High-performance liquid chromatography of proteins on compressed, non-porous agarose beads. I. Hydrophobic-interaction chromatography

Polysaccharides as separation media for the separation of proteins, peptides and stereoisomers of amino acids

Reliable separation of peptides, amino acids and proteins as accurate as possible with the maximum conformation and biological activity is crucial and essential for drug discovery. Polysaccharide, as one of the most abundant natural biopolymers with optical activity on earth, is easy to be functionalized due to lots of hydroxyl groups on glucose units. Over the last few decades, polysaccharide derivatives are gradually employed as effective separation media. The highly-ordered helical structure contributes to complex, diverse molecular recognition ability, allowing polysaccharide derivatives to selectively interact with different analytes. This article reviews the development, application and prospects of polysaccharides as separation media in the separation of proteins, peptides and amino acids in recent years. The chiral molecules mechanism, advantages, limitations, development status and challenges faced by polysaccharides as separation media in molecular recognition are summarized. Meanwhile, the direction of its continued development and future prospects are also discussed.

Preparation of cell-encapsulation devices in confined microenvironment

The entrapment of cells into hydrogel microdevice in form of microparticles or microfibers is one of the most appealing and useful tools for cell-based therapy and tissue engineering. Cell encapsulation procedures allow the immunoisolation of cells from the surrounding environment, after their transplantation and the maintenance of the normal cellular physiology. Factors affecting the efficacy of microdevices, which include size, size distribution, morphology, and porosity are all highly dependent on the method of preparation. In this respect, microfluidic based methods offer a promising strategy to fabricate highly uniform and morphologically controlled microdevices with tunable chemical and mechanical properties. In the current review, various cell microencapsulation procedures, based on a microfluidics, are critically analyzed with a special focus on the effect of the procedure on the morphology, viability and functions of the embedded cells. Moreover, a brief introduction about the optimal characteristics of microdevice intended for cell encapsulation, together with the currently used materials for the production is reported. A further challenging application of microfluidics for the development of "living microchip" is also presented. Finally, the limitations, challenging and future work on the microfluidic approach are also discussed.

Stellan Hjertén's contribution to the development of monolithic stationary phases

This overview is presented to celebrate the birthday of one of the luminaries of the separation science and my friend - Stellan Hjertén. He made significant contributions to a variety of areas in separation science such as electrophoresis, LC, and CEC to name just a few. Since the scope of his work was enormous, this review will focus only on a single aspect of his scientific activities, the design and applications of monolithic materials. During the years starting from 1989, Stellan Hjertén published many excellent papers concerning the preparation of acrylamide chemistry-based monoliths and their use in both micro-HPLC and CEC. The following text details his works in the field.

A model of flow-through pore formation in methacrylate ester-based monolithic columns

The main factors affecting the porosity of methacrylate-ester based monolithic columns were investigated. We prepared 23 monolithic capillary columns with porosity controlled by varying the proportions of butyl methacrylate and ethylene dimethacrylate monomers and of 1,4-butanediol and 1-propanol as the porogen solvent in the polymerization mixtures by thermally initiated in-situ polymerization in fused-silica capillaries. Using mixture design software, we systematically varied the composition of the polymerisation mixtures to find significant factors affecting flow-through pore formation. Multivariate analysis of the experimental data obtained for the fabricated columns yielded a model for prediction of the flow-through porosity in monolithic beds as a function of the composition of the polymerization mixture used to prepare polymethacrylate monolithic capillary columns. The mean error of prediction was lower than 8% for eight columns prepared independently of the original set of 15 columns used to derive the flow-through model. The flow-through porosity increases with increasing concentration of the binary porogen solvent mixture, the concentration of 1,4-butanediol being the main factor enhancing flow-through pore formation. On the other hand, increasing concentrations of the hydrophobic monomer butyl methacrylate and increasing concentrations of 1-propanol have a negative effect on flow-through pore formation. The capillary columns prepared with a high proportion of flow-through pores and a minimum amount of mesopores can be used for fast gradient separations of both low-molecular weight compounds and biopolymers.

