As inhabitants of rugged shores, mussels have an amazing capacity to stick to rocks, despite the constant pounding of waves. These organisms are also notorious for sticking to ships, glass, and, well, just about anything–even Teflon.
Researchers at Purdue University in West Lafayette, Ind., say they have uncovered the secret to what makes mussel glue so strong. It’s iron. Once they understand the glue’s chemistry, researchers might develop more effective antifouling paints to prevent mussels, barnacles, and other hangers-on from sticking to ships. Another payoff could be stronger biomaterials, particularly sutures and other wound-closing products.
Chemists have long known that mussels secrete an adhesive that consists of a hardened matrix of proteins. “These are really tough fibers,” says polymer chemist Robin Garrell of the University of California, Los Angeles. Exactly how these proteins link together to give the material, called byssus, its strength has remained unclear.
Jonathan Wilker and his colleagues at Purdue decided to investigate. They collected hundreds of common blue mussels off the coast of Maine and placed them on glass sheets in laboratory tanks of salt water. After the mussels fixed themselves to the glass, the researchers cut the mollusks free and scraped off the glue left behind.
Chemical analyses of the material revealed a high concentration of iron, which the mussels extract from seawater. The researchers then harvested the adhesive’s precursor molecules directly from the mussels. When the team added iron to a solution of the proteins, a tightly bound mesh formed. What’s more, it was a chemical match with the mussels’ own byssus, the researchers report in the Jan. 16 Angewandte Chemie.
The team also determined how iron interacts with side chains on the protein molecules. The side chains, called dihydroxyphenylalanine (DOPA), have an affinity for iron. As it turns out, a single iron atom can bind to three DOPA side chains. The iron-DOPA complex then reacts with oxygen, forming highly reactive chemical agents called radicals, Wilker says. Radicals often serve as polymerization agents in industrial processes, so it’s possible they cross-link byssus molecules into a tough material and perhaps also attach the material to surfaces, he proposes.
“This explains why iron is so concentrated” in mussels’ glue, Garrell says.
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Even so, iron may not be the key cross-linking agent. Previous studies have shown that enzymes also stitch together the glue proteins.
Nonetheless, coatings that interfere with iron’s binding to proteins could be a boon for the Navy and the shipping industry. Because the extra drag these creatures cause eats up fuel, mariners spend billions of dollars annually trying to keep organisms from accumulating on hulls.
One more thing, says Wilker: Existing coatings “kill off everything in water. It would be nice if we could develop something that’s nontoxic.”
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