Adsorption of proteins on metal surfaces studied by ellipsometric and capacitance measurements

Abstract

The adsorption of proteins on metal surfaces has been studied with simultaneous ellipsometric, capacitance and open-cell potential measurements. The effect of the nature of the metal on the amount and the configuration of adsorbed lysozyme and ovalbumin has been investigated. Lysozyme and ovalbumin were adsorbed on well-defined evaporated metal films of platinum, titanium, and zirconium. The metals were characterized in terms of surface free energy, open-cell rest potential, and surface texture. All three metals were found to have a similar surface free energy and to be perfectly smooth at a 600 Å level. The open-cell rest potential was +350, -140 and -560 mV (saturated calomel electrode) for platinum, titanium and zirconium, respectively. The adsorbed protein layer was characterized through its surface concentration, optical thickness, refractive index, and relative surface coverage as a function of time. The nature of the metals was shown significantly to determine the configuration of the adsorbed protein molecules. Lysozyme was adsorbed in a two-layer structure, with more extended molecules for the metals, in the order platinum, titanium, and zirconium. Compared with the number of lysozyme molecules on platinum, the number of molecules is slightly smaller than on titanium and considerably larger on zirconium. Ovalbumin was adsorbed in a monolayer structure, with more extended molecules for the metals, in the order platinum, titanium, and zirconium. The number of ovalbumin molecules was markedly smaller on titanium and zirconium than on platinum. The densities of both the adsorbed lysozyme and ovalbumin molecules decrease for the metals in the order platinum, titanium, and zirconium. These results are consistent with electrostatic interactions between protein charges and the characteristic charge of the metal as determining factors for the amount and configuration of the adsorbed molecules. However, the adsorption of lysozyme on platinum also indicates that non-electrical forces might be important. The results are discussed in terms of some possible models for the adsorption of proteins on surfaces.