Siderophores and mussel foot proteins: the role of catechol, cations, and metal coordination in surface adhesion.

Metal coordination, hydrogen bonding, redox reactions, and covalent crosslinking are seemingly disparate chemical and physicochemical processes that are all accomplished in natural materials by the catechol functional group. This review focuses on the reactivity of catechols in tris-2,3-dihydroxybenzoyl-containing microbial siderophores and synthetic analogs, as well as Dopa-(3,4-dihydroxyphenylalanine)-containing mussel foot proteins that adhere to surfaces in aqueous conditions. Mussel foot proteins with a high content of Dopa and cationic amino acids, Lys and Arg, adhere strongly to mica, an aluminosilicate mineral, in aqueous conditions. The siderophore cyclic trichrysobactin, tris-(2,3-dihydroxybenzoyl-D-Lys-L-Ser) and related synthetic analogs in which the tri-Ser macrolactone is replaced by Tren, tris-(2-aminoethyl)amine, also adheres strongly to mica. Variation in the nature of the catechol and cationic groups in synthetic analogs reveals a synergism between the cationic amino acid and the catechol, required for strong aqueous adhesion. Autoxidation and iron(III)-catalyzed oxidation of 2,3-dihydroxy and 3,4-dihydroxy catechols are also considered. These siderophore analogs provide a platform to understand catechol interactions and reactivity on surfaces, which may ultimately improve the design of synthetic materials that address diverse challenges in medicine, materials science, as well as other disciplines, in which surface adhesion in aqueous conditions is important.

Defining the Catechol-Cation Synergy for Enhanced Wet Adhesion to Mineral Surfaces.

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.

Catechol oxidation: considerations in the design of wet adhesive materials.

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 O2 was investigated with a Clark-type oxygen electrode. The results are consistent with a mechanism in which O2 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 O2 establishes that catechols functionalized with electron withdrawing groups have slower rates of oxidation by O2, whereas catechols with electron donating groups have faster rates of oxidation by O2. The Hammett correlation allows for selection of functionalized catechols with redox properties ideally suited for adhesive or crosslinking applications.

BIOLOGICAL ADHESIVES. Adaptive synergy between catechol and lysine promotes wet adhesion by surface salt displacement.

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 siderophores­bacterial iron chelators­consist 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 energies (E(ad) ≥-15 millijoules per square meter) to mica in saline pH 3.5 to 7.5 and resist oxidation. The adjacent catechol-lysine placement provides a "one-two punch," whereby lysine evicts hydrated cations from the mineral surface, allowing catechol binding to underlying oxides.