Mussel adhesion is mediated by foot proteins (mfp) rich in a catecholic amino acid, 3, 4-dihydroxyphenylalanine (dopa), capable of forming strong bidentate interactions with a variety of surfaces. A facile tendency toward auto-oxidation, however, often renders dopa unreliable for adhesion. Mussels limit dopa oxidation during adhesive plaque formation by imposing an acidic, reducing regime based on thiol-rich mfp-6, which restores dopa by coupling the oxidation of thiols to dopaquinone reduction.

The adhesion of mussel foot proteins (Mfps) to a variety of specially engineered mineral and metal oxide surfaces has previously been investigated extensively, but the relevance of these studies to adhesion in biological environments remains unknown. Most solid surfaces exposed to seawater or physiological fluids become fouled by organic conditioning films and biofilms within minutes. Understanding the binding mechanisms of Mfps to organic films with known chemical and physical properties therefore is of considerable theoretical and practical interest. Using selfassembled monolayers (SAMs) on atomically smooth gold substrates and the surface forces apparatus, we explored the forcedistance profiles and adhesion energies of three different Mfps, Mfp-1, Mfp-3, and Mfp-5, on (i) hydrophobic methyl (CH 3 )-and (ii) hydrophilic alcohol (OH)-terminated SAM surfaces between pH 3 and pH 7.5. At acidic pH, all three Mfps adhered strongly to the CH 3 -terminated SAM surfaces via hydrophobic interactions (range of adhesive interaction energy = −4 to −9 mJ/m 2 ) but only weakly to the OH-terminated SAM surfaces through H-bonding (adhesive interaction energy ≤ −0.5 mJ/m 2 ). 3, 4-Dihydroxyphenylalanine (Dopa) residues in Mfps mediate binding to both SAM surface types but do so through different interactions: typical bidentate H-bonding by Dopa is frustrated by the longer spacing of OHSAMs; in contrast, on CH 3 -SAMs, Dopa in synergy with other nonpolar residues partitions to the hydrophobic surface. Asymmetry in the distribution of hydrophobic residues in intrinsically unstructured proteins, the distortion of bond geometry between H-bonding surfaces, and the manipulation of physisorbed binding lifetimes represent important concepts for the design of adhesive and nonfouling surfaces. M arine mussels are experts at wet adhesion, achieving strong and durable attachments to a variety of surfaces in their chemically heterogeneous habitat. Adhesion is mediated by a byssus, which is essentially a bundle of leathery threads that emerge from the living mussel tissue at one end and are tipped by flat adhesive plaques at the other. Byssal plaques consist of a complex array of proteins (mostly mussel foot proteins, Mfps), each of which has a distinct localization and function in the structure, but all share the unusual modified amino acid 3, 4-dihydroxyphenylalanine (Dopa) (Fig. 1).Of the dozen or so known mussel foot proteins, Mfp-1, Mfp-3, and Mfp-5 have been shown to exhibit remarkable binding to mineral surfaces such as mica and TiO 2 (1). The versatility of mussel adhesion to surfaces with wide-ranging chemical and physical properties has inspired much research dedicated to understanding the mechanism of mussel adhesion and to developing biomimetic coatings and adhesives for wide-ranging industrial and biomedical applications, the latter including paints for coronary arteries (2), fetal membrane sealants (3), cell encapsulants (4), bone glues (5), and for securing transplants for diabetics (6).The catecholic moiety of Dopa (Fig. 1) ...

Mussels have a remarkable ability to attach their holdfast, or byssus, opportunistically to a variety of substrata that are wet, saline, corroded, and/or fouled by biofilms. Mytilus edulis foot protein-5 (Mefp-5) is one of several proteins in the byssal adhesive plaque of the mussel M. edulis. The high content of 3,4 dihydroxyphenylalanine (Dopa) (~30 mol%) and its localization near the plaque-substrate interface have often prompted speculation that Mefp-5 plays a key role in adhesion. Using the surface forces apparatus, we show that on mica surfaces Mefp-5 achieves an adhesion energy approaching Ead = ~− 14 mJ/m2. This exceeds the adhesion energy of another interfacial protein, Mefp-3, by a factor of 4–5 and is greater than the adhesion between highly oriented monolayers of biotin and streptavidin. The adhesion to mica is notable for its dependence on Dopa, which is most stable under reducing conditions and acidic pH. Mefp-5 also exhibits strong protein-protein interactions with itself as well as with Mefp-3 from M. edulis.

