Although
Dopa-Fe<sup>3+</sup> complexation is known to play an
important role in mussel adhesion for providing mechanical properties,
its function at the plaque/substrate interface, where actual surface
adhesion occurs, remains unknown, with regard to interfacial mussel
adhesive proteins (MAPs) type 3 fast variant (fp-3F) and type 5 (fp-5).
Here, we confirmed Dopa-Fe<sup>3+</sup> complexation of interfacial
MAPs and investigated the effects of Dopa-Fe<sup>3+</sup> complexation
regarding both surface adhesion and cohesion. The force measurements
using surface forces apparatus (SFA) analysis showed that intrinsic
strong surface adhesion at low pH, which is similar to the local acidified
environment present during the secretion of adhesive proteins, vanishes
by Dopa-Fe<sup>3+</sup> complexation and alternatively, strong cohesion
is generated in higher pH conditions similar to seawater. A high Dopa
content increased the capacity for both surface adhesion and cohesion,
but not at the same time. In contrast, a lack of Dopa resulted in
both weak surface adhesion and cohesion without significant effects
of Fe<sup>3+</sup> complexation. Our findings shed light on how mussels
regulate Dopa functionality at the plaque/substrate interface, in
response to the microenvironment, and might provide new insight for
the design of mussel-inspired biomaterials