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 sh ear rates by means of optical waveguide lightmode spectroscopy. At low ionic strength, increases of the adsorbed amount linear with time sug gest that the adsorption proceeds without surface exclusion. Within th e concentration range investigated, the duration of this linear regime was inversely proportional to the bulk concentration. The Linear regi me 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 ion ic strength the linear regime was not observed, suggesting an electros tatically driven self-assembly process at low ionic strength. This con clusion is supported by the linear arrays of lysozyme at graphite surf aces observed with STM (Haggerty, L.; Lenhoff, A. M. Biophys. J. 1993, 64, 886). Moreover, the adsorption rate at low ionic strength is fast er than that predicted by diffusion across the diffusion boundary laye r, suggesting that the transport is accelerated by electrostatic attra ction. This was confirmed by an estimation of the electrostatic free e nergy contribution.