Direct measurement of protein dipoles reveals the mechanism for like-charge adsorption on silica

Negatively charged particles are expected to adsorb with difficulty onto negatively charged surfaces. However, some proteins do. In this work, we use ferritin as a model protein particle to study its adsorption onto a negatively charged silica surface. Using Kelvin Probe Force Microscopy (KPFM), we provide direct measurement of the dipole moment of the protein ferritin on silica. The positive dipole value indicates that the negative end of the protein faces the surface. In this immobilization mechanism, our results unveil the pivotal role of sodium cations in the buffer solution. Changing the iron loading inside the ferritin, appears to add positive charges and the dipole is modulated without inducing structural changes. Comparison of quartz crystal microbalance results with KPFM reveals a strong correlation between rigidity and internal charge distribution. Our measurements provide insights on the relationship between dipole, protein adsorption, and the contribution of internal charge distribution.