Function and structural stability of protein adsorbed to swellable organosilica

Abstract

Abstract Swellable organically modified silica (SOMS) is a mesoporous sol-gel derived material that spontaneously swells >6 mL/g upon addition organic liquids. Adsorption of 8 different proteins to swollen-state SOMS was studied as a function of adsorbate molecular size and solution pH. Adsorption capacity was dependent on molecular size with the highest capacity of 690 mg/g measured for wheat germ acid phosphatase (58 kDa), a mid-size protein. Adsorption capacities were generally independent of pH suggesting protein-protein interactions do not play a major role in the adsorption mechanism. Rate of adsorption was measured at different protein loadings and fit to a pseudo second-order rate equation. Adsorption rates were independent of loading state when the pH was near a protein's isoelectric point (pI), but varied when the pH differed from the pI of adsorbate. Adsorption was found to irreversible for all proteins when rinsed with buffer solution or ethanol. Infrared spectroscopy of adsorbed cytochrome c and myoglobin indicated that both proteins maintain a native folded state that is stabilized to thermal denaturation. Adsorbed streptavidin was found to bind FITC-biotin (63% binding site saturation) suggesting that adsorbed proteins are accessible to small molecules in bulk solution. Acid phosphatase showed catalytic activity when adsorbed to swollen SOMS, however, specific activity of the bound enzyme was 10-fold lower compared to free enzyme in solution. Adsorption to SOMS appears to be a one-step, reagent-free method to irreversibly immobilize small to mid-sized proteins. Data supports an adsorption mechanism where protein molecules become sterically entrapped in the flexible mesoporous material.