Adsorption behavior and enzymatically or chemically induced cross‐linking of a mussel adhesive protein

Abstract

The adsorption behavior of the mussel adhesive protein Mytilus edulis foot protein‐1 (Mefp‐1) has been investigated on a negatively charged polar SiO2 surface and an electrically inert non‐polar CH3‐terminated thiolated gold surface. How the structure of adsorbed Mefp‐1 is changed upon chemically and enzymatically induced cross‐linking using sodium periodate (NaIO4) and catechol oxidase, both of which transform DOPA residues in Mefp‐1 into highly reactive o‐quinones, was also investigated. The results are compared with those resulting from addition of Cu2+ to adsorbed Mefp‐1, which forms complexes with and catalyses oxidation of DOPA residues, previously suggested to participate in the cohesive and adhesive properties of the byssus thread of M. edulis. By combining surface plasmon resonance (SPR) and quartz crystal microbalance/dissipation (QCM‐D) measurements, the effects of these agents were investigated with respect to changes in the amount of coupled water, the viscoelastic properties (rigidity) and the hydrodynamic thickness of the protein adlayers. The layer of Mefp‐1 formed on the bare CH3‐terminated surface was elongated, flexible and coupled hydrodynamically a substantial amount of water, whereas Mefp‐1 formed a rigidly attached adlayer on the SiO2 surface. Upon enzymatically and chemically induced cross‐linking of Mefp‐1 formed on the CH3 surface, the rigidity of the adlayer(s) increased significantly. A similar increase in the rigidity was observed also upon addition of Cu2+, suggesting that the high level of metal ions present in the byssus thread might be essential for the cohesive and adhesive properties of this protein. For the mass‐uptake kinetics of enzymatically induced cross‐linking, three different phases were observed and are interpreted as competition between binding of protein and release of coupled water. For the reaction with NaIO4 and Cu2+, only release of water affected the coupled mass. The importance of this type of information for an improved understanding of the strong adhesion and cohesive properties in marine environments is discussed.