sequentially increasing the concentration of NaCl in the elution buffer. In the initial cation-exchange step quantitative removal of lactoferrin (LF) and lactoperoxidase (LPO) was achieved with some simultaneous binding of immunoglobulins (Ig). The immunoglobulins were separated from the other two proteins by desorbing with a low concentration of NaCl (< or = 0.4 M), whereas lactoferrin and lactoperoxidase were co-eluted in significantly purer form, e.g. lactoperoxidase was purified 28-fold over the starting material, when the NaCl concentration was increased to 0.4-1 M. The anion-exchanger adsorbed beta-lactoglobulin (beta-LG) selectively allowing separation from the remaining protein.

Microcalorimetric studies on the interaction mechanism between proteins and hydrophobic solid surfaces in hydrophobic interaction chromatography: effects of salts, hydrophobicity of the sorbent, and structure of the protein

This study examines the effects of different salts as well as the influence of the relative hydrophobicities of different sorbents on the adsorption processes of proteins in hydrophobic interaction chromatography (HIC). Comparative data acquired by the equilibrium binding analysis and by isothermal titration microcalorimetry (ITC) are presented. In particular, thermodynamic parameters, including the enthalpy changes, related to the interactions between several globular proteins and various Toyopearl 650 M sorbents under solvent conditions containing either 2.0 M ammonium sulfate or 2.0 M sodium sulfate at pH 7.0 and 298.15 K have been evaluated in terms of the molecular properties of these systems. The results reveal that the dependence of the free energy change, deltaGads, for protein adsorption to HIC sorbents on the salt composition can be mainly attributed to the enthalpy changes associated with protein and sorbent dehydration and hydrophobic interactions. Differences in binding mechanisms between the n-butyl- and phenyl-HIC sorbents were evident. In the latter case, the participation of pi-pi hydrophobic interactions leads to significant differences in the associated enthalpy and entropy changes. Furthermore, an increase in the hydrophobicity of either the sorbent or the protein resulted in more negative values for the free energy change, which arose mostly from dehydration processes. Entropic effects favoring HIC adsorption increased with an increase in the exposed nonpolar surface area of the protein. Consequently, an increased contribution from the entropy change to the respective change in free energy occurs when HIC sorbents or proteins of higher hydrophobicity are employed, with these larger entropy changes consistent with a change in the interaction mechanism from a binding event dominated by adsorption to a partitioning-like process. Data extracted from the ITC measurements also provided insight into the interaction mechanisms that occur between proteins and hydrophobic solid surfaces, yielding information that can be applied to the HIC purification of proteins according to the concept of critical hydrophobicity of the system and its thermodynamic consequences.

Affinity chromatography of proteins on non-porous copolymerized particles of styrene, methyl methacrylate and glycidyl methacrylate

Non-porous particles having an average diameter of 2.1 microm were prepared by co-polymerization of styrene, methyl methacrylate and glycidyl methacrylate, which was abbreviated as P(S-MMA-GMA). The particles were mechanically stable due to the presence of benzene rings in the backbone of polymer chains, and could withstand high pressures when a column packed with these particles was operated in the HPLC mode. The polymer particles were advantaged by immobilization of ligands via the epoxy groups on the particle surface that were introduced by one of the monomers, glycidyl methacrylate. As a model system, Cibacron Blue 3G-A was covalently immobilized onto the non-porous copolymer beads. The dye-immobilized P(S-MMA-GMA) particles were slurry packed into a 1.0 cm x 0.46 cm I.D. column. This affinity column was effective for the separation of turkey egg white lysozyme from a protein mixture. The bound lysozyme could be eluted to yield a sharp peak by using a phosphate buffer containing 1 M NaCl. For a sample containing up to 8 microg of lysozyme, the retained portion of proteins could be completely eluted without any slit peak. Due to the use of a shorter column, the analysis time was shorter in comparison with other affinity systems reported in the literature. The retention time could be reduced significantly by increasing the flow-rate, while the capacity factor remained at the same level.

