Peak broadening in protein chromatography with monoliths at very fast separations

Monoliths for fast bioseparation and bioconversion and their applications in biotechnology

Monoliths have consolidated their position in bioseparation. More than 200 different applications have been reported in the past two decades and their advantages compared to conventional chromatography demonstrated. These include the high mass transfer efficiency due to the convective flow enabled by the macroporous character of the matrix. Recently plasmid DNA and viruses were separated with high efficiency and cryogels and monolithic superporous agarose were developed for capture of proteins from crude homogenates and separation of microorganisms or lymphocytes. Currently four companies manufacture monoliths mainly for analytical applications although monoliths with a volume of 0.8 liter are commercially available and 8 L are available as prototypes. A book entitled "Monolithic materials: preparation, properties and applications" was published in 2003 and became standard reference of the status of this area. This review focuses on the progress in monoliths that goes beyond the scope of this reference book. Less progress has been made in the field of bioconversions in spite of the fact that monolithic supports exhibit better performance than beads in enzymatic processing of macromolecules. It appears that the scientific community has not yet realized that supports for these applications are readily available. In addition, monoliths will further substantially advance bioseparations of both small and large molecules in the future.

Mass transfer characteristics of plasmids in monoliths

The hydrodynamic properties and pore-structure of monoliths based on functionalized poly(glycidyl methacrylate-ethylene dimethacrylate) were characterised by pulse response experiments using different probes representing a wide range of molecular mass. On a small scale, band spreading was found to be caused to the extent of more than 90% by extra-column effects. These monoliths have large channel diameters, providing a suitable chromatography adsorbent for processing of large molecules. Dynamic and static binding capacity for plasmid DNA was investigated. For our model plasmid, consisting of 4.9 kbp, a capacity of 7 mg/mL was observed in comparison to 0.3 mg/mL for a conventional medium designed for protein separation. When plasmids were loaded on the monolith a gradual increase in pressure drop was observed. The channels filled up and the cross-sectional area available for liquid flow decreased. Therefore, a higher pressure drop was observed during elution. This is caused by (i) shrinking of the channels as effect of the high salt concentration, (ii) high viscosity of the mobile phase due to high concentration of plasmids, and (iii) an increase of the hydrodynamic radius of the plasmid with salt concentration from 45 nm at 150 mM to 70 nm at 2 M NaCl, as measured by dynamic light scattering. These types of monoliths are considered to be the preferred adsorbents for plasmid separation.

Characterization of a continuous supermacroporous monolithic matrix for chromatographic separation of large bioparticles

A continuous supermacroporous monolithic chromatographic matrix has been characterized using a capillary model, experimental breakthrough curves, and pressure drop experiments. The model describes the convective flow and its dispersive mixing effects, mass transfer resistance, pore size distribution, and the adsorption behavior of the monolithic matrix. It is possible to determine an effective pore size distribution by fitting the capillary model to experimental breakthrough curves and pressure drop experiments. The model is able to describe the flow rate dependence of the experimental breakthrough curves. Mass transport resistance was due to: (i) dispersive mixing effects in the convective flow in the pores; and (ii) slow diffusion in the stagnant film covering the surface within each pore, under adsorption conditions. The monolithic matrix can be described by a very narrow pore size distribution, illustrating one of the advantages of the gel. A broader pore size distribution results in increased band broadening. This can be studied easily using the model developed in this investigation.

