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The effect of in-place regeneration of dye-ligand adsorbents on protein adsorption characteristics is presented. Regeneration with chemical treatments and time of exposure determined the protein capacity of the adsorbent, but no effect was observed on its protein binding affinity. Fixed-bed adsorption of bovine serum albumin and its selectivity with respect to lysozyme was studied. Breakthrough curves were measured for protein adsorption on fixed-bed columns and analyzed by a simple model to determine the relevant rate constants for the adsorption process. It was found that forward adsorption rate constant increased exponentially with the chemical treatment exposure time. Column linear gradient elution studies showed that adsorbent selectivity decreased with the chemical treatment exposure time due mainly to column loss of adsorption capacity. The implications of the results on the design and optimization of dye-ligand chromatographic processes are discussed.

Arthrobacter simplex ATCC 6946 (viable cells) was immobilized in a calcium polygalacturonate gel. The trapped cells were used for repeated batchwise bioconversion of steroids. Reichstein's compound S and hydrocortisone were dehydrogenated introducing a double bond between C1 and C2 of ring A. The products 1-dehydro S and prednisolone, respectively, were identified by high pressure liquid chromatography. Steroid dehydrogenase activity increased in the system when an artificial electron acceptor, such as menadione (vitamin K3) was present in the reaction mixture. An airlift-type reactor was used to bioconvert up to 90% of substrate in 15 min, under optimal conditions. The gel entrapped cell preparations were used for repeated batch bioconversion during 30 days; 69 batch bioconversions for Reichstein's compound S were performed during 15 days of operation of the reactor. The operational stability of the process and the feasibility of repeated batch bioconversions was shown to be comparable to similar processes.

A method for af®nity membrane column design, based on the analytical solution of the Thomas model for frontal analysis in membrane column adsorption, was developed. The method permits to ®nd the operating conditions to reach a 93.5% of the column capacity as operating capacity, using a sharpness restriction for the system breakthrough curve.The solution of the model is presented in a graphic form and can be used in a wide range of operational conditions, provided that four design restrictions are ful®lled. The application of the method was illustrated using experimental data and a simple procedure. The implications of the results on the design and optimiztion of af®nity membrane chromatographic columns are discussed. List of symbolsA membrane column cross-sectional area, cm 2 c solute concentration in the bulk phase, M c 0 solute concentration in the bulk phase at column inlet, M c à solute concentration in the bulk phase at equilibrium, M I 0 modi®ed zero-order Bessel function of the ®rst kind d p average pore diameter, cm D diffusion coef®cient, cm 2 s À1 F volumetric¯ow-rate, ml min À1 k 1 forward adsorption rate constant, M À1 s À1 k À1 reverse adsorption rate constant, s À1 K d dissociation constant, M L length of membrane column, lm L m membrane thickness, lm n dimensionless number of transfer units for overall process P molecule of protein PS complex between protein and ligand adsorbent q average protein concentration, M q à adsorbate concentration at equilibrium, M q m maximum binding capacity of the membrane, based on the solid volume, M r dimensionless separation factor S ligand adsorption site t time, s z axial distance along the membrane column Greek letters vtY L dimensionless solute concentration e void fraction C dimensionless ef¯uent volume s dimensionless time v linear velocity, cm s À1 IntroductionAf®nity membrane chromatography is a novel puri®ca-tion method that exploits the biospeci®c interactions between a protein and a ligand to economically purify proteins present at very low concentrations in complex solutions [1]. Af®nity membranes were developed to overcome the limitations encountered in conventional commercial processes using beads [2±5]. Af®nity membranes operate in convective mode, which can signi®-cantly reduce diffusion limitations commonly encountered in column chromatography. As a result, higher throughput and faster processing times are achieved in membrane systems [6]. Axial and radial diffusion, sorption kinetics, and nonuniformities in membrane porosity and thickness have been shown to affect af®nity membrane performance key factors such as breakthrough curve (BTC) sharpness and residence time. Degradation of membrane performance can be minimized working with axial Peclet number greater than 40, and radial Peclet numbers smaller than 0.04. Stacking more than 30 membranes averages outmembrane porosity and thickness nonuniformities [7]. Under these conditions, the membrane system performance can be predicted using the analytic solution of the

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