EFFECT OF SURFACE WETTABILITY ON PROTEIN ADSORPTION AND LATERAL DIFFUSION - ANALYSIS OF DATA AND A STATISTICAL-MODEL

The initial adsorption of two proteins, ferritin and fibrinogen, from buffer solution onto hydrophilic and hydrophobic silicon surfaces was measured at room temperature with off null ellipsometry in situ at a time resolution of 0.1 second under non-diffusion limited conditions. The kinetics of the initial fibrinogen adsorption showed a lag-phase followed by a non-linear accelerated binding. The kinetics of ferritin adsorption at the hydrophobic surface showed a lag phase and an initial non-linear accelerated rate of adsorption whereas adsorption at the hydrophilic surface was proportional to t(0.77), after an extended lag-phase. The binding rate decreased abruptly and the rate of adsorption became proportional to the logarithm of time. Saturation levels of binding were lower at the hydrophilic surface. Surface diffusion of adsorbed proteins was measured by fluorescence recovery after photobleaching of fluorescein-labelled proteins. Diffusion of ferritin was seen on both surfaces with a diffusion constant a factor of 1000 lower than in bulk solution. Surface diffusion of fibrinogen was not detectable on either of the surfaces within a time scare of 0-100 seconds. The spatial distribution of ferritin was examined by transmission EM. The number of sites of initial adsorption of ferritin was 3 x 10(9) per cm(2) at the hydrophilic and 2 x 10(10) per cm(2) at the hydrophobic surface. The final distribution of adsorbed ferritin was close to random at the hydrophobic surface whereas scattered large clusters and small clusters were seen at the hydrophilic surface. A statistical model is presented that explains: (i) the lag-phase and the non-linear rate of adsorption, (ii) the sudden decrease in adsorption rate and (iii) the logarithmic rate of adsoption.