Chromatographic performance of monolithic and particulate stationary phases. Hydrodynamics and adsorption capacity

Monolithic chromatographic support structures offer, as compared to the conventional particulate materials, a unique combination of high bed permeability, optimized solute transport to and from the active surface sites and a high loading capacity by the introduction of hierarchical order in the interconnected pore network and the possibility to independently manipulate the contributing sets of pores. While basic principles governing flow resistance, axial dispersion and adsorption capacity are remaining identical, and a similarity to particulate systems can be well recognized on that basis, a direct comparison of sphere geometry with monolithic structures is less obvious due, not least, to the complex shape of theskeleton domain. We present here a simple, widely applicable, phenomenological approach for treating single-phase incompressible flow through structures having a continuous, rigid solid phase. It relies on the determination of equivalent particle (sphere) dimensions which characterize the corresponding behaviour in a particulate, i.e. discontinuous bed. Equivalence is then obtained by dimensionless scaling of macroscopic fluid dynamical behaviour, hydraulic permeability and hydrodynamic dispersion in both types of materials, without needing a direct geometrical translation of their constituent units. Differences in adsorption capacity between particulate and monolithic stationary phases show that the silica-based monoliths with a bimodal pore size distribution provide, due to the high total porosity of the material of more than 90%, comparable maximum loading capacities with respect to random-close packings of completely porous spheres.

Computational fluid dynamics simulations yielding guidelines for the ideal internal structure of monolithic liquid chromatography columns

A theoretical calculation of the separation performance of a (hypothetical) micro-structured monolithic LC column is presented, confirming that the polydispersity effect in parallel bundle columns can theoretically be eliminated to a very large extent by radially redistributing the mobile phase fluid at regular intervals. It is demonstrated that the flow can be redistributed in such a way that the advantage coming from the suppression of the polydispersity effect largely exceeds the losses caused by the additional pressure-drop and band broadening. The presently considered micro-structured column would allow to perform N > 100,000 plate separations in a few hundred of seconds, i.e., about an order of magnitude faster than the best possible packed bed and monolithic HPLC columns, while offering the same mass loadability. This clearly demonstrates that the currently available LC columns are still far away from the absolute resolution limit of the ideal, fully optimised LC column.

A correlation for the pressure drop in monolithic silica columns

To gain insight into how the pressure drop in monolithic silica columns is determined by the microscopic details of the pore structure, a series of well-validated computational fluid dynamics simulations has been performed on a simplified model structure, the so-called tetrahedral skeleton column. From these simulations, a direct correlation between the pressure drop and two main structural properties (skeleton thickness and column porosity) of the monolithic skeleton could be established. The correlation shows good agreement with the experimental pressure-drop data available from the literature on silica monoliths, especially when a correction for the flow-through pore size heterogeneity is made. The established correlation also yields a much more accurate representation of the relation between the flow resistance and the bed porosity than does the Kozeny-Carman model, making it much better suited for porosity optimization calculations.

Direct chromatographic capture of enzyme from crude homogenate using immobilized metal affinity chromatography on a continuous supermacroporous adsorbent

A continuous supermacroporous matrix has been developed allowing direct capture of enzyme from non-clarified crude cell homogenate at high flow-rates. The continuous supermacroporous matrix has been produced by radical co-polymerization of acrylamide, allyl glycidyl ether and N,N'-methylene-bis(acrylamide) which proceeds in aqueous solution of monomers frozen inside a column (cryo-polymerization). After thawing, the column contains a continuous matrix having interconnected pores of 10-100 microm size. Iminodiacetic acid covalently coupled to the cryogel is a rendering possibility for immobilized metal affinity chromatographic purification of recombinant His-tagged lactate dehydrogenase, (His)6-LDH, originating from thermophilic bacterium Bacillus stearothermophilus, but expressed in Escherichia coli. The large pore size of the adsorbent makes it possible to process particulate-containing material without blocking the column. No preliminary filtration or centrifugation is needed before application of crude extract on the supermacroporous column. A total of 210 ml crude homogenate, 75 ml of it non-clarified, was processed on a single 5.0 ml supermacroporous column at flow speeds up to 12.5 ml/min without noticeable impairment of the column properties. Mechanically the cryogel adsorbent is very stable. The continuous matrix could easily be removed from the column, dried at 70 degrees C and kept in a dry state. After rehydration and reinsertion of the matrix into an empty column, (His)6-LDH was purified as efficiently as on the newly prepared column. The procedure of manufacturing the supermacroporous continuous cryogel is technically simple. Starting materials and initiators are cheap and available and are simply mixed and frozen under specified conditions. Altogether these qualities reveal that the supermacroporous continuous cryogels is a very interesting alternative to existing methods of protein purification from particulate-containing crude extracts.

