The relationship between hydrophobic and electrostatic interactions in protein adsorption was studied for the various sulfonated microspheres, and it was compared with that for carboxylated microspheres. Hydrophobic interaction governed the adsorption in the low sulfonated microspheres whereas electrostatic interaction governed it in the high sulfonated microspheres. The transition point was observed when the two forces were exactly balanced, but it was shifted to the left compared with the case of the carboxylated microspheres. The above adsorption experiments with different kinds of surface functionality revealed the following general mechanism of protein adsorption. The protein adsorption occurs mainly by hydrophobic interaction when the hydrophobic surface is slightly modified with weak or strong acid, while it occurs primarily by hydrogen bonding (or electrostatic) interaction when the surface is mostly modified with weak (or strong) acid. Adsorption by electrostatic interaction is higher than that by any other interactions, but the rate of adsorption is slowest. Bovine serum albumin adsorbed onto carboxylated microspheres always undergoes an irreversible conformational change to the end-on mode, while this would not occur around the transition point in the case of the sulfonated microspheres. Copyright (C) 1998 Elsevier Science B.V.
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