Microphase Behavior and Enhanced Wet-Cohesion of Synthetic Copolyampholytes Inspired by a Mussel Foot Protein

Abstract

Numerous attempts have been made to translate mussel adhesion to diverse synthetic platforms. However, the translation remains largely limited to the Dopa (3,4-dihydroxyphenylalanine) or catechol functionality, which continues to raise concerns about Dopa's inherent susceptibility to oxidation. Mussels have evolved adaptations to stabilize Dopa against oxidation. For example, in mussel foot protein 3 slow (mfp-3s, one of two electrophoretically distinct interfacial adhesive proteins in mussel plaques), the high proportion of hydrophobic amino acid residues in the flanking sequence around Dopa increases Dopa's oxidation potential. In this study, copolyampholytes, which combine the catechol functionality with amphiphilic and ionic features of mfp-3s, were synthesized and formulated as coacervates for adhesive deposition on surfaces. The ratio of hydrophilic/hydrophobic as well as cationic/anionic units was varied in order to enhance coacervate formation and wet adhesion properties. Aqueous solutions of two of the four mfp-3s-inspired copolymers showed coacervate-like spherical microdroplets (ϕ ≈ 1−5 μm at pH ∼4 (salt concentration ∼15 mM). The mfp-3s-mimetic copolymer was stable to oxidation, formed coacervates that spread evenly over mica, and strongly bonded to mica surfaces (pull-off strength: ∼17.0 mJ/m 2 ). Increasing pH to 7 after coacervate deposition at pH 4 doubled the bonding strength to ∼32.9 mJ/m 2 without oxidative cross-linking and is about 9 times higher than native mfp-3s cohesion. This study expands the scope of translating mussel adhesion from simple Dopa-functionalization to mimicking the context of the local environment around Dopa. M arine mussels (Figure 1a) attach to hard surfaces, e.g., mineral and metal, in the intertidal zone where waves with and without suspended sand often exceed 25 m/sec velocities. 3,4-Dihydroxyphenylalanine (Dopa), a main constituent in mussel foot proteins (mfps) and substantially contributing to wet adhesion, has been incorporated in synthetic polymers to mimic the bio wet-adhesion. 1−5 However, other constitutional features of mfps, e.g., cationic residues (lysine, K), anionic residues (aspartic acid, D), nonionic polar residues (asparagine, N), and nonpolar residues (alanine, A), have not typically been included in mussel-inspired synthetic wet-adhesion systems. 1,2Here, we studied the microphase behavior and wet-adhesion of copolyampholytes with fixed catechol content and varied other key functionalities. Potential effects of aromatic moieties (Tyr, Trp) besides Dopa in mfp-3s have not been specifically tested in the present structural design of the model copolyampholytes. Conditions for the experiments were adjusted according to the microenvironmental conditions of adhesive protein deposition under the mussel's foot including acidic to neutral pH and ionic strength of ≤100 mM. 3,4 In mussel adhesion, polyelectrolyte adhesive proteins or mfps are presented to target surfaces after being condensed as a dense fluid by complex coacervation, a critical ste...

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