Mussels have been extensively researched over the past years. Besides their ability to attach themselves to rock surfaces, they secrete 3,4-dihydroxy-phenylalanine (DOPA)-rich proteins in strong water adhesion conditions. Numerous factors affect the underwater adhesion of mussels. They include the role of catechol in the H-bonding formation and oxidation-induced cross-linking, hydrophilicity/hydrophobicity balance, and the polymeric backbone polarity. Despite this knowledge, coupled with the design of better artificial hydrogels and adhesives, the underlying mechanisms governing adhesion of mussel foot-byssus and the role of other amino acids remain underexplored.
The discovery of the synergic effects of catechol moiety and charge amino groups in mussel foot-byssus proteins (Mfps) present viable and promising insights for understanding the adhesion of mussel foot-byssus in underwater conditions. It has been discovered that lysine residues are effective in the removal of hydrated layers and promote the hydrogen bond formation of the nearby catechol groups with the surface sequentially. Previous findings have also revealed the strong influence of acidic solutions on the synergy effects, with lower pH values producing stronger adhesion. This has been mainly attributed to the oxidation prohibition of catechols under acidic conditions.
However, whether the spatial proximity of lysine and DOPA residues is the pre-requisite for the synergic effects and whether lower pH value plays other roles other than oxidation prevention are still open questions. Therefore, understanding the interactions of surfaces modified with multiple amino groups and catechol with planer surface and their abilities at various pH values to mimic natural adhesive processes is important. Equipped with this knowledge, researchers at Jianghan University and Chinese Academy of Sciences: Associate Professor Youbing Mu, Mr. Pengzhou Mu, Dr. Xiao Wu and Professor Xiaobo Wan investigated the synergetic effects of curved surfaces modified with multiple catechols and amino cations. Their main objective was to prove that despite the importance of spatial proximity, it may not be a key requirement of synergic effects between catechol and amino groups in underwater adhesion. The work is currently published in the journal, Applied Surface Science.
In their approach, the underwater experiments to study the synergistic effects were carried out at the nanoscale level using atomic force microscopy (AFM). Two separate probes were grafted on the AFM tips: monomers bearing catechol and amine cation groups and those carrying the two groups separately and isolated from each other by the monomers without modification. The facets of the synergetic effects were investigated and discussed. Moreover, the influence of the proximity in space as well as pH conditions on the synergetic effects was also studied in detail.
The authors found out that the synergic effects exhibited two facets. First, the two groups resulted in stronger adhesion in electrolyte solutions than when the catechols acted alone. Interestingly, the effect never required a close proximity of the amine cations and the catechols, confirming that proximity is not a prerequisite for synergistic effects. Second, integrating the catechol and cation groups into one molecule and placing them in close proximity enhanced the adhesion force in low pH solution, and on the other hand, lowering pH value was not effective to promote stronger adhesion when catechol and amino groups are separated from each other in space, suggesting that the influence of synergistic effects on spatial proximity is conditional and depends on the pH of the electrolyte solution. Thus, the acid conditions contributed to adhesion more than just by protecting the catechol from oxidation.
In summary, the two facets of synergistic effects of catechol and amine cation in underwater adhesion conditions were reported: Spatial proximity emerged as a non-prerequisite for synergic effects since the two groups did not have to be next to each other to promote strong adhesion between the surfaces; more interestingly, at lower pH values, spatial proximity became important and could promote even stronger adhesion. The insights offered a better understanding of the reasons as to why most DOPA and lysine moieties are close to each other in Mfps and why Mfps secretion is accompanied by a pH plump. In a statement to Advances in Engineering, Professor Xiaobo Wan explained the study would advance mussel foot-byssus-related studies and inspire more genius designs of underwater applicable adhesives.
Mu, Y., Mu, P., Wu, X., & Wan, X. (2020). The two facets of the synergic effect of amine cation and catechol on the adhesion of catechol in underwater conditions. Applied Surface Science, 530, 146973.