New ligand, N-(2-pyridylmethyl)aminoacetate, for use in the immobilised metal ion affinity chromatographic separation of proteins

A new chelating compound has been developed for use in the immobilised metal ion affinity chromatographic separation of proteins. The tridentate ligand, sodium N-(2-pyridylmethyl)aminoacetate (carbpyr), 1, was prepared via a one-step synthesis from 2-picolylamine, 3 and then immobilised onto Sepharose CL-4B through the epoxide coupling procedure. The binding behaviour of the resulting IMAC sorbent, following chelation with Cu2+ ions to a density of 152 micromol Cu2+ ions/g gel was characterised by frontal analysis experiments using horse heart myoglobin (HMYO) at pH 7.0 and pH 9.0. From the derived isotherms, the adsorption capacity, q(m), for the binding of HMYO to immobilised Cu2+-N-(2-pyridylmethyl)aminoacetate (im-Cu2+-carbpyr)-Sepharose CL-4B at these pH values was found to be 1.92 and 1.91 micromol/g sorbent, respectively, whilst the dissociation constants K(D) were 0.0092 x 10(-6) M and 0.0062 x 10(-6) M at pH 7.0 and pH 9.0, respectively, indicating that the HMYO-im-Cu2+-N-(2-pyridylmethyl)aminoacetate complex was more stable under alkaline conditions, although the binding capacity in terms of micromol protein/g gel remained essentially unchanged. The selectivity features of the im-Cu2+-carbpyr-Sepharose CL-4B sorbent were further characterised in terms of the binding properties with several human serum proteins at pH 5.0, pH 7.0 and pH 9.0.

Examination of the protein binding behaviour of immobilised copper (II)-2,6-diaminomethylpyridine and its application in the immobilised metal ion affinity chromatographic separation of several human serum proteins

A new metal ion chelator has been developed for use in the immobilised metal ion affinity chromatography (IMAC) of proteins. The aromatic tridentate ligand 2,6-diaminomethylpyridine (bisampyr), 1, was prepared as the dihydrochloride salt, via a two step synthesis from 2,6-pyridinedimethanol, 2, and immobilised onto Sepharose CL-4B through an epoxide coupling procedure. The resulting sorbent was chelated with Cu2+ ions to a density of 420 micromol Cu2+ ions per g gel and then characterised by frontal analysis using the protein, horse heart myoglobin (HMYO), at pH 7.0 and 9.0. From the resulting adsorption isotherms, the adsorption capacity, qm, for HMYO at pH 7.0 and pH 9.0 with the immobilised Cu2+-bisampyr Sepharose sorbent was found to be 1.27 micromol protein/g gel and 1.43 micromol protein/g gel, whilst the corresponding dissociation constants, K(D)s, were 18.0 x 10(-6) M and 16.0 x 10(-6) M respectively. The results confirm that the HMYO-Cu2+-bisampyr complex had similar stability at these pH values. This finding is in contrast with the situation observed with some other commonly used IMAC chelating ligates such as Cu2+-iminodiacetic acid (Cu2+-IDA) or Cu2+-nitrilotriacetic acid (Cu2+-NTA). Using human serum proteins, the interactive properties of the immobilised Cu2+-bisampyr Sepharose sorbent were further characterised at pH 5.0, 7.0 and 9.0 with specific reference to the binding behaviour of albumin, transferrin, and alpha2-macroglobulin.

