pH-dependent adsorption and conformational change of ferritin studied on metal oxide surfaces by a combination of QCM-D and AFM

In this work, we have identified key parameters for controlling the surface density of ferritin on metal oxides and observed a conformational change of the protein shell of ferritin occurring exclusively at pH 4. The quartz crystal microbalance with dissipation (QCM-D) monitoring technique was used to examine protein adsorption on gold and Ti, Si-, Ta-, Al-, and Nb-oxide surfaces. A comparative study of the adsorption to Ti-oxide was made using atomic force microscopy (AFM). The surface density of ferritin was controlled by a variation in the solution pH (from 2 to 8) and the isoelectric point (Ip) of the surface. On the basis of these findings, clear trends in the effect of electrostatic interactions between the ferritin proteins and between ferritin and the surface were found, making it possible to tune the surface density of ferritin through the choice of solution pH and the Ip of the surface. Furthermore, the influence of the pH on the viscoelastic properties of the adsorbed ferritin layer was examined using QCM-D. Conformational changes of the protein shell of ferritin were found to occur at pH 4, which is close to the Ip of ferritin (4.6), but not for lower or higher values of the pH. This conformational change of ferritin was seen clearly in dissipation per frequency value obtained from the QCM-D technique.