Dependence of pore diffusivity of protein on adsorption density in anion-exchange adsorbent

Experimental study of zeolitic diffusion by use of a concentration-dependent surface diffusion model

Surface diffusivity in adsorption and ion exchange processes is probably the most important property studied expensively in the literature but some aspects, especially its dependence on solid phase concentration, is still an open subject to discussion. In this study a new concentration-dependent surface diffusion model, equipped with a flexible double selectivity equilibrium relationship is applied on the removal of Pb²⁺, Cr³⁺, Fe³⁺ and Cu²⁺ from aqueous solutions using a natural zeolite. The model incorporates the Chen-Yang surface diffusivity correlation able to deal with positive and negative dependence with surface coverage. The double selectivity equilibrium relationship successfully represents the experimental equilibrium data, which follow Langmurian isotherm type for Pb²⁺, sigmoidal for Cr³⁺ and Fe³⁺ and linear for Cu²⁺. The concentration-dependent surface diffusion model was compared with the constant diffusivity surface diffusion model and found to be moderately more accurate but considerably more useful as it provides more insights into the diffusion mechanism. The application of the model resulted in an average deviation of 8.56 ± 6.74% from the experimental data and an average solid phase diffusion coefficients between 10⁻⁹ and 10⁻¹⁰ cm²/s. The results showed that the diffusion of metal ions in the zeolite structure is unhindered following the surface diffusion mass transfer mechanism.

Approaches to high-performance preparative chromatography of proteins

Preparative liquid chromatography is widely used for the purification of chemical and biological substances. Different from high-performance liquid chromatography for the analysis of many different components at minimized sample loading, high-performance preparative chromatography is of much larger scale and should be of high resolution and high capacity at high operation speed and low to moderate pressure drop. There are various approaches to this end. For biochemical engineers, the traditional way is to model and optimize a purification process to make it exert its maximum capability. For high-performance separations, however, we need to improve chromatographic technology itself. We herein discuss four approaches in this review, mainly based on the recent studies in our group. The first is the development of high-performance matrices, because packing material is the central component of chromatography. Progress in the fabrication of superporous materials in both beaded and monolithic forms are reviewed. The second topic is the discovery and design of affinity ligands for proteins. In most chromatographic methods, proteins are separated based on their interactions with the ligands attached to the surface of porous media. A target-specific ligand can offer selective purification of desired proteins. Third, electrochromatography is discussed. An electric field applied to a chromatographic column can induce additional separation mechanisms besides chromatography, and result in electrokinetic transport of protein molecules and/or the fluid inside pores, thus leading to high-performance separations. Finally, expanded-bed adsorption is described for process integration to reduce separation steps and process time.

Investigation of pore diffusion hindrance of monoclonal antibody in hydrophobic interaction chromatography using confocal laser scanning microscopy

In this article, hindrance of intraparticle mass transfer during the adsorption of a monoclonal antibody (mAb) on Butyl Sepharose 4FF was investigated under different conditions. In addition to common fluid phase measurements, confocal laser scanning microscopy (CLSM) was applied to evaluate the respective intraparticle concentration profiles. In order to ensure that the observed intraparticle protein distributions are not disturbed by artefacts of CLSM, microscopic data are carefully analysed considering signal attenuation and competitive adsorption between labelled and native species. Using this setup, lower protein concentration in the inner particle region was observed even after long equilibration times in protein solution. Since the observed phenomenon showed a dependency on the amount of adsorbed protein, we assumed that the intraparticle diffusion was hindered by the adsorbed protein molecules. We propose hypotheses on the diffusion hindrance, and compare the experimental results with model-based simulations of single particles that include novel terms for the description of hindered diffusion.

Fabrication and characterization of superporous cellulose bead for high-speed protein chromatography

Novel superporous cellulose (SC) matrix has been fabricated by water-in-oil emulsification-thermal regeneration using granules of calcium carbonate as porogenic agents. As a control, microporous cellulose (MC) bead was fabricated in the absence of calcium carbonate. Simultaneously, double cross-linking was applied to enhance the mechanical strength of the particles. The photographs by scanning electron microscopy of the SC bead illustrated that there were more "craters" of several microns scattering on the surface of the beads. It led to a higher water content and effective porosity of the SC medium. The two beads were then modified with diethylaminoethyl (DEAE) group to prepare anion exchangers. The dynamic uptake results of bovine serum albumin (BSA) exhibited that the pore diffusivity of BSA in the DEAE-SC bead was two to three times larger than that in the DEAE-MC bead. In addition, the column packed with the DEAE-SC showed lower backpressure, higher column efficiency and dynamic binding capacity than the column packed with the DEAE-MC at a flow rate range of 150-900cm/h. Moreover, the column efficiency of the DEAE-SC column was independent of flow velocity up to a flow rate of 1200cm/h. All the results exhibited the superior characteristics of the SC bead as a potential medium for high-speed protein chromatography.

Modeling of the whole expanded-bed protein adsorption process with yeast cell suspensions as feedstock

Expanded bed adsorption of bovine serum albumin (BSA) directly from a feedstock containing whole yeast cells has been investigated with an anion-exchanger DEAE Spherodex M. In the presence of 6% (w/w) yeast cells, the axial liquid-phase dispersion coefficient was found in the order of 10(-6) m2/s, which felled into the common range of 1.0 x 10(-6)-1.0 x 10(-5) m2/s observed previously without the use of cell suspensions as mobile phase. We found that the static and dynamic binding capacity of BSA decreased with increasing the yeast cell concentration due to the competitive adsorption of cells onto the outer surface of the anion-exchanger. However, because of the small size of the adsorbent, the large pore diffusivity of protein and the favorable column efficiency (low axial dispersion coefficient), the dynamic binding capacity of BSA in the presence of 6% (w/w) cells in the expanded bed reached 86% that of the equilibrium adsorption density. Then, the whole expanded bed adsorption process of BSA in the presence of cells, including feedstock loading, washing and elution steps, was predicted using a mathematical model with parameters all determined independently. In the elution stage, the steric mass-action adsorption isotherm with salt concentration as one of the model parameters was used to predict the step-gradient elution process with salt concentration increases. Computer simulations showed that the model was in good agreement with the experimental results for the whole operation process.