The degree of aggregation in solution controls the adsorbed amount of mussel adhesive proteins on a hydrophilic surface

The adsorption behaviour of the mussel adhesive protein Mefp-1 on a hydrophilic surface was studied by Surface Plasmon Resonance (SPR) at pH values of 4.5 and 6.5 under aerobic conditions and at an ion strength of 0.1 M NaCl. In this environment Mefp-1 molecules aggregate by crosslinking, likely via Dopa–Dopa quinone charge transfer interactions. The initial rate of aggregation increases with increasing pH, as could be derived from Photon Correlation Spectroscopy measurements. The degree of aggregation determines the adsorption plateau value of Mefp-1. Step-like adsorption curves have been found at pH 6.5, as well as at pH 4.5, which can be interpreted as the adsorption of an ad-layer of Mefp-1 aggregates onto the initially adsorbed Mefp-1 layer on the surface. The rate of formation of this second layer increases with increasing pH and Mefp-1 concentration. The affinity of the ad-layer for the first adsorbed layer appears to be much smaller than the affinity of the first layer for the surface (Poly Vinyl Alcohol). Probably, also the ad-layer formation proceeds by the establishment of specific crosslinks with the first layer of adsorbed Mefp-1.