Monolithic columns in high-performance liquid chromatography

Monolithic media have been used for various niche applications in gas or liquid chromatography for a long time. Only recently did they acquire a major importance in high-performance column liquid chromatography (HPLC). The advent of monolithic silica standard- and narrow-bore columns and of several families of polymer-based monolithic columns has considerably changed the HPLC field, particularly in the area of narrow-bore columns. The origin of the concept, the differences between their characteristics and those of traditional packed columns, their advantages and drawbacks, the methods of preparation of monoliths of different forms, and the current status of the field are reviewed. The actual and potential performance of monolithic columns are compared with those of packed columns. Monolithic columns have considerable advantages, which makes them most useful in many applications of liquid chromatography. They are extremely permeable and offer a high efficiency that decreases slowly with increasing flow velocity.

The suitability of DEAE-Cl active groups on customized poly(GMA-co-EDMA) continuous stationary phase for fast enzyme-free isolation of plasmid DNA

The creation of a commercially viable and a large-scale purification process for plasmid DNA (pDNA) production requires a whole-systems continuous or semi-continuous purification strategy employing optimised stationary adsorption phase(s) without the use of expensive and toxic chemicals, avian/bovine-derived enzymes and several built-in unit processes, thus affecting overall plasmid recovery, processing time and economics. Continuous stationary phases are known to offer fast separation due to their large pore diameter making large molecule pDNA easily accessible with limited mass transfer resistance even at high flow rates. A monolithic stationary sorbent was synthesised via free radical liquid porogenic polymerisation of ethylene glycol dimethacrylate (EDMA) and glycidyl methacrylate (GMA) with surface and pore characteristics tailored specifically for plasmid binding, retention and elution. The polymer was functionalised with an amine active group for anion-exchange purification of pDNA from cleared lysate obtained from E. coli DH5alpha-pUC19 pellets in RNase/protease-free process. Characterization of the resin showed a unique porous material with 70% of the pores sizes above 300 nm. The final product isolated from anion-exchange purification in only 5 min was pure and homogenous supercoiled pDNA with no gDNA, RNA and protein contamination as confirmed with DNA electrophoresis, restriction analysis and SDS page. The resin showed a maximum binding capacity of 15.2 mg/mL and this capacity persisted after several applications of the resin. This technique is cGMP compatible and commercially viable for rapid isolation of pDNA.

Preparation of monolithic columns with target mesopore-size distribution for potential use in size-exclusion chromatography

The main factors affecting the mesopore porosity of methacrylate-ester based monolithic columns were investigated. We prepared 40 monolithic capillary columns with porosity controlled by varying the proportions of butyl methacrylate (BMA) and ethylene dimethacrylate (EDMA) monomers and of 1,4-butanediol (BUT) and 1-propanol (PROP) as the porogen solvent in the polymerisation mixtures by thermally initiated in situ polymerisation in fused-silica capillaries. Using mixture design software, we systematically varied the composition of the polymerisation mixtures to find significant factors affecting mesopore formation. Multivariate analysis of the experimental data obtained for the fabricated columns yielded a model for prediction of the mesopore porosity in monolithic beds as a function of the composition of the polymerisation mixture used to prepare polymethacrylate monolithic capillary columns. The mean absolute deviation of predicted porosities is 0.029 for most of the columns, with only eight columns showing deviations exceeding 0.050. The main factor affecting the mesopore porosity proved to be the combination of the concentration of hydrophobic monomer (BMA) and the concentration of the less-polar solvent, 1-propanol, in the porogen mixture. The proportion of mesopores in the monolithic capillary columns increases with increasing concentration of 1-propanol and with decreasing concentration ratios of the cross-linker (EDMA) to monomer (BMA) and of BUT to PROP porogenic solvents.

