Marine adhesive proteins: natural composite thermosets

Abstract

Marine environments are severely challenging for the performance and durability of synthetic adhesives. Factors commonly associated with adhesive failure are weak boundary layers (water, oxides), adhesive erosion and swelling. For many permanently attached marine organisms such as barnacles, mussels, oysters, etc., however, underwater adhesion is 'business-as-usual'. Knowledge about the chemistry and bioprocessing of these marine adhesives will provide profound insights for the evolution of a new generation of environmentally safe, water-resistant adhesives. Despite their apparent structural diversity, marine adhesives are essentially analogous to composite thermosets, that is, the adhesive consists of fibre, filler and catalyst molecules that are dispersed in a cross-linked resin rendering it resistant to heat and solvents. The fibres and fillers in these composites are variable. e.g. collagen, fibroin, chitin present as fibres, and sand, shell, air and water present as fillers. The precured resins of seven organisms including members of the Mollusca, Annelida, and Platyhelminthes have now been isolated and partially sequenced. These are proteins with basic isoelectric points, high levels of the amino acid 3,4-dihydroxyphenyl-L-alanine (DOPA), and an extended, flexible conformation. The DOPA functional group in particular is thought to play a key role in (a) the chemisorption of these polymers to surface underwater, and (b) covalent cross-linking or setting of the adhesive, the latter reaction catalysed by the enzyme catecholoxidase. Much more needs to be done to explore the details of the adhesive processing and delivery strategies used by these organisms.