Biotechnol. Prog. 1994, 70, 76-81
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Binding of Lysozyme onto a Cation-Exchange Microporous Membrane Containing Tentacle-Type Grafted Polymer Branches Satoshi Tsuneda, Hironori Shinano, Kyoichi Saito,*and Shintaro Furusaki Department of Chemical Engineering, Faculty of Engineering, University of Tokyo, Hongo, Tokyo 113, Japan
Takanobu Sugo Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute, Takasaki, Gunma 370-12, Japan
Ion-exchange adsorption of lysozyme to the sulfonic acid (S03H) group on polymer chains grafted onto microporous polyethylene hollow-fiber membranes was examined. The lysozyme solution was forced to permeate across the hollow fiber. Diversely anchored S03H groups, i.e., SP and SS groups, were introduced into the membrane by reaction of the glycidyl methacrylate-grafted membrane with propanesultone and sodium sulfite, respectively. The resulting SP and SS group-containing membranes, designated as SP-Tand SS-Tfibers, respectively, had 95 and 77 96 water flux of the original membrane, respectively. The binding capacity of lysozyme as a function of the S03H group density was compared between the SP-T and SS-Tfibers from measurement of the ion-exchange breakthrough curves during the permeation of lysozyme solution across the SP-T and SS-T fibers. The binding capacity of lysozyme to the SP-T fiber remained constant, independent of the SP group density, whereas that to the SS-T fiber increased linearly with increasing SS group density. This difference was explained by means of a model whereby lysozyme adheres onto the SP group-containing grafted polymer branches, while the SS group-containing grafted polymer branches hold lysozyme in a tentaclelike manner.
Introduction The microporous ion-exchangemembrane is a promising alternative to macroporous ion-exchange beads because proteins with lower diffusivity are transported to the ionexchange group by means of convection. Radiationinduced grafting is a powerful method by which one may attach specific functional moieties to a microporous membrane (Tsuneda et al., 1992). Appropriate selection of conditions for grafting a monomer onto a microporous membrane, such as pre-irradiation or simultaneous irradiation and vapor- or liquid-phase contact with the monomer, enabled us to maintain feasible flux after grafting (Yamagishi et al., 1991a). The chemical and physical structures of the grafted polymer branches determine the water permeability and protein adsorptivity of the resulting microporous membrane. In addition, interaction of proteins with the ion-exchange group on the grafted polymer branches should be clarified so as to design functional membranes capable of efficiently collecting proteins. A cation-exchange group has been used for the purification and separation of proteins (Church et al., 1986; Kat0 et al., 1987). The sulfonic acid (S03H) group is representative of a strongly acidic cation-exchange group. Several schemes of introduction of the S03H group into the polymeric materials have been proposed; sulfonation includes direct attack of concentrated sulfuric acid on the benzene ring, copolymerization of S03Hgroup-containing monomers, reaction of the diols with propanesultone, and ring-opening of the epoxides with sodium sulfite. In a previous study (Shinanoet al., 19931,the sulfopropylgroup
* Author
t o whom correspondence should be addressed.
8756-7938/94/3010-0076$04.50/0
((CH2)3S03H)was introduced into a microporous membrane, and ion exchange during permeation of biomacromolecular (lysozyme) and metal ions (Cu ion) was compared with regard to its rate and capacity. Selection of the S03H group is required to obtain a microporous cation-exchange membrane suitable for processing biological fluids such as broth and serum under water permeability and protein adsorptivity criteria. In this study, diversely anchored S03H groups were introduced into the polymer chains grafted on the pore surface of the microporous membrane, and then a solution containing lysozyme as a model protein was permeated across the S03H group-containing membrane through the pores to determine ion-exchange adsorption rate and capacity. The objectiveof our study was threefold: (1)to introduce the three variations of the S03H group into microporous membranes via radiation-induced grafting and subsequent chemical modifications; (2) to evaluate the binding rate and capacity from measurement of the breakthrough curves of lysozyme during permeation across the S03H group-containing microporous membrane; and (3) to discuss the binding structure of lysozyme to the grafted polymer branches containing the SO3H group.
Experimental Procedures Materials. Microporous hollow-fiber membrane used as a trunk polymer was made of polyethylene with a nominal 0.34-pm pore diameter and 719% porosity. The inner and outer diameters of this hollow fiber were 1.95 and 3.01 mm, respectively. Sodium styrenesulfonate (SSS, CH2=CHCeH&03Na) was purchased from Tosoh Co. and purified before grafting. Technical grade glycidyl methacrylate (GMA,CHYCCH~COOCH~CHOCH~ and 2-hy-
0 1994 American Chemical Society and American Institute of Chemical Engineers
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Bbtechnoi. Rog., 1994, Vol. IO, No. 1
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