| With unique advantages compared with conventional synthetic adhesives, bioadhesion has inspired wide researches on the involved biological adhesion systems and also promoted the developments of biomimetic or bio-inspired adhesive materials. However, one of the major problems in the current application of biomimetic adhesives is that the adhesion performance is usually not stable in moisturized environment. Marine mussels are well known for their abilities of firmly attaching to various substrates in lakes and highly saline oceans, which offers a perfect solution against humidity. It has been found that the formation of mussel adhesion is attributed to the proteins secreted inside the byssal plaque, named mussel foot proteins (Mfps), and further attributed to the unusual amino acid 3,4-dihydroxyphenyl-L-alanine (Dopa) contained in Mfps. The Dopa content usually correlates with the adhesion capacity of an Mfp. Among the studies of mussel adhesion, the concerning adhesion mechanism, or the molecular interaction dominating the Mfp binding, is the most fundamental and complicated issue, which has not been fully figured out yet. A thorough exploration of the Mfp adhesion mechanisms is therefore critical in building the foundation for the development of biomimetic functional material with great adhesive performance, well adaptive capacity and environmentally friendly features.This thesis focuses on the Mfp adhesion mechanisms. Specifically, the investigations are carried out into three parts including:the adhesion mechanism of interfacial Mfp, the Mfps adhesion on organic films and the role of Dopa-boronate complex in Mfp adhesion. Experimentally, a surface forces apparatus (SFA) with nano-scale resolution is utilized in the direct measurements of the adhesion between the Mfp and the substrate surface and meanwhile the cohesion between two Mfps, under different solution or surface conditions.The adhesion mechanism of interfacial Mfp is investigated with the Mfp-5 secreted by Mytilus edulis, which outperforms all the other Mfps in terms of Dopa content of up to 27 mol%. The SFA results show that Mfp-5 is the most adhesive Mfp known so far. At acidic pH, the adhesion energy of Mfp-5 to mica is as high as-13.7 mJ/m2, and the cohesion energy between two symmetrical Mfp-5 films is up to-2.5 mJ/m2. However, as Dopa is oxidized to Dopaquinone, the protein adhesion can be dramatically reduced or even eliminated. The results indicate that the adhesion of Mfp-5 to mica is dominated by the bidentate hydrogen bonding between the catechol group in Dopa of the protein and the oxygen atoms on the mica surface, which proves again the significant dependence of Mfp adhesion on catecholic chemistry in Dopa. However, Mfp adhesion is not rigidly proportional to the Dopa content, and it still can be influenced by the protein conformation when the adhesive contact is made. In terms of the interaction between Mfps, Mfp-5 is observed to tightly bind with another interfacial protein Mfp-3, which implicates that besides being active in the adhesion in mussel plaque, Mfp-5 also serves as a crucial "mediator" of interfacial molecules. Under auto-oxidation condition at pH 5.5 or 7.5, a strong cohesion is also detected between two oxidized Mfp-5 layers when the surfaces are left in contact overnight. This cohesion is most likely due to the Dopaquinone-mediated covalent cross-linking.The Mfp’s adhesion to organic surfaces is investigated by using two chemically different self-assembled monolayers (SAMs) as substrates, which respectively contain nonpolar (hydrophobic) CH3-and polar (hydrophilic) OH-headgroups. The SFA results show that at the CH3-ended SAM surfaces, all three different Mfps (i.e. Mfp-1, Mfp-3 and Mfp-5) exhibit strong adhesion energies ranging from-4 to-9 mJ/m2. The strongest adhesion is measured by Mfp-3 with the most hydrophobic components rather than Mfp-5 with the most Dopa. Therefore, this adhesion must be attributed to the hydrophobic interaction between the hydrophobic residues, which are contained in all three Mfps, and the alkyl surface. Moreover, the asymmetric distribution of three more hydrophobic tryptophan residues at Mfp-3’s C terminus might also contribute to the enhanced adsorption of Mfp-3 molecules at the CH3-SAM interface. Besides, it has been demonstrated for the first time that the coating protein Mfp-1 can bridge two chemically asymmetric surfaces with the adhesion strength comparable to the hydrogen bonding of interfacial Mfps, which implies the great adaptivity of Mfp adhesion. At the OH-ended SAM surfaces, only weak adhesions are measured in the three Mfps, which is probably due to the formation of weaker monodentate hydrogen bonds with shorter lifetimes as the protein approaches the surface. This finding reveals the potential control strategy for Dopa-mediated wet adhesion.The role of Dopa-boronate complex in Mfp adhesion is investigated with a buffer solution containing borate. The SFA results show that a time-dependent adhesion between Mfp-5 and mica can be reversibly achieved at an auto-oxidation pH of 7.5 with 0.1 M boric acid. When the buffer is changed to pH 3 without borate, the Mfp-5 adhesion is recovered to nearly full level with negligible degradation, which means that the auto-oxidation of the protein at neutral pH is retarded along with the formation of Dopa-boronate complex. The following test, in which the boric acid is replaced with phenylboronic acid, further rules out the possibility of the boronate group mediating the adhesion at pH 7.5. Thus, the incredible phenomenon of reversible and strong adhesion of Mfp-5 to mica in pH 7.5/borate buffer is indeed dominated by the hydrogen bonding of Dopa, and further benefits from the reversible and fast complexation of borate to Dopa as well as the dissociation chemistry of Dopa-boronate complex. Moreover, two conditions are required for using borate to realize Mfp adhesion, which are compression and working at neutral or basic pH only. |
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