Adsorption kinetics of hen egg white lysozyme at pH 7.4
onto a Si(Ti)O2 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
μg/cm2 and was followed by a second regime with surface
exclusion. 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.