Adsorption and relaxation kinetics of albumin and fibrinogen on hydrophobic surfaces: Single-species and competitive behavior

We report the kinetic behavior of albumin and fibrinogen adsorption and relaxation from gentle shearing flow and phosphate buffer onto C16 self-assembled monolayers. The adsorption kinetics were generally transport-limited; however, the ultimate coverages depended on the rates at which protein molecules arrived at the surface, suggesting that interfacial relaxations determined the ultimate coverage. Of particular note was a dependence of the ultimate coverage of both proteins on the wall shear rate, in addition to the influence of the bulk solution concentration. Analysis of single protein experiments revealed interfacial protein relaxation rates of 0.12 and 0.15 nm(2) molecule(-1) s(-1) for albumin and fibrinogen, respectively. These rates were constant over the range of experimental conditions and represent the initial relaxation rates after protein adhesion to the surface. The initial protein footprints were consistent with the free solution protein dimensions and, in the case of albumin, grew over a factor of 5 as the protein relaxed. For fibrinogen, relaxations were less extensive, increasing the footprint by a factor of 3. The extents of relaxation and the sizes of the protein footprints during the Linear regime of spreading suggest that interfacial denaturing contributes significantly to the relaxation process, in addition to simple reorientations. The albumin relaxation behavior was shown, in addition to its influence on albumin coverage, to affect the coverage of fibrinogen in competitive situations. When the C16 layer was passivated with albumin prior to fibrinogen adsorption, short albumin exposures (still sufficient to cover the C16 surface) were ineffective at preventing fibrinogen adsorption. Prolonged incubation of albumin layers in albumin solution or buffer dramatically reduced subsequent fibrinogen adhesion.