Ion Valency as a Molecular Switch for Salt‐Resistant Underwater Adhesion

Achieving underwater adhesion remains challenging due to the disruption of interfacial interactions by hydration layers and the ionic environment. This study shows how high adhesion in a saline environment can be achieved in adhesive peptide systems relying on π–π and cation‐π interactions using multivalent ions. Monovalent ions (K + ) disrupt native peptide‐peptide interactions, drastically reducing adhesion strength. Conversely, multivalent ions (Mg 2+ and Y 3+ ) enable robust interfacial adhesion by forming stable π‐cation‐π networks, effectively compensating for disrupted native pairings. The adhesion enhancement by Y 3+ is particularly pronounced, highlighting its unique capability for multidentate bridging. Molecular dynamics simulations and quantum mechanical analyses confirm that Y 3+ ions stabilize extended interfacial interactions, enabling stronger stress dissipation during tensile deformation. Additionally, NMR spectroscopy supports these observations by demonstrating significant cation‐dependent perturbations of aromatic (Phe) and cationic (Lys) peptide residues. A thermodynamic model further elucidates the competitive binding dynamics underpinning adhesion modulation and capturing all experimental trends. This work provides detailed molecular insights into ion valency effects on cation‐π mediated underwater adhesion, guiding the development of bio‐inspired materials with tailored ionic responsiveness suitable for biomedical and technological applications in saline environments.