Examination of protein adsorption in fluidized bed and packed bed columns at different temperatures using frontal chromatographic methods

The influences of various experimental parameters on the dynamic adsorption capacity (DAC) and the dynamic adsorption rate (DAR) of a biomimetic affinity silica-based adsorbent in fluidized and packed bed columns operated under plug flow conditions and at different temperatures have been investigated with different inlet concentrations of hen egg white lysozyme (HEWL) and human serum albumin (HSA). The DACs as well as the DARs of both the fluidized and packed beds were examined at 10% saturation (i.e., at the QB value) and the experimental data compared with the corresponding data obtained from batch equilibrium adsorption procedures. Parameters examined included the fluid superficial velocity and protein concentration and their effect on the binding capacity and column efficiency. Consistent with various results reported from this and other laboratories on the behavior of biospecific affinity adsorbents derived from porous silica and zirconia particles, adsorbents prepared from Fractosil 1000 were found to exhibit appropriate rheological characteristics in fluidized bed systems under the experimental conditions. Moreover, changes in temperature resulted in a more significant effect on the breakthrough profiles of HSA compared to HEWL with the immobilized Cibacron Blue F3G-A with Fractosil 1000 adsorbent. This result suggests that temperature effects can possibly be employed profitably in some processes as part of a strategy to enhance column performance with fluidized bed systems for selective recovery of target proteins. At relatively low superficial velocities of the feed, the DARs with HEWL and HSA were similar for both the fluidized and packed bed column systems, whereas, at high superficial velocities, the DARs for these proteins were larger with the packed bed columns.

Comparative studies on the isothermal characteristics of proteins adsorbed under batch equilibrium conditions to ion-exchange, immobilised metal ion affinity and dye affinity matrices with different ionic strength and temperature conditions

In these investigations, the influence of a range of experimental parameters on the isothermal characteristics of hen egg white lysozyme (HEWL) and human serum albumin (HSA) adsorbed to several different adsorbents has been examined. The adsorbents were selected to encompass the same basic types of silica support matrices, but with the ligand properties and surface characteristics adjusted so that the dominant mode of interaction between the protein and the ligand involved either electrostatic binding (i.e. as ion-exchange interaction with polyaspartic acid immobilised onto glycidoxypropyl-modified Fractosil 1000), mixed-mode binding with both hydrophobic and electrostatic effect contributing to the protein-ligand interaction (i.e. as dye-affinity interactions with Cibacron Blue F3G-A immobilised onto Lichroprep DIOL or onto glycidoxypropyl-modified Fractosil 1000), or lone pair coordination binding (i.e. as immobilised metal ion affinity (IMAC) interactions with Cu2+ ions complexed with iminodiacetic acid immobilised onto glycidoxypropyl-modified Fractosil 1000). In each case, the adsorbents exhibited similar ligand densities and had the same particle size ranges and silica surface pretreatment. The effect of the ionic strength of the adsorption buffer and temperature on the isothermal adsorption behaviour under batch equilibrium binding conditions of the two test proteins were determined. Consistent with previous observations with soft gel ion exchangers and triazine dye-based adsorbents that are used in packed bed chromatographic systems, the capacities of the silica-based ion-exchange adsorbents, as well as the Cibacron Blue F3G-A dye affinity adsorbents, for both HSA and HEWL were reduced as the salt concentration was increased under batch equilibrium binding conditions. Moreover, with both of these classes of adsorbents, as the ionic strength was increased under constant temperature conditions, the isothermal adsorption dependencies progressively approximated more closely a Langmuirean model of independent binding site interactions, typical of a mono-layer binding process. In contrast, with the silica-based immobilised metal ion affinity adsorbents as the ionic strength was increased the adsorption behaviour appeared to follow a Freundlich model, indicative of positive cooperativity in the binding process. In parallel experiments, the effect of changes in temperature under iso-ionic strength conditions was examined. With increasing temperature, different patterns of isothermal adsorption behaviour for both test proteins were observed, with the magnitude of these trends depending on the type of interaction involved between the immobilised ligand and the protein. Utilising first order Van't Hoff relationships to analyse the experimental data for these protein-ligand interactions, the apparent changes in enthalpy and entropy for these interactions have been derived from the dependency of the change in the apparent Gibbs free energy on 1/T.

