Adhesion of Mussel Foot Protein‑3 to TiO<sub>2</sub> Surfaces: the Effect of pH

The underwater adhesion of marine mussels relies on mussel foot proteins (mfps) rich in the catecholic amino acid 3,4-dihydroxyphenylalanine (Dopa). As a side chain, Dopa is capable of strong bidentate interactions with a variety of surfaces, including many minerals and metal oxides. Titanium is among the most widely used medical implant material and quickly forms a TiO<sub>2</sub> passivation layer under physiological conditions. Understanding the binding mechanism of Dopa to TiO<sub>2</sub> surfaces is therefore of considerable theoretical and practical interest. Using a surface forces apparatus, we explored the force–distance profiles and adhesion energies of mussel foot protein 3 (mfp-3) to TiO<sub>2</sub> surfaces at three different pHs (pH 3, 5.5 and 7.5). At pH 3, mfp-3 showed the strongest adhesion force on TiO<sub>2</sub>, with an adhesion energy of ∼−7.0 mJ/m<sup>2</sup>. Increasing the pH gives rise to two opposing effects: (1) increased oxidation of Dopa, thus, decreasing availability for the Dopa-mediated adhesion, and (2) increased bidentate Dopa-Ti coordination, leading to the further stabilization of the Dopa group and, thus, an increase in adhesion force. Both effects were reflected in the resonance-enhanced Raman spectra obtained at the three deposition pHs. The two competing effects give rise to a higher adhesion force of mfp-3 on the TiO<sub>2</sub> surface at pH 7.5 than at pH 5.5. Our results suggest that Dopa-containing proteins and synthetic polymers have great potential as coating materials for medical implant materials, particularly if redox activity can be controlled.