Directed immobilization of peptide ligands to accessible pore sites by conjugation with a placeholder molecule

When small ligands are immobilized onto a porous chromatography medium, only a limited number of binding sites contributes to the interaction with the target molecule. The main part of the ligand molecules is distributed on sites that are not accessible for the target protein due to steric hindrance. To direct the ligand into a well-accessible position, the ligand was conjugated to a large molecule that acted as a placeholder during the immobilization step. Then the placeholder molecule was cleaved off and washed out. Two linear peptides with affinity for lysozyme and human blood coagulation factor VIII, respectively, were studied as model systems. The protected peptide ligand was covalently linked to a 20-kDa poly(ethylene glycol) molecule containing an acid-labile linker. After selective deprotection of the peptide and purification, immobilization of this conjugate on a preactivated chromatography matrix was performed alternatively through the free N-terminus, the epsilon-amino group of lysine, or the sulfohydryl group of cysteine. After the immobilization reaction, the spacer molecule and remaining protecting groups were cleaved off and the gels were tested by affinity chromatography. This novel immobilization technique substantially increased the binding capacity and the ligand utilization for the target protein, and site-specific immobilization could be demonstrated.

Chromatography of microbial cells using continuous supermacroporous affinity and ion-exchange columns

Continuous supermacroporous chromatographic columns with anion-exchange ligands [2-(dimethylamino)ethyl group] and immobilized metal affinity (IMA) ligands (Cu2+-loaded iminodiacetic acid) have been developed allowing binding of Escherichia coli cells and the elution of bound cells with high recoveries. These poly(acrylamide)-based continuous supermacroporous columns have been produced by radical co-polymerization of monomers in aqueous solution frozen inside a column (cryo-polymerization). After thawing, the column contains a continuous matrix (so-called cryogel) with interconnected pores of 10-100 microm in size. The large pore size of the matrix makes it possible for E. coli cells to pass unhindered through a plain column containing no ligands. E. coli cells bound to an ion-exchange column at low ionic strength were eluted with 70-80% recovery at NaCl concentrations of 0.35-0.40 M, while cells bound to an IMA-column were eluted with around 80% recovery using either 10 mM imidazole or 20 mM EDTA solutions, respectively. The cells maintain their viability after the binding/elution procedure. These preliminary results indicate that microbial cells can be handled in a chromatographic mode using supermacroporous continuous columns. These columns are easy to manufacture from cheap and readily available starting materials, which make the columns suitable for single-time use.

Short monolithic columns as stationary phases for biochromatography

Monolithic supports represent a novel type of stationary phases for liquid and gas chromatography, for capillary electrochromatography, and as supports for bioconversion and solid phase synthesis. As opposed to individual particles packed into chromatographic columns, monolithic supports are cast as continuous homogeneous phases. They represent an approach that provides high rates of mass transfer at lower pressure drops as well as high efficiencies even at elevated flow rates. Therefore, much faster separations are possible and the productivity of chromatographic processes can be increased by at least one order of magnitude as compared to traditional chromatographic columns packed with porous particles. Besides the speed, the nature of the pores allows easy access even in the case of large molecules, which make monolithic supports a method of choice for the separation of nanoparticles like pDNA and viruses. Finally, for the optimal purification of larger biomolecules, the chromatographic column needs to be short. This enhances the speed of the separation process and reduces backpressure, unspecific binding, product degradation and minor changes in the structure of the biomolecule, without sacrificing resolution. Short Monolithic Columns (SMC) were engineered to combine both features and have the potential of becoming the method of choice for the purification of larger biomolecules and nanopartides on the semi-preparative scale.

