Influence of surface charge on protein adsorption at an amphoteric surface: Effects of varying acid to base ratio

Plasma polymerization of 1,2-diaminocyclohexane (DACH) results in an amino functional surface which is not stable under ambient conditions. The aging process has previously been assumed to be an oxidation initiated by residual polymer radicals. We here propose that the surface process is not an oxidation but a carboxylation. Primary amino groups react with atmospheric carbon dioxide to give carbamate groups which are stabilized by salt formation with adjacent ammonium groups. We have used a novel electrokinetic technique to measure electroosmosis and monitor the aging process. While the total number of ionizable surface groups was constant, a gradual change in the ratio of basic to acid sites at the surface was found. This amphoteric surface which spontaneously changed its ratio of basic to acid sites was used as substrate to investigate the effect of surface charge on adsorption of two proteins: human serum albumin (HSA) and lysozyme. It was found that lysozyme adsorbs in much higher amounts that HSA irrespective of the charge at the surface. Whereas adsorption of HSA was not dependent on the change in surface charge, adsorption of lysozyme decreased with aging, i.e., as the ratio of acid to base sites at the surface approached unity. An interesting finding was that more lysozyme was adsorbed the more positively charged the surface, even at pH values where the protein carried a strong net positive charge. (C) 1996 Academic Press, Inc.