Influence of covalent attachment of low molecular weight substances on the rejection and adsorption properties of crystalline proteinaceous ultrafiltration membranes

Abstract

Many bacteria possess crystalline cell surface layers (S-layers) as their outermost cell envelope component. High-resolution electron microscopical and permeability studies on S-layers revealed pores with a size from 2–8 nm. Each type of S-layer lattice showed pores of identical size and morphology. By depositing S-layer fragments on appropriate microfiltration membranes and crosslinking the S-layer protein with glutaraldehyde, it was possible to produce ultrafiltration membranes with a crystalline and isoporous active filtration layer. Due to a surplus of free carboxyl groups on the surface of the S-layer lattice and in the pore areas, these composite S-layer ultrafiltration membranes (SUMs) showed a net negative charge. Upon converting the free carboxyl groups into neutral or positively charged groups by amidation, both the adsorption and rejection properties of SUMs could be altered. As expected, neutral SUMs showed the lowest unspecific protein adsorption. Membrane modification studies further demonstrated that in addition to the molecular size, the physicochemical properties of the immobilized compounds and the resulting interactions with the test proteins in solution determine the rejection characteristics of SUMs.