Atomic force microscopy studies of the influence of convex and concave nanostructures on the adsorption of fibronectin.

Atomic force microscopy (AFM)-based force spectroscopy was used to analyze the adsorption of bovine plasma fibronectin on periodically grooved nanostructures (groove/summit width: 90 nm; depth: 120 nm). We present a simple procedure that allowed us to directly compare the local protein density and conformation for the convex summits, the concave grooves and planar reference regions of the substrate. At a bulk fibronectin concentration of 5 μg/ml, the amount of adsorbed protein per surface area was significantly higher in all regions of the nanostructure than on the planar reference, and fibronectin tended to adsorb preferentially in the concave grooves. The increased surface concentration resulted in an additional stabilization of the molecules by protein-protein interactions and a lower degree of denaturized fibronectin in the nanostructured regions. The stabilization was less pronounced in concave regions, indicating that the increased contact area in the grooves counteracted the stabilization by increased protein-substrate interactions and must be compensated for by additional protein-protein interactions. Less favorable sites were occupied at higher bulk fibronectin concentrations (25 μg/ml, 100 μg/ml), and a high degree of native folded fibronectin was observed in both the nanostructured and planar regions. Our results demonstrate that the amount of adsorbed fibronectin per surface area can be increased if a substrate is provided with a topographic nanostructure. Our results also show that the local conformational state of fibronectin is determined by the locally different interplay of protein-protein and protein-substrate interactions.