Adsorption of fibrinogen onto hydrophobic and hydrophilic quartz surfaces was studied by ellipsometry and transmission electron microscopy (TEM) of negatively stained proteins. The initial adsorption at the hydrophobic surface, measured by ellipsometry, can be described by an apparent forward rate constant k l of 2 x l o 4 M -' s-'. This constant was timedependent and is therefore considered as a rate coefficient. The apparent forward rate coefficient of adsorption to a hydrophilic surface was both time-dependent and concentration-dependent, indicating a history-dependent process of adsorption. Plateau levels of adsorption were concentration-dependent a n d lower a t t h e h y d r o p h i l i c q u a r t z s u r f a c e (1.2 pmol/cm2) than at the hydrophobic surface (1.8 pmol/cm2). These surface concentrations correspond to rather tightpacked monolayers of molecules adsorbed end-on. The initial desorption can be described by a first order rate constant = s-'), down to S0-90% of the initial surface concentration. The dissociation rate then decreased (k1 = s-') result-ing in an apparently stable level of adsorbed protein. Slow changes of the binding strength of adsorbed proteins was seen during 24-72 h adsorption time. Dev i a t i o n s f r o m a n i d e a l e q u i l i b r i u m isotherm were seen both in the time dependence and as concavities in a Scatchard plot, suggesting intermolecular cooperativity. A t low bulk concentrations a heterogeneous distribution of fibrinogen molecules was found at the surface below monolayer coverage. The supramolecular structure was characterized by the formation of end-to-end dimers and trimers laying down at the surface. At higher surface concentration adsorbed molecules showed polycrystalline structure with repeated nearest neighbordistances at 16 nm. The distribution of adsorbed fibrinogen molecules indicates that surface-adsorbed fibrinogen may form a two-phase system, containing significant amounts of water.
The atypical kinetics and concentration dependence of fibrinogen adsorption may thus be due to properties of a twodimensional phase separation from a three-dimensional liquid bulk.