Casin immobilization and enzymolysis behaviors at different solid-aqueous interfaces by quartz crystal microbalance

A quartz crystal microbalance (QCM) with dissipation monitoring was used to characterize the adsorption and subsequent trypsin-catalyzed hydrolysis of casin molecules at several typical interfaces, including Au, SiO ₂ and polystyrene coated chip surfaces. The results showed that a high non-specific adsorption amount of 127.9 ng/cm ² was observed for casin on polystyrene chip surface, while no apparent adsorption occurred on native Au and SiO ₂ chips. After activation and modification, casin was covalently immobilized on Au and SiO ₂ chip surfaces with the amounts of 178 and 1718 ng/cm ², respectively. The subsequent enzymolysis of attached casin by injection of typsin was also monitored using QCM. After trypsin treatment, casin molecules and its hydrolysate peptide adsorbed on polystyrene surface could be efficiently removed with surfactant, without trypsin pre-treatment, however, only 36% of the adsorbed casin was removed by surfactant. A method for measuring activity of trypsin at solid-aqueous interfaces was developed. The measured specific hydrolytic activities of trypsin for immobilized casin on Au and SiO ₂ chip surfaces are 0.010 and 0.157 U/mg, respectively, which was lower than that for free casin in solution with the specific activity of 17.8 U/mg.