Byssal threads of the blue mussel attach to a wet, solid surface using adhesive proteins that overcome the surface’s attraction to water molecules.

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Blue mussels (Mytilus edulis) are bivalves that attach to rocks in wave-battered intertidal seashores. They attach using stringy fibers that emerge from their protective shells, secreted by glands on the soft bodies inside. These fibers, called byssal threads, stick to the rock with a mussel-produced adhesive comparable in strength to human-made glues but without carcinogens, such as formaldehyde. The mussel glue can also cure under water.

A key feature of the blue mussel’s unique adhesive chemistry is the presence of the amino acid 3,4-dihydroxyphenylalanine, with its reactive catechol functional group (two hydroxyl groups sticking out from a benzene ring) that forms strong bonds with catechols on adjacent molecules and with metal atoms present in the surface of most natural solid substrates. Another key feature is the ability of catechol chains to overcome a solid surface’s otherwise strong preference for water molecules (which is why conventional adhesives fail on wet surfaces).

In addition to catechol chains, the interfacial adhesive protein, fp-3F, is located in surface of mussels and contains a great quantity of dopa, which helps make underwater adhesion possible. The shape of dopa has been imitated for several years to make underwater adhesives. However, there has been a limitation in making underwater adhesives as strong as the natural adhesives of mussels because it has been challenging to balance between surface adhesion, which is attraction between surface and adhesives, and cohesion of adhesive molecules.

It is now known that the amino acid lysine, which has a positive electric charge, helps aid dopa in strong underwater adhesion. These molecules have synergetic effect on mussel adhesions in various conditions, and are either bound to or apart from each other at a specific location. The distance between dopa and lysine affected their synergy on surface adhesion and cohesion differently.

When these two molecules are adjacent to each other, surface adhesion increased greatly. Lysine enhances underwater surface adhesion by attracting water molecules, which disrupt underwater adhesion, in the surface and water molecules around dopa. Lysine does this in part by disrupting ferric ion, a mediator for cohesion, from approaching dopa electrically and structurally.

New mussel-inspired adhesives, which have wide-ranging applications from surgical glues to wood composites, currently use soy as an inexpensive, accessible feedstock. These human-made adhesives work by blocking certain amino acids in soy proteins that are not present in mussel proteins, such as glutamic acid, so that the resulting compound bears a closer resemblance to that of mussel proteins.

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“Intensive studies have found that 3,4-dihydroxyphenylalanine (Dopa) is one of the key molecules for underwater mussel adhesion. Although basic mechanisms of mussel adhesion have been elucidated, little is known about how mussels control the balance between surface adhesion and cohesion, which is critical for successful adhesion without peeling and/or tearing. In this work, we focused on lysine (Lys) molecules which are frequently flanked to Dopa residues in interfacial adhesive proteins, specifically their synergy and anti-synergy on surface adhesion and cohesion” (Shin et al. 2020:168)

Journal article
The position of lysine controls the catechol-mediated surface adhesion and cohesion in underwater mussel adhesionJournal of Colloid and Interface Science Volume 563, 15 March 2020, Pages 168-176March 15, 2020
Mincheol Shin, Ji Yeon Shin, Kyeounghak Kim, Byeongseon Yang, Jeong WooHan, Nak-Kyoon Kim, Hyung Joon Cha

Journal article
Mussel-Inspired Surface Chemistry for Multifunctional CoatingsScienceOctober 18, 2007
H. Lee, S. M. Dellatore, W. M. Miller, P. B. Messersmith

“Pounding waves are no match for the mighty mussel, that produces strong, flexible threads that cling to rocks…mussels secrete a unique amino acid called dihydroxyphenylalanine…Researchers have developed a new group of adhesives for wood products inspired by the ability of mussels to cling to rocks using thread-like tentacles. These threads are proteins that retain powerful adhesive properties even in water.” (ScienceDaily 2005)

Web page
Wood Glue Inspired by Mussels: Chemist's Glue Borrows Unique Amino Acid from Mollusk

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