Mass spectrometric detection in chromatography. Trends and perspectives

The newest results in the use of miscellaneous mass spectrometric detection methods in various chromatographic techniques are compiled and critically evaluated. Examples of application in clinical chemistry, health care, and in the analysis of pharmaceuticals, environmental pollutants, foods and food products are given. The benefits and drawbacks of MS detection are discussed, and future trends are briefly discussed.

Characterization of silica-based monoliths with bimodal pore size distribution

Band dispersion was studied and the retention thermodynamics addressed for insulin and angiotensin II on C18 silica monoliths with a bimodal pore size distribution, covering linear mobile-phase velocities up to 1 cm/s and different temperatures. These data suggest that the influence of average column pressure on retention (between 0 and 10 MPa) is not negligible. Plate height curves were interpreted with the van Deemter equation by assuming an independent contribution from mechanical and non-mechanical dispersion mechanisms. This analysis revealed diffusion-limited mass transfer in the mesoporous silica skeleton which, in turn, allowed us to calculate an equivalent dispersion particle diameter (d(disp) = 3 microm) using the C-term parameter of the van Deemter equation. The resulting superposition of reduced plate height curves for monolithic and particulate beds confirmed that this view presents an adequate analogy. The macroporous interskeleton network responsible for the hydraulic permeability of a monolith was translated to the interparticle pore space of particulate beds, and an equivalent permeability particle diameter (d(perm) = 15 microm) was obtained by scaling based on the Kozeny-Carman equation.

Effect of experimental conditions on strong biocomplimentary pairing in high-performance monolithic disk affinity chromatography

The effect of flow-rate on quantitatively determined binding parameters for several biocomplementary pairs in affinity mode high-performance monolithic disk affinity chromatography (HPMDAC) has been investigated using frontal analysis approach. Affinity interactions were evaluated from linearized adsorption isotherms and dynamic dissociation constants of the complexes K(diss.) and the theoretical adsorption capacities Q(max) were calculated. HPMDAC isolation of a typical protein trypsin from both buffered solution and artificial mixture as well as biospecific extraction of antibodies against bovine serum albumin and recombinant protein G from such complex mixtures as blood serum and cellular lysate were examined. Immobilized counterparts soybean trypsin inhibitor, bovine serum albumin, and human immunoglobulin G were used in chromatographic experiments. The maximum adsorption capacities obtained at different flow-rates were compared with those determined at static conditions. The dependence of quantitative parameters on the surface density of immobilized ligands has also been explored. Finally, a series of experiments was carried out to evaluate the dependence of dynamic affinity binding on temperature for two complementary pairs.

Direct synthesis of peptides on convective interaction media monolithic columns for affinity chromatography

Solid-phase peptide synthesis was performed on glycidyle methacrylate-co-ethylene dimethacrylate monoliths using Fmoc chemistry. The native epoxy groups were amino-functionalized by reaction with ethylenediamine or ammonia ions. A peptide directed against human blood coagulation factor VIII was synthesized as a model peptide. Amino acid analysis revealed the correct amino acid ratio as present in the sequence. The ligand density of 5 micromol/mL was equal to that achieved with conventional peptide immobilization via epoxy groups. These supports were directly used as peptide affinity chromatography matrixes. The functionality of the CIM monolithic supports was proven by affinity chromatography of factor VIII. The ammonia-functionalized support performed with low hydrophobicity and did not show unspecific adsorption of proteins.

Affinity monoliths generated by in situ polymerization of the ligand

An affinity monolith with a novel immobilization strategy was developed leading to a tailored pore structure. Hereby the ligand is conjugated to one of the monomers of the polymerization mixture prior to polymerization. After the polymerization, a monolithic structure was obtained either ready to use for affinity chromatography or ready for coupling of additional ligand to further increase the binding capacity. The model ligand, a peptide directed against lysozyme, was conjugated to glycidyl methacrylate prior to the polymerization. With this conjugate, glycidyl methacrylate, and ethylene dimethacrylate, a monolith was formed and tested with lysozyme. A better ligand presentation was achieved indicated by the higher affinity constant compared to a conventional sorbent.