Pore structural characterization of monolithic silica columns by inverse size-exclusion chromatography

In this work, a parallel pore model (PPM) and a pore network model (PNM) are developed to provide a state-of-art method for the calculation of several characteristic pore structural parameters from inverse size-exclusion chromatography (ISEC) experiments. The proposed PPM and PNM could be applicable to both monoliths and columns packed with porous particles. The PPM and PNM proposed in this work are able to predict the existence of the second inflection point in the experimental exclusion curve that has been observed for monolithic materials by accounting for volume partitioning of the polymer standards in the macropores of the column. The appearance and prominence of the second inflection point in the exclusion curve is determined to depend strongly on the void fraction of the macropores (flow-through pores), (b) the nominal diameter of the macropores, and (c) the radius of gyration of the largest polymer standard employed in the determination of the experimental ISEC exclusion curve. The conditions that dictate the appearance and prominence of the second inflection point in the exclusion curve are presented. The proposed models are applied to experimentally measured ISEC exclusion curves of six silica monoliths having different macropore and mesopore diameters. The PPM and PNM proposed in this work are able to determine the void fractions of the macropores and silica skeleton, the pore connectivity of the mesopores, as well as the pore number distribution (PND) and pore volume distribution (PVD) of the mesopores. The results indicate that the mesoporous structure of all materials studied is well connected as evidenced by the similarities between the PVDs calculated with the PPM and the PNM, and by the high pore connectivity values obtained from the PNM. Due to the fact that the proposed models can predict the existence of the second inflection point in the exclusion curves, the proposed models could be more applicable than other models for ISEC characterization of chromatographic columns with small diameter macropores (interstitial pores) and/or large macropore (interstitial pore) void fractions. It should be noted that the PNM can always be applied without the use of the PPM, since the PPM is an idealization that considers an infinitely connected porous medium and for materials having a low (<6) pore connectivity the PPM would force the PVD to a lower average diameter and larger distribution width as opposed to properly accounting for the network effects present in the real porous medium.

Preparation of uniform-sized agarose beads by microporous membrane emulsification technique

Uniform-sized agarose beads were prepared by membrane emulsification technique in this study. Agarose was dissolved in boiling water (containing 0.9% sodium chloride) and used as water phase. A mixture of liquid paraffin and petroleum ether containing 4 wt% of hexaglycerin penta ester (PO-500) emulsifier was used as oil phase. At 55 degrees C, the water phase permeated through uniform pores of microporous membrane into the oil phase by a pressure of nitrogen gas to form uniform W/O emulsion. Then the emulsion was cooled down to room temperature under gentle agitation to form gel beads. The effect of oil phase, emulsifier, especially temperature on the uniformity of the beads were investigated and interpreted from interfacial tension between water phase and oil phase. Under optimized condition, the coefficient variation (C.V.) showing the size distribution of the beads was under 15%. This was the first report to prepare uniform agarose beads by membrane emulsification, and to investigate the effect of temperature on the size distribution of the droplets and beads. The beads with different size can be prepared by using membranes with different pore size, and the result showed that there was a linear relationship between the average diameter of beads and pore size of the membranes; beads with diameter from 15 to 60 microm were able to obtain in this study.

Dispersion effects in preparative polymethacrylate monoliths operated in radial-flow columns

Monolithic media have found widespread use as excellent tools for fast analytical separations of small molecules, proteins, pDNA and viruses. Polymethacrylate monoliths with large channels are attractive for capturing large molecules, like immunoglobulins, DNA, and viruses. For preparative purposes, these monoliths are operated in radial flow mode. Band spreading in monoliths is extremely low and mostly dominated by the contribution of extra column effects. The model used here had a single axial dispersion coefficient which lumps together extra column effects and the intrinsic band spreading of the monolithic material to characterize the adsorption of proteins and pDNA on polymethacrylate ion-exchange monoliths. Due to the fact that the performance of the monolith was unaffected by the velocity within the applied range, and due to highly favourable adsorption isotherms, a constant pattern model could be applied to predict preparative runs on radial flow units assuming axial flow for modelling.

Chromatographic media for bioseparation

Bioseparation processes are dominated by chromatographic steps. Even primary recovery is sometimes accomplished by chromatographic separation, using a fluidized bed instead of a fixed bed. In this review, the action principles, features of chromatography media regarding physical and chemical properties will be described. An attempt will be made to establish categories of different media. Characteristics for bioseparation are the large pores and particle sizes. To achieve sufficient capacity for ultralarge molecules, such as plasmids or nanoparticles, such as viruses monoliths are the media of choice. In these media, the mass transport is accomplished by convection, and thus, the low diffusivity can be overcome. Common to all modern chromatography media is the fast operation. There are examples where a residence time of less then 3 min, is sufficient to reach the full potential of the adsorbent.

