Diversified Material Designs in Biological Underwater Adhesives - Structural Interfaces and Attachments in Biology - page 180

acids: Gly, Lys, and DOPA. The primary structure of the other basic protein
(18 kDa) contains degenerate copies of a consensus repeat with a large variation
in the individual sequences. The acidic protein is almost completely occupied by
Ser, with a sporadic insertion of Tyr. Only one variant is found in the C-terminal
basic sequence, which contains six Cys.
It is presumed that the function of the tubeworm's cement surface coupling to
foreign materials is due to DOPA and Pho-Ser, analagous to the mussel byssal
system. Metal-coordination bonding and Cys-DOPA cross-linkage are suggested to
be involved with setting the cement [ 64 ]. The cement contains Mg 2+ and Ca 2+ in an
extractable form in the EDTA solution, and EDTA-treatment actually reduces
adhesive force and compressive characteristics [ 65 ]. The detection of Cys-DOPA
in the whole cement

[ 63 ] also suggests the occurrence of intermolecular

cross-linkage.
The components found in the tubeworm cement look like water-soluble poly-
electrolyte solutes. It diffuses into the surrounding water if a specific mechanism is
not provided. However, the physicochemical properties of the cement proteins,
occurrence of divalent cation species, and pH-shift in the secretory gland and
seawater might fulfill conditions for complex coacervation [ 64 ]. Complex coacer-
vate seems to have good properties for removing the water boundary layer and
spreading onto material surfaces.

9.7 Impacts to Artificial Material Design

Most research aimed at learning about biological adhesives (Fig. 9.7 ) has been
focused on amino acids, DOPA, or the side chain, catechol group. In three decades
of challenges, it has been shown that intermolecular cross-linking with several
styles and surface coupling to metal/metal oxide are practical for materials design
with DOPA. Numerous forms of gel and coating materials have been made from
synthetic peptide mimics, protein extracts from the mussel foot, recombinant forms
of proteins, and DOPA/catechol-incorporated synthetic polymers.
There are two main ways to introduce DOPA into a backbone of material. These
are: post-conversion of Tyr to DOPA by the enzyme or oxidants, and the direct
incorporation of DOPA. The former style has been used for synthetic peptides [ 22 ]
and the recombinant forms [ 66 , 67 ], while the latter has been principally possible in
chemical synthesis [ 68 - 70 ] and protein extracts from the animal [ 71 ]. Recently,
in vivo conversion of Tyr to DOPA in the recombinant form of a eukaryotic host
was also reported as an additional case of the latter [ 72 ].
Prior studies have primarily focused on a covalent cross-linkage, and
post-conversions from Tyr have been used to produce some materials. The
post-conversion not only includes conversion to DOPA, but also the subsequent
conversion to the quinone and the immediate formation of any covalent cross-linking
at an elevated pH. Initial research has increased our experience for controlling the