Characterisation of silica-based heparin affinity sorbents from equilibrium binding studies on plasma fractions containing thrombin

The binding properties of rigid heparin sorbents, synthesised by end-point-attachment of heparin onto aminopropylderivatised silica through reductive amination, were characterised through batch-adsorption studies employing human plasma fractions containing thrombin. Thrombin was quantified using a chromogenic assay that had been specially modified for these studies. These investigations yielded information regarding the maximum adsorption capacities/stoichiometries and binding affinities for thrombin present in complex protein mixtures. Of the two types of heparin-silica evaluated, heparin-Fractosil 1000, with a pore size of 1000 A, displayed a capacity of 2.4 mol of thrombin/mol of heparin (mol T-mol H). This stoichiometry was significantly higher than the value of 1.8 mol T-mol H obtained for the commercial soft gel heparin-Sepharose CL-6B. Furthermore, the heparin-Fractosil 1000 sorbents were superior in capacity and binding site accessibility to heparin-LiChroprep Si60 sorbents, where the smaller pore size of 60 A largely restricts the ligand-protein interactions to the outer surface of the sorbent particles. Nevertheless, heparin-LiChroprep Si60 sorbents were useful, in that they simulated a non-porous particle system, in which intra-pore diffusion effects are eliminated. The batch adsorption results with these sorbents indicated that the adsorption involved both high and low binding affinity characteristics. This bimodal binding mechanism was also evident with the commercial heparin-Sepharose sorbent. Binding stoichiometries and affinities in the high concentration range were similar to values reported for a largely non-specific electrostatic thrombin-heparin interaction. Dissociation constants in the nanomolar range were observed in the low concentrations range. This stronger binding affinity is more similar to highly specific bio-affinity interactions. Thus, the results indicated that heparin-thrombin interactions with these systems involve both a weak electrostatic and a strong biospecific interaction component.

Characterisation of silica-based heparin-affinity adsorbents through column chromatography of plasma fractions containing thrombin

Different heparin-silicas, synthesised in this laboratory via directional end-point attachment of heparin (H) onto various amino-derivatised silicas, have been evaluated in packed-bed and expanded-bed column chromatographic experiments using crude preparations of the therapeutic protein, thrombin (T). Adsorbent capacities, determined through batch adsorption experiments, were verified by employing frontal analysis in packed-bed systems. The performance of these adsorbents was also investigated in terms of thrombin purification factors and recoveries. The potential of the heparin-silicas was further examined in the expanded-bed column chromatographic mode using a scaled-up procedure. With heparin-Fractosil 1000 adsorbents, capacities of around 100,000 U thrombin/ml adsorbent could be achieved. Heparin-Fractosil 1000 adsorbents of intermediate heparin content (around 4 mg heparin/ml sorbent) displayed binding stoichiometries similar to that of the commercial heparin-Sepharose (2.6-2.7 mol T/mol H). Furthermore, binding stoichiometries were largely unaffected by increasing the heparin content on the heparin-Fractosil 1000 adsorbents from 0.8 to 4.6 mg of heparin/ml of sorbent. This result suggests that optimal binding site accessibilities for the thrombin-heparin interaction occurs at lower ligand density values. The binding capacity values determined from frontal analysis were confirmed by the recovery data, thus indicating minimal irreversible adsorption. Specific activities of ca. 2100 U/mg were obtained for thrombin when affinity-purified on these heparin-LiChroprep Si60 or heparin-Fractosil 1000 adsorbents. These values were higher than the maximum achievable purity obtained through alternative, multi-step chromatographic purification procedures reported by other investigators. These results indicated that the packed-bed performances with these silica-based adsorbents were superior to currently available commercial soft gel adsorbents, with the more dense heparin-silicas exhibiting very good potential for use in expanded-bed applications.

