Mussels use a variety of 3, 4-dihydroxyphenyl-l-alanine (DOPA) rich proteins specifically tailored to adhering to wet surfaces. Synthetic polypeptide analogues of adhesive mussel foot proteins (specifically mfp-3) are used to study the role of DOPA in adhesion. The mussel-inspired peptide is a random copolymer of DOPA and N5 -(2-hydroxyethyl)-l-glutamine synthesized with DOPA concentrations of 0–27 mol% and molecular weights of 5.9–7.1 kDa. Thin films (3–5 nm thick) of the mussel-inspired peptide are used in the surface forces apparatus (SFA) to measure the force–distance profiles and adhesion and cohesion energies of the films in an acetate buffer. The adhesion energies of the mussel-inspired peptide films to mica and TiO2 surfaces increase with DOPA concentration. The adhesion energy to mica is 0.09 μJ m−2 molDOPA−1 and does not depend on contact time or load. The adhesion energy to TiO2 is 0.29 μJ m−2 molDOPA−1 for short contact times and increases to 0.51 μJ m−2 molDOPA−1 for contact times >60 min in a way suggestive of a phase transition within the film. Oxidation of DOPA to the quinone form, either by addition of periodate or by increasing the pH, increases the thickness and reduces the cohesion of the films. Adding thiol containing polymers between the oxidized films recovers some of the cohesion strength. Comparison of the mussel-inspired peptide films to previous studies on mfp-3 thin films show that the strong adhesion and cohesion in mfp-3 films can be attributed to DOPA groups favorably oriented within or at the interface of these films.

Mussels have a remarkable ability to attach their holdfast, or byssus, opportunistically to a variety of substrata that are wet, saline, corroded, and/or fouled by biofilms. Mytilus edulis foot protein-5 (Mefp-5) is one of several proteins in the byssal adhesive plaque of the mussel M. edulis. The high content of 3,4 dihydroxyphenylalanine (Dopa) (~30 mol%) and its localization near the plaque-substrate interface have often prompted speculation that Mefp-5 plays a key role in adhesion. Using the surface forces apparatus, we show that on mica surfaces Mefp-5 achieves an adhesion energy approaching Ead = ~− 14 mJ/m2. This exceeds the adhesion energy of another interfacial protein, Mefp-3, by a factor of 4–5 and is greater than the adhesion between highly oriented monolayers of biotin and streptavidin. The adhesion to mica is notable for its dependence on Dopa, which is most stable under reducing conditions and acidic pH. Mefp-5 also exhibits strong protein-protein interactions with itself as well as with Mefp-3 from M. edulis.

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