Sorption of proteins by slightly cross-linked polyelectrolyte hydrogels: Kinetics and mechanism

Sorption of proteins such as cytochrome c, lysozyme or protamine by slightly cross-linked poly(acrylic acid) and bovine serum albumin by slightly cross-linked poly(N,N-dimethyl-N-ethylaminoethyI methacrylate bromide) hydrogels in salt-free and saline aqueous solutions was studied. The polyanionic hydrogel uptakes the proteins at pH below their isoelectric points while polycationic one at pH above them. As a result highly swollen original hydrogel transforms into relatively compact cross-linked polyelectrolyte-protein complex. Sorption of proteins by slightly cross-linked polyelectrolyte hydrogels is a chemically drawn diffusion process. The driving force of the process comes from the gain in the free energy of the interpolyelectrolyte coupling reaction between the protein and oppositely charged segments of the polyelectrolyte network. Apparently the mechanism of protein uptake is similar to that earlier proposed for linear polyelectrolytes. It involves a relay-race transfer of protein molecules from one fragment of polyelectrolyte network to the other without radial mixing via interpolyelectrolyte exchange reaction. As a result core-shell constructs consisting of an outer weakly swollen complex shell and a highly swollen hydrogel core are formed at intermediate stages of the process. The rate of sorption is determined by the rate of the interpolyelectrolyte exchange reaction, that is the rate of the formation of free fragments of polyelectrolyte network (vacancies) on the inner complex-hydrogel boundary. The amount of vacancies depends on the area of this boundary. Kinetic curves of protein sorption by hydrogels in neutral salt-free solutions could not be fitted under the terms of Fickian diffusion but can be expressed in terms of the kinetic equation derived for a frontal heterogeneous reaction. At the same time kinetics of protein sorption obeys to Fick's diffusion law when the salt concentration increased.