Purification of recombinant human basic fibroblast growth factor: stability of selective sorbents under cleaning in place conditions

Human basic fibroblast growth factor (bFGF) was produced from recombinant Escherichia coli by high-cell-density cultivation. In order to develop a purification strategy for large-scale purification, chromatographic sorbents with different anionic functional groups were compared in terms of selectivity for bFGF and stability under cleaning in place (CIP) conditions. Heparin-Sepharose CL-6B, Fractogel EMD-SO3- 650 (S) and SP-Sepharose (high performance) were found suitable for this purpose with decreasing selectivity in that order. Each sorbent was treated eight times under CIP conditions employing both 0.2 and 1.0 M NaOH, in order to study modifications of these sorbents. Heparin-Sepharose displayed more than 50% loss of capacity after the first CIP treatment and decreasing selectivity with each cycle. Both cation exchangers displayed almost constant results regarding selectivity and capacity. The Fractogel EMD-SO3- exhibited only slightly lower selectivity for bFGF than Heparin-Sepharose and the highest capacity of all sorbents tested. Agglomeration of bFGF at low salt concentrations was a serious problem. By direct application of pooled fractions from Fractogel EMD-SO3- onto Heparin-Sepharose a highly pure product was obtained; however, the recovery after Heparin-Sepharose was only 30%.

Removal of endotoxins by affinity sorbents

Histidine, histamine and polymyxin B affinity sorbents were employed for the removal of Escherichia coli-derived endotoxins. Their effectiveness was compared with those of poly-L-lysine-Sepharose and DEAE-Sepharose. All sorbents reduced the concentration of endotoxins from an E. coli culture filtrate to tolerable levels. However, their effectiveness was not higher than that of the anion exchanger, which displayed clearance rates of up to 15,000. Endotoxin removal from protein solutions depended on the net charge of the desired protein. Lysozyme as a model for positively charged proteins enhanced endotoxin removal. In contrast, only low initial contamination levels (< 34 EU/ml) were reduced to tolerable levels from bovine serum albumin (BSA) as the negatively charged protein model owing to competition of BSA and endotoxins for adsorption sites. Hence also a low BSA recovery was observed after the treatment whereas the lysozyme recovery was almost 100%. At pH values below the isoelectric point of BSA, endotoxin removal was also more effective. The best conditions for the decontamination were found at neutral pH and low ionic strength (< or = 20 mM phosphate). Ionic forces between ligands and endotoxins are dominant at this ionic strength; hydrophobic interactions are not very effective. Hence the selectivities of all sorbents towards endotoxins are not exceptionally high. DEAE-anion exchangers are the most suitable sorbents for the removal of endotoxins from solutions accommodating positively charged proteins owing to their low cost and high capacity. Poly-L-lysine-Sepharose was most effective for the removal of small amounts of endotoxins from solutions of negatively charged proteins. The "affinity ligands" histamine, histidine and polymyxin B were effective for the removal of endotoxins from E. coli filtrate; however, their effectiveness decreased dramatically in the presence of BSA and it was lower than for poly-L-lysine- and DEAE-Sepharose in the presence of lysozyme.

High-performance liquid chromatography of amino acids, peptides and proteins. CXXXVIII. Adsorption of horse heart cytochrome c onto a tentacle-type cation exchanger

Determination of the change in the Gibb's free energy from the adsorption isotherm associated with the interaction between a biomolecule and an ion-exchange resin is often achieved by assuming that a Langmuirean model prevails. However, the adsorption of horse heart cytochrome c onto the tentacle-type cation exchanger LiChrospher 1000 SO3- at pH 4.00 showed an isotherm of rectangular form. In this case the Langmuirean model is not applicable. In this paper, we propose an alternative way to deal with this situation, whereby the adsorption capacity of the adsorbent with a defined protein sample is studied as a function of displacing-ion concentration. The experimental conditions over defined ranges are then selected in order to relate this function to the change in the Gibb's free energy for the interaction between the protein and the ion exchanger. Additional comments about the general utility of the on-line adsorption vessel system employed to determine the adsorption isotherms are also made.

