Adhesion mechanisms of the mussel foot proteins mfp-1 and mfp-3

Abstract

Mussels adhere to a variety of surfaces by depositing a highly specific ensemble of 3,4-dihydroxyphenyl-l-alanine (DOPA) containing proteins. The adhesive properties of Mytilus edulis foot proteins mfp-1 and mfp-3 were directly measured at the nano-scale by using a surface forces apparatus (SFA). An adhesion energy of order W approximately 3 x 10(-4) J/m(2) was achieved when separating two smooth and chemically inert surfaces of mica (a common alumino-silicate clay mineral) bridged or "glued" by mfp-3. This energy corresponds to an approximate force per plaque of approximately 100 gm, more than enough to hold a mussel in place if no peeling occurs. In contrast, no adhesion was detected between mica surfaces bridged by mfp-1. AFM imaging and SFA experiments showed that mfp-1 can adhere well to one mica surface, but is unable to then link to another (unless sheared), even after prolonged contact time or increased load (pressure). Although mechanistic explanations for the different behaviors are not yet possible, the results are consistent with the apparent function of the proteins, i.e., mfp-1 is disposed as a "protective" coating, and mfp-3 as the adhesive or "glue" that binds mussels to surfaces. The results suggest that the adhesion on mica is due to weak physical interactions rather than chemical bonding, and that the strong adhesion forces of plaques arise as a consequence of their geometry (e.g., their inability to be peeled off) rather than a high intrinsic surface or adhesion energy, W.

Fig. 1.

Schematic drawing of a byssal thread attached to a substrate.

Fig. 2.

Mussels on mica. (a) All three mussel species adhered to mica. (b) Enlargement of square area in a showing the mussel with byssal threads connected to both mica sheet and bearing the weight of a mica sheet with three congeners by means of only three byssal threads.

Fig. 3.

AFM of mfp-1 adsorbed on mica. (a) Tapping mode AFM image of a mfp-1 coated mica surface after drying. (b) Magnified rectangular area in a showing ≈1-nm-thick flat layers (light patches, P) on the bare mica (dark areas). The bare mica regions are likely due to shrinking upon dehydration of the mfp-1 layer during drying.

Fig. 4.

Two mica surfaces bridged by mfp-3. (Left) Fringes of equal chromatic order (FECO) images during an mfp-3 bridging experiment. (Right) Schematic drawings of corresponding molecular processes occurring at the junction. Triangles represent the likely binding sites on the molecules. (a) Two mica surfaces in flat adhesive contact in air. (b) Same surfaces after an mfp solution was injected between them. (c) A configuration of surfaces immediately after the jump out from adhesive contact. The white vertical line indicates the original wavelength, λ, of mica–mica contact (corresponding to D = 0).

Fig. 5.

Normal forces, F⊥, measured on separating two mica surfaces (Out curves) as a function of surface separation, D, in solutions of mfp-1 (open circles) and mfp-3 (filled triangles). The left ordinate gives the measured force, F⊥/R (normalized by the radius of the surfaces, R). The right ordinate gives the corresponding adhesion energy per unit area, W, between two flat surfaces, defined by W = F⊥/3πR (27). Attractive adhesion forces were measured only in solutions of mfp-3, whereas the forces were always repulsive and reversible upon approach in solutions of mfp-1. Also, note that the repulsive “hard walls” for mfp-1 films were always farther out than for mfp-3 films.

Fig. 6.

Successive force runs measured between mica surfaces bridged by mfp-3 after the first separation. Open symbols are the forces on approach; filled symbols are the forces on separation after different compressive loads and/or various contact times. The dashed lines show the bridging force curves of Fig. 5. (Inset) Adhesion vs. contact time; the horizontal arrow shows the adhesion value of the first separation.

Fig. 7.

Measured normal forces, F⊥, as a function of surface separation, D, between an mfp-1-coated mica surface and a bare mica surface in buffer solution before (circles) and after (squares and diamonds) shearing. Open symbols are the forces on approach; filled symbols are the forces on separation.

Fig. 8.

Schematic drawing of the interactions and rearrangements between a protein adsorbed on a mica surface and a bare mica surface under compression (a), separation without shear (b), shear without separation (c), and separation after shear (d). Triangles represent the binding sites on the protein molecules.