The two facets of the synergic effect of amine cation and catechol on the adhesion of catechol in underwater conditions

Many factors govern the underwater adhesion of mussels, among which the synergic effect of charged amino group and catechol moiety in mussel foot-byssus proteins has been discovered recently. Here we show that although very important, spatial proximity might not be the prerequisite for the synergic effect between amino and catechol groups in underwater adhesion. By designing probes grafted with monomer integrated with both groups and with monomers carrying the two groups separately, we revisted the synergic effect at nanoscale level. It is found that the synergic effect has two facets: first, the presence of both groups enable stronger adhesion in electrolyte solutions than the presence of catechols alone, and this effect does not require catechols and amine cations to be next to each other in space. Second, only when the catechol and cation are paired and next to each other, the adhesion force could be further enhanced in low pH solutions, which means the dependence of synergic effect on the spatial proximity of catechols and amine cations is conditional to the acidity of the electrolyte solution. We reason that besides the already discovered effect in protecting catechol from oxidation, acid condition contributes to underwater adhension by interfering with the formation of the hydration layer, which helps the eviction of absorbed cations and the coordination of catechol sequentially when the catechol and amino cation are spatially next to each other.