The Contribution of DOPA to Substrate-Peptide Adhesion and Internal Cohesion of Mussel-Inspired Synthetic Peptide Films

Abstract

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 N(5) -(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 TiO(2) surfaces increase with DOPA concentration. The adhesion energy to mica is 0.09 μJ m(-2) mol(DOPA) (-1) and does not depend on contact time or load. The adhesion energy to TiO(2) is 0.29 μJ m(-2) mol(DOPA) (-1) for short contact times and increases to 0.51 μJ m(-2) mol(DOPA) (-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.

Figure 1

The mussel-inspired DOPA-containing random copolymer used in these studies (top). Schematic of the surfaces in the SFA experiments in an asymmetric configuration used to measure the adhesion of the mussel-inspired peptide to TiO₂ (bottom).

Figure 2

QCM measurement of the frequency and dissipation shift during the adsorption of a 20 μg mL⁻¹ solution of 18 mol% DOPA mussel-inspired peptide to TiO₂. The final frequency shift of 52 Hz corresponds to an adsorbed mass of 0.28 μg cm⁻². Within 20 min 95% of the final mass has adsorbed.

Figure 3

Representative force runs comparing the interactions between thin films of a) mfp-3 and b) 18 mol% DOPA mussel-inspired peptide measured in the SFA. The mussel-inspired peptide is similar to mfp-3 in DOPA concentration and molecular weight. In each case the surfaces were left in contact for >30 min before separating. Mfp-3 data is reproduced from Lin et al.^[22]

Figure 4

Representative force runs showing the adhesion of mussel-inspired peptide to mica left) and TiO₂ (right) with increasing DOPA concentration and contact times as indicated in each force curve.

Figure 5

Summary of the adhesion energy of the peptide films to TiO₂ and mica as a function of concentration. Adhesion to mica is not affected by contact time, but adhesion to TiO₂ increases with contact time. Error bars are one standard deviation of 6–10 measurements for the mica and TiO₂ (1 min). Due to time constraints during each experiment there were not enough data points for the TiO₂ (60 min) for a valid statistical analysis and so the error bars shown are ±20%, which was the largest error found in any of the other measurements.

Figure 6

Pressure (P = −dW/dD) vs. distance for the separation of 18 and 10 mol% DOPA mussel-inspired peptide on TiO₂ after a contact time of 1 h. The region of constant pressure at 50 kPa for 18 mol% DOPA peptide and 15 kPa for 10 mol% DOPA peptide are characteristic of a phase transition. Inset) The original separation force-distance profiles. The pressure-distance curves were generated by differentiating the data in the grey box.

Figure 7

Representative force runs showing the interaction between 27 mol% DOPA mussel-inspired peptide films at pH 5.5 a) before and b) after adding the oxidizing agent periodate. Insets show the oxidative state of the DOPA residues.

Figure 8

The interaction between 27 mol% DOPA mussel-inspired peptide films as a function of pH. Top: Representative force run at pH 5.5 with freshly deposited mussel-inspired peptide showing strong adhesion. Middle: Representative force run between the same peptide films after increasing the pH to 7.5. Bottom: Representative force run between the same peptide films after reducing the pH back to 5.5 and adding a thiol containing polymer. Adhesion has been restored but with lower adhesion strength than originally measured. Insets show possible interactions responsible for the adhesive behavior of the DOPA.

Figure 9

Illustration of the proposed binding mechanism of DOPA to TiO₂ and mica surfaces. DOPA and DOPAquinone, to a lesser extent, can form bidentate binuclear complexes with the TiO₂ surface (left). In contrast, on mica (right) the interaction with DOPA is much less specific and may result from the hydrogen bonding of the phenolic OH groups to the oxygen atoms of the cleaved mica surface. DOPAquinone has no H to donate.