High-performance liquid chromatography of amino acids, peptides and proteins. CXXVI. Modelling of protein adsorption with non-porous and porous particles in a finite bath

Analytical solutions for a mathematical model describing dynamic adsorption processes of proteins onto non-porous adsorbent particles in a finite bath are presented. The model, based on the Langmuir adsorption isotherm, has been applied to experimental data obtained with affinity and ion-exchange adsorbents. The external film mass transfer resistance, as well as the rate of surface interaction between proteins and adsorbents, have been taken into account. The model has been extended to the case of adsorption onto porous particles by employing a linear driving force approximation for describing mass transfer in the pore fluid. This approach enables the derivation of an effective overall liquid phase mass transfer coefficient, permitting subsequent adaptation of the analytical solutions developed for non-porous particles. The evaluation of the effective liquid phase mass transfer coefficients is also described. Examples of a comparison between predicted and experimental dynamic adsorption curves for both dye-affinity and ion-exchange systems are presented. The application of the model for predicting the optimum operating conditions is discussed.

High-performance liquid chromatography of amino acids, peptides and proteins. CXXXII. Optimisation of operating parameters for protein purification with chromatographic columns

In large-scale chromatography, process optimisation is one of the key elements for success. This paper presents a method for determining the optimum operating parameters for affinity and ion-exchange chromatographic columns when used for protein purification. Based on a mathematical model developed as part of our association investigations, computer programs have been developed to describe the dynamic relationships acting within the chromatographic system. Two basic operating parameters, the flow-rate and the effluent concentration at which the adsorption stage is terminated, can be optimised to give a maximum production rate. The sample loading volume and the processing time then can be determined. The effect of washing conditions on the production rate and the yield is also discussed. Examples are given for a specific system where the optimisation is based on the yield and the percentage utilisation of the column capacity. Contour plots are generated to aid the determination of the range of controlling parameters, and to guide further system design.

Protein purification by dye-ligand chromatography

Dye-ligand chromatography has developed into an important method for large-scale purification of proteins. The utility of the reactive dyes as affinity ligands results from their unique chemistry, which confers both the ability to interact with a large number of proteins as well as easy immobilization on typical adsorbent matrices. Reactive dyes can bind proteins either by specific interactions at the protein's active site or by a range of non-specific interactions. Divalent metals participate in yet another type of protein-reactive dye interactions which involve the formation of a ternary complex. All of these types of interactions have been exploited in schemes for protein purification. Many factors contribute to the successful operation of a dye-ligand chromatography process. These include adsorbent properties, such as matrix type and ligand concentration, the buffer conditions employed in the adsorption and elution stages, and contacting parameters like flowrate and column geometry. Dye-ligand chromatography has been demonstrated to be suitable for large-scale protein purification due to their high selectivity, stability, and economy. Also, the issue of dye leakage and process validation of large-scale dye-ligand chromatography has been discussed. Reactive dyes have also been applied in high performance liquid affinity chromatographic techniques for protein purification, as well as non-chromatographic techniques including affinity partition, affinity membrane separations, affinity cross-flow filtration, and affinity precipitation.

High-performance liquid chromatography of amino acids, peptides and proteins. CIX. Investigations on the relation between the ligand density of cibacron blue immobilized porous and non-porous sorbents and protein-binding capacities and association constants

