Protein interactions with polyelectrolyte multilayers: Interactions between human serum albumin and polystyrene sulfonate/polyallylamine multilayers

The interactions between polystyrenesulfonate (PSS)/polyallylamine (PAH) mu ltilayers with human serum albumin (HSA) were investigated by means of scan ning angle reflectometry (SAR). We find that albumin adsorbs both on multil ayers terminating with PSS (negatively charged) or PAH (positively charged) polyelectrolytes. On films terminating with PSS only, an albumin equivalen t monolayer is found whereas when PAH constitutes the outer layer, albumin interacts with the multilayer in such a way as to form a protein film that extends over thicknesses that can be as high as four times the largest dime nsion of the native albumin molecule. Once the protein film is formed, it i s found that when the albumin solution is replaced by a pure buffer solutio n of same ionic strength as the adsorption solution almost no desorption ta kes place. On the other hand, when a buffer solution of higher ionic streng th is brought in contact with the albumin film, a significant amount of ads orbed proteins is released. One also observes that, for albumin solutions o f a given protein concentration, the adsorbed amount depends on the ionic s trength of the adsorption solution. On surfaces terminating with PAH, the a dsorbed protein amount first increases rapidly but passes through a maximum and decreases with the ionic strength. The ionic strength corresponding to the maximum of the adsorbed albumin amount itself depends on the albumin c oncentration. On the other hand, on films terminating with PSS the adsorbed amount increases with the salt concentration before leveling-off. These re sults show that the underlying complexity of concentration and pH dependent adsorption/desorption equilibria often simply termed "protein adsorption" is the result of antagonist competing interactions that are mainly of elect rostatic origin. We also propose two microscopic models, that are compatibl e with our experimental observations.