Preparation of strong anion-exchange chromatographic packings based on monodisperse polymeric beads and their application in the separation of biopolymers

A new hydrophilic strong anion-exchange (SAX) stationary phase for HPLC has been synthesized by chemical modification of macroporous 8.0-microm monodisperse poly(glycidylmethacrylate-co-ethylenedimethacrylate) beads (P(GMA/EDMA)). The stationary phase was evaluated in detail to determine its ion-exchange properties, separability, reproducibility, hydrophilicity, and the effect of column loading and pH on the separation and retention of proteins. It was found to have an ion-exchange chromatographic (IEC) retention mechanism. The highest dynamic protein loading capacity of the synthesized SAX packing for BSA was 22.6 mg g(-1). Five proteins were separated within 6.0 min using the synthesized SAX resin. The SAX resin was also used for rapid separation and purification of recombinant human stem cell factor (rhSCF) from a crude extract solution in only one step. The purity of the purified of rhSCF was >92.4%.

Properties and performance of novel high-resolution/high-permeability ion-exchange media for protein chromatography

There is continued interest in the development of stationary phases for protein chromatography that can provide high resolution at elevated flow rates of the mobile phase. When using porous particles, resolution and dynamic binding capacity decline rapidly as the flow rate is increased. Monolithic columns have been developed to overcome these limitations. However, there are difficulties in manufacturing homogeneous larger scale monoliths. In this paper we investigate the morphology and performance characteristics of columns based on new ion exchangers obtained by mechanically disrupting continuous beds of acrylamido-based polymeric media. Near colloidal suspensions of loose particles obtained with this procedure can be flow-packed in ordinary chromatography columns resulting in beds of unexpectedly high hydraulic permeability. Columns up to 2.2 cm in diameter were studied with both Q and S functionalized media. The hydraulic permeability and interparticle porosity of these columns were rather high. The permeabilities of the S and Q media were 1.5 x 10(-13) and 2.4 x 10(-13) m2, respectively, while the corresponding porosities were 60 and 70%. These porosity values are similar to those of monoliths, suggesting that these particles assemble under flow to give high-porosity bridged structures. The structure of these packed beds was further characterized by embedding small packed columns in resins and obtaining sections for microscopic observation. The sections reveal the presence of small aggregates of non-porous 1-3 microm particles, surrounded by flow channels several micrometers in size. The height equivalent to a theoretical plate under isocratic and gradient elution conditions and the dynamic binding capacity were determined for several proteins and were found to be virtually independent of flow.

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.

Silica gel-based monoliths prepared by the sol-gel method: facts and figures

There is a great deal of interest in continuous beds as stationary phases for both HPLC and CEC. There are various ways to prepare monoliths, by polymerization of organic species or by polymerization of silicon alkoxides. The former method has recently been reviewed, while silica based monoliths are now commercially available. The purpose of this paper is to deal with the problems associated with silica based monoliths. The most important problem is obviously the cracking and the shrinkage of the bed during drying. The second problem is monolith cladding. Much literature has been published but no definitive solution is available and thus a wide research area remains open. Monoliths are a compromise between loadability, permeability and mass transfer kinetics. Due to the better mass transfer properties of a monolithic skeleton over distinct particles, high flow rates and high speed separations are possible.

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.

Hydrophobic interaction chromatography of proteins

In this article, an overview of hydrophobic interaction chromatography (HIC) of proteins is given. After a brief description of protein hydrophobicity and hydrophobic interactions, we present the different proposed theories for the retention mechanism of proteins in HIC. Additionally, the main parameters to consider for the optimization of fractionation processes by HIC and the stationary phases available were described. Selected examples of protein fractionation by HIC are also presented.

Monoliths as stationary phases for separation of proteins and polynucleotides and enzymatic conversion

Monoliths are considered as a novel generation of stationary phases. They were applied for capillary electrochromatography and liquid chromatography exploiting every action principle such as ion-exchange, affinity recognition, reversed-phase, and hydrophobic interaction. The fast separation was explained by convective transport of the solutes through the bed. The contribution of this mode of transport is similarly explained as done for the beds packed with particles with gigapores. For monolithic beds, the concept of an ultrashort bed was frequently used. This mode of operation allows very short separation time. In many cases a gradient elution is necessary to achieve separation. Examples of applications for protein and polynucleotide separation performed on monoliths are given. Enzymatic conversion was described showing the examples of several immobilzed enzymes.

