Adhesive Properties of Adsorbed Layers of Two Recombinant Mussel Foot Proteins with Different Levels of DOPA and Tyrosine

Abstract

Using a surface forces apparatus and an atomic force microscope, we characterized the adhesive properties of adsorbed layers of two recombinant variants of Perna viridis foot protein 5 (PVFP-5), the main surface-binding protein in the adhesive plaque of the Asian green mussel. In one variant, all tyrosine residues were modified into 3,4-dihydroxy-Lphenylalanine (DOPA) during expression using a residuespecific incorporation strategy. DOPA is a key molecular moiety underlying underwater mussel adhesion. In the other variant, all tyrosine residues were preserved. The layer was adsorbed on a mica substrate and pressed against an uncoated surface. While DOPA produced a stronger adhesion than tyrosine in contact with the nanoscopic Si 3 N 4 probe of the atomic force microscope, the two variants produced comparable adhesion on the curved macroscopic mica surfaces of the surface forces apparatus. These findings show that the presence of DOPA is not a sufficient condition to generate strong underwater adhesion. Surface chemistry and contact geometry affect the strength and abundance of protein−surface bonds created during adsorption and surface contact. Importantly, the adsorbed protein layer has a random and dynamic polymer-network structure that should be optimized to transmit the tensile stress generated during surface separation to DOPA surface bonds rather than other weaker bonds.

Supramolecular β‐Sheet Suckerin–Based Underwater Adhesives

“…Adhesion is thus attributed due to the progressive stretching of flexible polymer‐like segments in a random network of proteins, eventually leading to protein bond rupture. [ 14,15 ] The network is both cross‐linked (e.g., via cation–π bonds or amyloidogenic interactions inducing β‐sheet formation) and attached to the Si 3 N 4 probe and TiO 2 substrate (e.g., Dopa–surface bonds). Adhesive peaks were due to the rupture of protein bonds, either a protein–protein cross‐link or protein–surface bond.…”

“…Adhesive peaks were due to the rupture of protein bonds, either a protein–protein cross‐link or protein–surface bond. The maximum adhesive force F 0 on the order of 0.1–1.0 nN is consistent with the rupture of single bonds, [ 11e,14 ] whereas the adhesive energy W = −∫ FdD characterizes the adhesive properties of the protein network, cumulating the effect of breaking multiple bonds of different types. Both F 0 and W showed a large variation among different measurements, even when all experimental parameters such as AFM tip velocity, or dwell time and maximum load at contact were kept constant (Figure S6c,d, Supporting Information).…”

“…Surface adhesion ultimately depends on the density and strength of the bonds that must be broken to separate the surfaces, generating failure planes at the surfaces [ 12b ] and fractures within the protein layer. [ 14 ] The interplay between surface bonds and crosslinks provides additional opportunities to engineer wet adhesives at the molecular level, e.g., to direct micro‐fracture within the protein layer where cohesive molecular domains can provide additional mechanical dissipation. [ 7b,15 ]…”

Structure and sequence features of mussel adhesive protein lead to its salt-tolerant adhesion ability

“…Besides, the adhesion ability of dipeptides with Lys at the N terminus (Lys-Dopa) is stronger than C terminus (Dopa-Lys), indicating that the cooperative Lys and Dopa binding to hydrophilic surfaces strongly depends on the protein sequences (29). As for the adhesive proteins, increasing studies also showed that recombinant protein enriched with Tyr rather than Dopa has strong adhesive ability (32)(33)(34)(35). All these studies are consistent with our findings based on MD simulations and lap shear experiments.…”

“…In addition, the Dopa-based peptide containing insufficient Lys exhibits limited adhesion to hydrophilic surface (30). Moreover, increasing studies showed that the peptides containing Lys and unmodified Tyr can also adsorb to hydrophilic surfaces (31)(32)(33)(34)(35). Together, although the crucial role of Dopa to enhance the mussel attachment by promoting the protein cross-linking has been well established (36)(37)(38), its effect on adhesive interaction is under debate.…”