Diversified Material Designs in Biological Underwater Adhesives - Structural Interfaces and Attachments in Biology - page 173

9.4.2 Versatile DOPA

Several studies on model peptides, proteins, or single amino acids indicate that
DOPA strongly couples to metal/metal oxide with the strength nearly comparable
to that of covalent bonding [ 19 ]. The highest content of DOPA in the two
surface coupling proteins of the adhesive disk is in agreement with a significant
role of DOPA for coupling to foreign metal/metal oxide surfaces in the natural
system.
Results of in vitro studies have indicated the potential of DOPA to form several
cross-linkages, including nucleophilic addition of thiolate of Cys to form Cys-
DOPA [ 20 ], Michael addition via DOPA quinone [ 21 , 22 ] to form di-DOPA and
Lys-DOPA, and coordination bonding via metal ion, mostly Fe 3+ [ 23 ]. Molecular
assembly and curing are considered to be essential functions to form the bulk of
the disk, the coating layer, and linking between separate microstructures, thus
intermolecular cross-linkage is assumed to be crucial for function. The occurrence
of cross-links was indicated by the results of in vivo analyses, which include
spectroscopic evidence [ 24 ] and the finding of co-localization of Fe 3+ with DOPA
(a suggestion for their coordination bonding) [ 25 ], the detection of 1 mol% 5- S-
Cys-DOPA out of all of the DOPA in the hydrolysate of foot-print remained onto
glass substratum [ 2 ], and the detection of 5, 5 0 -diDOPA at a ratio of 1 per 1,800 total
amino acids of the disk by rotational echo double resonance [ 26 ]. For instance,
surface force apparatus (SFA)-measurement indicated that the bulk protein with a
low concentration of Fe 3+ (

M) have homo-protein interaction with a strength of
2.2 mJ/m 2 that approaches the 10 mJ/m 2 energy measured for the strongest known
non-covalent protein-ligand interaction (biotin-avidin) [ 25 ]. Thus, several intermo-
lecular cross-linkages via DOPA-residues have practical responsibility to form the
bulk of the disk.
The curing not only forms the bulk of the adhesive, but is also responsible for
mechanical properties such as tensile strength at loading and recovery at unloading.
The metal-coordination bonding is actually reversible [ 27 ], thus it may be a good
design for durable bulk-formation.
The coating layer of byssal thread is dual-functional, since coating is known
to be essential for both extensibility and stiffness of the whole byssal thread [ 28 , 29 ]
in addition to protection from microbial degradation. The protein at the coating
layer has the third highest level of DOPA at 10-15 mol% among nine byssal
proteins. The co-localization of a metal ion, Fe 3+ , implies the significance of
metal-coordination bonding via DOPA. EDTA-treatment reduced the coating hard-
ness by 50%, indicating that the coordination bonding is, at least, involved in the
stiffness.
Both possible linking proteins contain lesser amounts of DOPA and are rich
in His/Asp and Cys, respectively. The abundant His and Asp residues in a linking
protein [ 13 ] are considered to form coordination bonding with metal ions Cu 2 þ
and Ca 2 þ , respectively, thus the protein is suggested to link other proteins
via intermolecular metal-coordination bonding. The other possible linking

m