Synthesis and wettability characteristics of model adhesive protein sequences inspired by a marine mussel

Three adhesive polyoctapeptides (X/Y-Gly-Tyr-Ser-Ala-Gly-Tyr-Lys)n (X, Thr and Ala: Y, Thr:Ala = 3:2), which are the C-terminal octapeptide sequences shared by all those consensus motifs of a ribbed mussel Geukensia demissa adhesive protein (RMAP), have been synthesized by the polycondensation of the active esters. The correlation between wettability and adhesion with the alternative substitution by hydrophobic Ala and hydrophilic Thr in the synthetic RMAPs in aqueous solution and as films on high and low surface free energy (sfe) surfaces has been investigated by surface chemical approaches. The side chain moieties of RMAPs arrange their local conformation to adapt to the inherent surface character, and they orientate on high and low sfe surfaces. On a high sfe glass the hydrophilic side chains of the RMAP molecules face toward the surface, and on a low sfe polyethylene (HDPE) the hydrophobic side chains face toward the surface. The bonding strengths of synthetic RMAPs on glass and HDPE with and without enzymatic oxidation have been examined. Increased starting concentration exhibited the strongest adhesive capability (7.5 kgf/cm2) on alumina due to the increased cohesion among RMAP molecules. As another factor to increase adhesive ability, the intermolecular cross-linking among RMAP molecules by tyrosinase increases their adhesive capability by about 20% on HDPE, glass, and alumina except iron.