A porous silica of nominal 5 microns particle diameter and 30 nm pore size (Nucleosil 300-5) and a non-porous silica of nominal 1.5 microns particle diameter were activated with 3-mercaptopropyltriethoxysilane (MPTS), followed by the immobilization of the triazine dye, Cibacron Blue F3GA. Various biomimetic dye sorbents with graduated ligand densities between 1 mumol/m2 and 0.01 mumol/m2 were prepared. The capacities and the association constants associated with the binding of lysozyme to these sorbents were determined by frontal analysis experiments [J. Chromatogr., 476 (1989) 205-225]. Due to the ability of the Cibacron Blue F3GA-modified silicas to act as mixed mode coulombic and hydrophobic interaction sorbents and the highly charged nature of the surface structure of lysozyme (pl 11), two mobile phase conditions were examined. In one case a 0.1 M phosphate buffer, pH 7.8, was used as the equilibration and loading buffer, in the second case 1 M sodium chloride-0.1 M phosphate buffer, pH 7.8 was employed as the equilibration and loading buffer to monitor the influence of ionic interactions. The elution was performed in each case with a 2.5 M potassium thiocyanate solution. With the porous silica dye sorbents and 1 M NaCl present in the loading buffer, the highest capacity was achieved when Cibacron Blue F3GA was immobilised to the level of 0.1 mumol/m2. In the case of the non-porous silica dye sorbents, the maximum protein capacity was achieved when 0.5 mumol/m2 dye were immobilised onto the support. Evaluation of the frontal breakthrough curves confirmed that the kinetics of adsorption of lysozyme onto the non-porous sorbent were substantially faster than the adsorption of lysozyme onto the porous sorbent due to the absence of pore diffusion effects in case of the non-porous support. Furthermore, the adsorption of lysozyme on both sorbents was faster when no salt was added to the loading buffer, indicating that there is either conformational or reorientation effects operating during the specific binding of the protein to the dye ligand, or that the interaction is proceeding through the participation of a second class of binding sites. The magnitude of the association constants, Ka, for the lysozyme-Cibacron Blue F3GA systems were found to be dependent on the ligand density of the sorbent. With decreasing ligand density, the protein-ligand interaction became stronger, e.g. Ka values became larger. These results confirm earlier observations on the effect of ligand steric compression on the affinate-ligand association constant, e.g. the protein needs sufficient space to interact with the ligand in an optimum way.(ABSTRACT TRUNCATED AT 400 WORDS)

High-performance liquid chromatography of amino acids, peptides and proteins. C. Characterisation of coulombic interactive regions on hen lysozyme by high-performance liquid anion-exchange chromatography and computer graphic analysis

The molecular characteristics of the dominant anion-exchange binding site of hen egg white lysozyme (HEWL) has been investigated using a combination of high-performance liquid chromatographic techniques and computer graphic analysis of the X-ray crystallographic structure. These studies have indicated that the site of highest electrostatic potential, in terms of the density of negatively charged amino acid side chains, is located around the catalytic cleft area. The four residues tentatively identified to be involved in the electrostatic binding domain were aspartic acid 48, 52, 101 and glutamic acid 35. The number of these charged groups correlated with the maximum value of the chromatographically determined retention parameter (Zc value). Variations in the range of experimental Zc values obtained under different elution conditions have been interpreted in terms of conformational flexibility of the structural domains of HEWL which result in the opening or closure of the catalytic cleft during the retention process.

High-performance liquid chromatography of amino acids, peptides and proteins. CIII. Mass transfer resistances in ion-exchange and dye-affinity chromatography of proteins

Adsorption equilibria and rate kinetics have been investigated for the binding of several proteins, with different molecular geometries, to several ion-exchange and dye-affinity chromatographic resins with varying pore size and protein accessibilities. The pore geometry was shown to play a significant role in the protein capacity and loadability of both the ion-exchange and dye-affinity resins. For example the Fractogel HW75-Cibacron Blue F3GA affinity sorbent had the greatest capacity for the small protein, lysozyme, compared to the other Fractogel HW-Cibacron Blue F3GA sorbents, and similarly, the ion-exchange resins, such as DEAE-Fractogel 65, bound more human serum albumin (HSA), as opposed to the larger protein, ferritin. The apparent diffusion of protein from the bulk phase to the ligands/ionic sites was calculated to be considerably restricted when the pore to protein size ratio was small, as is the case of DEAE Fractogel 65/ferritin system, and the dye-affinity Fractogel HW55/HSA system. In these circumstances, pore diffusivity was calculated to be up to 100-fold smaller than bulk diffusivity.

