Stabilizing Effects of Various Polyelectrolyte Multilayer Films on the Structure of Adsorbed/Embedded Fibrinogen Molecules: An ATR−FTIR Study

Abstract

The structural changes in fibrinogen as a consequence of its adsorption onto the surface of or its embedding into the interior of poly(allylamine hydrochloride) (PAH) or poly(styrenesulfonate) (PSS) multilayers are investigated by means of attenuated total reflection Fourier transform infrared (ATR−FTIR) spectroscopy. It is found that both adsorption and embedding preserve the secondary structure of the fibrinogen molecules. Furthermore, the interactions of the polyelectrolytes with the protein molecules prevent their aggregation, especially in the embedded state, at room temperature. Thus, it seems that the structure and the biological activity of proteins adsorbed on or embedded in polyelectrolyte multilayers could largely be preserved, which opens up great perspectives in the design of new bioactive surfaces. The nature and the extent of the polyelectrolyte−protein interactions are further studied via analysis of the thermotropic responses of the different architectures. It is found that both PAH- and PSS-terminated polyelectrolyte multilayers can elevate the onset temperature of the structural changes in adsorbed/embedded fibrinogen molecules by about 5 °C as compared with that for fibrinogen in solution. These polyelectrolytes also broaden the thermally induced structural transitions in the adsorbed/embedded fibrinogen molecules. The magnitude of these thermally induced structural changes is polyelectrolyte- and architecture-dependent. Whereas multilayer PAH−fibrinogen and multilayer PSS−fibrinogen constructions exhibit roughly the same large-scale thermally induced structural changes, in all architectures where fibrinogen is embedded the scale of these structural changes is restricted. The restriction becomes stronger as the presence of PSS at the polyelectrolyte−fibrinogen interfaces increases (PAH−fib−PAH < PAH−fib−PSS ≈ PSS−fib−PAH < PSS−fib−PSS). In the PSS−fib−PSS arrangement, the secondary structure of fibrinogen as determined from its infrared spectrum changes only slightly up to 90 °C. The underlying processes of the thermally induced structural changes is, in addition, different for fibrinogen molecules adsorbed onto or embedded into PAH-terminated polyelectrolyte multilayers. A tentative model based on “encapsulation” of the embedded protein by the polyelectrolytes is proposed to explain the observed features.