Absence of surface exclusion in the first stage of lysozyme adsorption is driven through electrostatic self-assembly

Adsorption kinetics of hen egg white lysozyme at pH 7.4 onto a Si(Ti)O-2 surface were measured at two ionic strengths and under different shear rates by means of optical waveguide lightmode spectroscopy. At low ionic strength, increases of the adsorbed amount linear with time suggest that the adsorption proceeds without surface exclusion. Within the concentration range investigated, the duration of this linear regime was inversely proportional to the bulk concentration. The Linear regime always stopped at a threshold value of 0.18 +/- 0.02 mu g/cm(2) and was followed by a second regime with surface exclusion. I At high ionic strength the linear regime was not observed, suggesting an electrostatically driven self-assembly process at low ionic strength. This conclusion is supported by the linear arrays of lysozyme at graphite surfaces observed with STM (Haggerty, L.; Lenhoff, A. M. Biophys. J. 1993, 64, 886). Moreover, the adsorption rate at low ionic strength is faster than that predicted by diffusion across the diffusion boundary layer, suggesting that the transport is accelerated by electrostatic attraction. This was confirmed by an estimation of the electrostatic free energy contribution.