Figure 3.

pH dependence of Fe³⁺-mediated cohesion between two symmetric mfp-1 (Mc) films. Representative force vs distance plot showing the interaction between two symmetric mfp-1 (Mc) films deposited at 50 μg/mL in 0.1 M sodium acetate buffer, pH 5.5, 0.25 M KNO₃, and 1 mM bis–tris with C_Fe³⁺ = 0 (gray) and 100 μM (blue) at pH 5.5. The cohesion between the mfp-1 (Mc) films was preserved after increasing the pH to 7.5 (magenta). (b) Representative force vs distance plot showing the interaction between two symmetric mfp-1(Me) films deposited at 20 μg/mL in 0.1 M sodium acetate buffer, pH 5.5, 0.25 M KNO₃, and 1 mM bis–tris with C_Fe³⁺ = 0 (gray) and 100 μM (blue) at pH 5.5. The surfaces showed a weak bridging cohesion (W_c < 0.2 mJ/m²) after increasing the pH to 7.5 (magenta). It should be noted that C_Fe³⁺ is the concentration of ferric cation in the bulk solution between the surfaces. Flushing with buffer at pH 7.5 removes iron from the bulk solution, however, to the extent that the preadsorbed protein films already had some bound Fe³⁺, the Dopa–Fe³⁺ complexes will be present in them.