High-performance liquid chromatography of amino acids, peptides and proteins. XCVII. The influence of the gradient elution mode and displacer salt type on the retention properties of closely related protein variants separated by high-performance anion-exchange chromatography

The influence of different elution modes, gradient times and flow-rates on the relative retention of closely related variants of carbonic anhydrase and ovalbumin has been investigated using high-performance ion-exchange chromatography. Three isoform species of carbonic anhydrase and four isoforms related to ovalbumin eluted by anion-exchange chromatography were characterised by isoelectric focusing and sodium dodecylsulphate-polyacrylamide electrophoresis. Gradient retention data were collected using several different alkali metal halides as the displacer salt, in order to systematically evaluate the effect on selectivity of different anions and cations in the series F-, Cl- and Br-, and Li+, Na+ and K+. While the selectivity between the different ovalbumin isoform species remained essentially constant with each displacer salt, solute Zc-values [J. Chromatogr., 458 (1988) 27] varied with the type of salt. In contrast, non-parallel retention plots were obtained for the carbonic anhydrase isoforms with the Zc values different for each isoform. Furthermore, significant differences in chromatographic behaviour for these proteins were observed between experiments carried out under gradient elution conditions with either varied gradient time and constant flow-rates or fixed gradient time and varied flow-rates. These results are discussed in terms of the influence of column residence time and protein-salt interactions of the solute's interactive ionotope and the concomitant effects these structural perturbations may have on chromatographic behaviour.

High-performance liquid chromatography of amino acids, peptides and proteins. XCV. Thermodynamic and kinetic investigations on rigid and soft affinity gels with varying particle and pore sizes: comparison of thermodynamic parameters and the adsorption behaviour of proteins evaluated from bath and frontal analysis experiments

The thermodynamic constants, associated with the interaction of three proteins with triazine dye affinity sorbents, have been derived from bath and frontal analysis experiments. In cases where mass-transfer restrictions are very high, calculation of the thermodynamic constants directly from frontal analysis experiments could not be achieved. In such cases, a portion of the adsorbate was always present in the effluent, a situation which has its effect as the split peak phenomenon. With Fractogel-based triazine dye affinity sorbents none of the test proteins applied in frontal analysis were adsorbed. A similar behaviour was observed for a Cellufine sorbent during the adsorption of human serum albumin and the Blue Sepharose CL6B sorbent during the adsorption of alcohol dehydrogenase, which displayed much slower apparent adsorption kinetics than observed in the bath experiments. These phenomena were shown to be associated with changes in the gel structure, caused in part by the column packing procedure. Silica-based sorbents performed better in the adsorption of lysozyme in the column mode than soft-gel affinity sorbents, as was evident in the higher capacities and steeper breakthrough curves. At high protein concentrations (feedstock concentration greater than 0.2 mg/ml) breakthrough curves obtained with small- and large-particle-size sorbents, but of constant pore size, were found to be identical. This finding demonstrates that the use of small-particle-size sorbents (e.g. particle diameter, dp less than or equal to 5 microns) for the preparative isolation of proteins may not be justified when operating in the overload mode. With other higher-molecular-weight proteins and the silica-based sorbent systems examined, the small-particle-size sorbents (dp = 5 microns) displayed less symmetrical shapes of their breakthrough curves than the larger-particle-size and soft-gel sorbents. This behaviour was further exacerbated when non-porous glass or silica-based sorbents were utilized. These non-porous affinity sorbents displayed nearly rectangular breakthrough shapes at the onset of the adsorption process, but comparatively slow adsorption kinetics became evident as saturation was approached. This phenomenon has been attributed to surface rearrangement and/or reorientation of the adsorbed proteins, particularly with sorbents of high ligand densities.