Improving our understanding of wet adhesion is crucial for developing next generation wet adhesives. Despite decades of efforts on developing better wet adhesives, water and moisture are still enemies of strong adhesion to polar surface; however, nature may have pointed the right direction to human beings: marine mussels achieve durable underwater adhesion using a suite of proteins that are peculiar in having high levels of 3, 4-dihydroxyphenylalanine (Dopa). The object of this research was to investigate the basic surface interactions employed by the mussels and roles of Dopa in the mussel adhesion. I used the surface forces apparatus (SFA) technique to measure the interactions between mussel foot protein-3 fast (Mfp-3 fast) and the model substrate, mica, as well as the interactions between various Mfps.

The adhesion of Mfp-3 fast to mica is closely coupled with Dopa redox and pH. Raising the pH from 3 to 7.5 diminishes the adhesion energy of Mfp-3 on mica and appears to be driven by the pH-dependent oxidation of Dopa. Once Dopaquinone forms, it quickly undergoes tautomerization to alpha,beta--dehydrodopa, which abolishes side-chain rotation and optical activity in the alpha carbon, and imposes severe local configurational constraints, leading to an increase of the hydrodynamic radius of a single protein chain. Addition of thiol-rich mussel foot protein-6 (Mfp-6) restores mfp-3 adhesion by coupling the oxidation of thiols to the reduction of Dopaquinones.

The strong cohesive interaction (Ead = -3 mJ/m2) between two Mfp-3 slow protein films over a wide range of pH (from 3 to 7.5) suggests that the hydrophobic Mfp-3 slow plays important roles in incorporating hydrophobic interactions and inter residue H-bonding to strengthen adhesives. The SFA tests on the adhesive properties of synthetic Mfp-5 derived short peptides with different overall charge and charge densities at different pH suggests that the electrostatic interactions also contribute to the adhesion of Mfp-5 to charged surfaces such as mica.