Peak broadening in protein chromatography with monoliths at very fast separations

Monoliths are stationary phases cast as a continuous medium which are interlaced by flow channels ramified with micropores. Pulse response experiments with bovine serum albumin as a model protein were applied for testing polymethacrylate-based monoliths, resulting in peak broadening that practically was not influenced by the chromatographic velocity. An empirical model was developed to describe peak broadening, allowing a term to account for the pore convection and a term for the pore diffusion. A diffusional distance lower than 10 nm was estimated. This corresponds to values observed with monodisperse 1-microm particles. Systematic investigations by changing the response time of the detector showed that the full potential of the monoliths could not be exploited, since the currently available chromatography systems are the limiting factor regarding the speed of data acquisition and virtual peak broadening by the infinite length of the detector. Inertia of the liquid and synchronization between liquid handling and electronic control introduced an additional disturbance. At the lowest possible response time, reliable peak data could be obtained up to a velocity of 35 cm/min. The pressure drop along the continuous bed was much smaller compared to a conventionally packed bed. Different flow patterns and significantly reduced eddy vortexes may be responsible for the high specific permeability.

Protein separation using non-porous sorbents

This article overviews the development of non-porous sorbents having small particle diameters which have proven effective for rapid analysis and micropreparative separation of proteins by liquid chromatography. Much attention is given to the preparation and application of silica- and polystyrene-based non-porous packings for various chromatographic modes, especially affinity chromatography. Modeling works on the prediction and parameter estimation for the dynamics of protein adsorption using non-porous sorbents are reviewed and briefly described. To conclude this review, future prospects of the application of non-porous sorbents are also presented.

Synthesis and chromatographic characterization of dextran-coated zirconia high-performance liquid chromatographic stationary phases

Porous zirconia particles made by the oil emulsion (OE) method and the polymerization-induced colloid aggregation (PICA) method have been coated with a small, carboxymethylated (approximately 5%) dextran polymer and crosslinked in place. The parameters of the coating process (dextran concentration, adsorption time and crosslinker concentration) have all been examined and an optimum value for each determined. The coated and uncoated materials were characterized by nitrogen sorptometry and size-exclusion chromatography (SEC) using solutes (polystyrenes and dextrans) of well-defined molecular masses. Nitrogen sorptometry results show that the PICA material has a much lower pore volume and smaller pore diameter than do the OE materials. Despite this, the elution volumes of the SEC probes change very little upon polymer coating the PICA material while the OE material shows a very large change upon coating.

Effect of bed compression on high-performance liquid chromatography columns with gigaporous polymeric packings

The behavior of chromatographic columns packed with gigaporous, highly cross-linked styrenic particles was investigated for use in protein separation by reversed-phase chromatography at high flow velocities. Stainless-steel columns which were 3.5 or 7.5 cm long and had an inner diameter of 0.46 mm were slurry packed with 8 or 20 mm diameter spherical particles of 4000 A mean pore size by using methanol as the packing fluid. It was found that the conditions employed during the packing process have a dramatic effect on the properties of such columns and that this can be attributed in part to the deformability of the particles. An increase in the packing pressure to approximately 6000 p.s.i. (41 MPa) resulted in a higher mass-transfer efficiency for the column with a concomitant decrease in permeability. This is ascribed to a decrease in the interstitial porosity with increasing packing pressure since the experimentally measured plate heights for these columns were found to agree quantitatively with theoretical predictions that relate changes in the interstitial porosity to intraparticle mass transfer. However, the theoretically derived relationship between porosity, permeability, and efficiency does not hold for columns packed at pressures higher than 6000 p.s.i., in which case the total column porosity was found to be high while the permeability and column efficiency were low. This behavior is explained by the formation of a low-porosity layer of highly compressed particles at the downstream end of the column during high pressure packing so that the assumption of axially uniform column properties used in the theoretical approach leads to very large errors.