Adsorption dynamics of native and alkylated derivatives of bovine serum albumin at air-water interfaces

The dynamics of adsorption of bovine serum albumin (BSA) and its derivatives at the air-water interface were investigated through the measurement of dynamic surface pressure using a Langmuir minitrough. BSA was chemically modified through reductive alkylation to yield methylated and pentylated BSA with different surface hydrophobicities. Adsorption isotherms (Gamma I)-A of native and modified BSAs at pH 7 were measured, and the surface density (Gamma) was calculated directly from the area of the spread monolayer produced with Trurnit's method. The areas per molecule for BSA and modified BSAs at sufficiently high surface pressures were found to be much larger than those corresponding to the molecular dimensions, thus indicating unfolding of the molecules at the interface. The modified BSAs were found to occupy much larger areas per molecule than the native BSA, possibly due to more unfolding. The rate of surface pressure increase was higher for higher bulk concentrations, especially at smaller times. The deviation from diffusion-controlled adsorption was found to occur at smaller times for smaller ionic strengths, and for pH 7 rather than for pH 5.5 (at the isoelectric point). The steady-state surface pressures for native BSA ranged from 10 to 17 mN/m for bulk concentrations of 0.5 to 10 ppm and were higher for higher bulk concentrations, higher ionic strengths, and at the isoelectric point. The initial rate of surface pressure increase and the steady-state value were highest for pentylated BSA and lowest for native BSA. A previously developed model (Narsimhan and Uraizee, Biotech. Prog. 8, 187, 1992) was able to predict well the effects of bulk concentration, pH, ionic strength, and surface hydrophobicity on the dynamics of adsorption. The model predictions of the dynamics of surface pressure agreed fairly well with the experimental data at short times and at longer times. However, the model was found to overpredict the surface pressure at intermediate times, probably because of the use of the simplifying assumption that the surface equation of state during adsorption is given by the steady-state I Gamma-Gamma isotherm. (C) 1996 Academic Press, Inc.