Mussel foot proteins (mfps) have been investigated as a source of inspiration for the design of underwater coatings and adhesives. Recent analysis of various mfps by a surface forces apparatus (SFA) revealed that mfp-1 functions as a coating, whereas mfp-3 and mfp-5 resemble adhesive primers on mica surfaces. To further refine and elaborate the surface properties of mfps, the force-distance profiles of the interactions between thin mfp (i.e. mfp-1, mfp-3 or mfp-5) films and four different surface chemistries, namely mica, silicon dioxide, polymethylmethacrylate and polystyrene, were measured by an SFA. The results indicate that the adhesion was exquisitely dependent on the mfp tested, the substrate surface chemistry and the contact time. Such studies are essential for understanding the adhesive versatility of mfps and related/similar adhesion proteins, and for translating this versatility into a new generation of coatings and (including in vivo) adhesive materials.

Formation of dehydrodopa in an adhesive mussel foot protein (mfp3) during autoxidation or periodate‐mediated oxidation influences film thickness on mica in the surface forces apparatus (SFA). Dehydrodopa‐dependent protein conformations in mussel plaque may enable different packing modes that are reversible.

Marine mussels of the genus Mytilus live in the hostile intertidal zone, attached to rocks, bio-fouled surfaces and each other via collagen-rich threads ending in adhesive pads, the plaques. Plaques adhere in salty, alkaline seawater, withstanding waves and tidal currents. Each plaque requires a force of several newtons to detach. Although the molecular composition of the plaques has been well studied, a complete understanding of supra-molecular plaque architecture and its role in maintaining adhesive strength remains elusive. Here, electron microscopy and neutron scattering studies of plaques harvested from Mytilus californianus and Mytilus galloprovincialis reveal a complex network structure reminiscent of structural foams. Two characteristic length scales are observed characterizing a dense meshwork (approx. 100 nm) with large interpenetrating pores (approx. 1 mm). The network withstands chemical denaturation, indicating significant cross-linking. Plaques formed at lower temperatures have finer network struts, from which we hypothesize a kinetically controlled formation mechanism. When mussels are induced to create plaques, the resulting structure lacks a well-defined network architecture, showcasing the importance of processing over self-assembly. Together, these new data provide essential insight into plaque structure and formation and set the foundation to understand the role of plaque structure in stress distribution and toughening in natural and biomimetic materials.

The biochemistry of mussel adhesion has inspired the design of surface primers, adhesives, coatings and gels for technological applications. These mussel-inspired systems often focus on incorporating the amino acid 3,4-dihydroxyphenyl-L-alanine (Dopa) or a catecholic analog into a polymer. Unfortunately, effective use of Dopa is compromised by its susceptibility to auto-oxidation at neutral pH. Oxidation can lead to loss of adhesive function and undesired covalent cross-linking. Mussel foot protein 5 (Mfp-5), which contains ∼30 mole % Dopa, is a superb adhesive under reducing conditions but becomes nonadhesive after pH-induced oxidation. Here we report that the bidentate complexation of borate by Dopa to form a catecholato-boronate can be exploited to retard oxidation. Although exposure of Mfp-5 to neutral pH typically oxidizes Dopa, resulting in a>95% decrease in adhesion, inclusion of borate retards oxidation at the same pH. Remarkably, this Dopa-boronate complex dissociates upon contact with mica to allow for a reversible Dopa-mediated adhesion. The borate protection strategy allows for Dopa redox stability and maintained adhesive function in an otherwise oxidizing environment.

Sessile marine mussels must “dry” underwater surfaces before adhering to them. Synthetic adhesives have yet to overcome this fundamental challenge. Previous studies of bio-inspired adhesion have largely been performed under applied compressive forces but these are poor predictors of an adhesive’s ability to spontaneously penetrate surface hydration layers. In a force-free approach to measuring molecular-level interaction via the surface water diffusivity, different mussel foot proteins were found to have differential abilities to evict hydration layers from the surfaces—a necessary step for adsorption and adhesion. It was anticipated that Dopa would mediate dehydration given its efficacy forbio-inspired wet adhesion. Instead, hydrophobic side-chains are found to be a critical component in bringing about protein-surface intimacy. This is the first direct measurement of interfacial water dynamics during force-free adsorptive interactions at solid surfaces, and offers guidance for engineering wet adhesives and coatings.