authors contributed equally to this work.One Sentence Summary: Siderophore-inspired, mussel foot protein mimetic adhesives resist oxidation and reveal a synergistic catechol-lysine interplay that enables wet adhesion to mineral surfaces Abstract: In physiological fluids and seawater, adhesion of synthetic polymers to solid surfaces is severely limited by high salt, pH, and hydration, yet these conditions have not deterred the evolution of effective adhesion by mussels. Mussel foot proteins provide insights about adhesive adaptations: notably, the abundance and proximity of catecholic Dopa (3,4-dihydroxyphenylalanine) and lysine residues hint at a synergistic interplay in adhesion. Certain siderophores-bacterial iron-chelators-consist of paired catechol and lysine functionalities thereby providing a convenient experimental platform to explore molecular synergies in bioadhesion. These siderophores and synthetic analogs exhibit robust adhesion (W adh ≥15mJ/m 2 )to mica in saline pH 3.5-7.5 and resist oxidation. The adjacent catechol-Lys placement provides a "1-2 punch", whereby Lys evicts hydrated cations from the mineral surface, allowing catechol binding to underlying oxides.

Mussel foot proteins (Mfps) exhibit remarkably adaptive adhesion and bridging between polar surfaces in aqueous solution despite the strong hydration barriers at the solid-liquid interface. Recently, catechols and amines-two functionalities that account for >50 mol % of the amino acid side chains in surface-priming Mfps-were shown to cooperatively displace the interfacial hydration and mediate robust adhesion between mineral surfaces. Here we demonstrate that (1) synergy between catecholic and guanidinium side chains similarly promotes adhesion, (2) increasing the ratio of cationic amines to catechols in a molecule reduces adhesion, and (3) the catechol-cation synergy is greatest when both functionalities are present within the same molecule.

Catechols are found as functional groups in Nature as the 3,4-dihydroxy isomer, generally with electron donating substituents and as the 2,3-dihydroxy isomer, with an electron withdrawing substituent. Adhesive properties of catechol materials rely on the vicinal diol configuration, yet conversely, cohesive properties of catechol materials depend on catechol oxidation that promote subsequent crosslinking reactions. While the pH-dependent oxidation of catechol by dioxygen is well recognized, a better understanding of substituent effects on catechol redox chemistry is important in the design of new catechol-containing functional materials. The pH-dependent oxidation kinetics of catechol and substituted catechols by O was investigated with a Clark-type oxygen electrode. The results are consistent with a mechanism in which O oxidizes both mono-deprotonated and fully deprotonated catechol anions. A linear Hammett correlation for the pH-independent second order rate constants for catechol oxidation by O establishes that catechols functionalized with electron withdrawing groups have slower rates of oxidation by O, whereas catechols with electron donating groups have faster rates of oxidation by O. The Hammett correlation allows for selection of functionalized catechols with redox properties ideally suited for adhesive or crosslinking applications.