Protein-associated water and secondary structure effect removal of blood proteins from metallic substrates

Removing adsorbed protein from metals has significant health and industrial consequences. There are numerous protein-adsorption studies using model self-assembled monolayers or polymeric substrates but hardly any high-resolution measurements of adsorption and removal of proteins on industrially relevant transition metals. Surgeons and ship owners desire clean metal surfaces to reduce transmission of disease via surgical instruments and minimize surface fouling (to reduce friction and corrosion), respectively. A major finding of this work is that, besides hydrophobic interaction adhesion energy, water content in an adsorbed protein layer and secondary structure of proteins determined the access and hence ability to remove adsorbed proteins from metal surfaces with a strong alkaline-surfactant solution (NaOH and 5 mg/mL SDS in PBS at pH 11). This is demonstrated with three blood proteins (bovine serum albumin, immunoglobulin, and fibrinogen) and four transition metal substrates and stainless steel (platinum (Pt), gold (Au), tungsten (W), titanium (Ti), and 316 grade stainless steel (SS)). All the metallic substrates were checked for chemical contaminations like carbon and sulfur and were characterized using X-ray photoelectron spectroscopy (XPS). While Pt and Au surfaces were oxide-free (fairly inert elements), W, Ti, and SS substrates were associated with native oxide. Difference measurements between a quartz crystal microbalance with dissipation (QCM-D) and surface plasmon resonance spectroscopy (SPR) provided a measure of the water content in the protein-adsorbed layers. Hydrophobic adhesion forces, obtained with atomic force microscopy, between the proteins and the metals correlated with the amount of the adsorbed protein-water complex. Thus, the amount of protein adsorbed decreased with Pt, Au, W, Ti and SS, in this order. Neither sessile contact angle nor surface roughness of the metal substrates was useful as predictors here. All three globular proteins behaved similarly on addition of the alkaline-surfactant cleaning solution, in that platinum and gold exhibited an increase, while tungsten, titanium, and stainless steel showed a decrease in weight. According to dissipation measurements with the QCM-D, the adsorbed layer for platinum and gold was rigid, while that for the tungsten, titanium, and stainless steel was much more flexible. The removal efficiency of adsorbed-protein by alkaline solution of SDS depended on the water content of the adsorbed layers for W, Ti, and SS, while for Pt and Au, it depended on secondary structural content. When protein adsorption was high (Pt, Au), protein-protein interactions and protein-surface interactions were dominant and the removal of protein layers was limited. Water content of the adsorbed protein layer was the determining factor for how efficiently the layer was removed by alkaline SDS when protein adsorption was low. Hence, protein-protein and protein-surface interactions were minimal and protein structure was less perturbed in comparison with those for high protein adsorption. Secondary structural content determined the efficient removal of adsorbed protein for high adsorbed amount.