Aromatic/aliphatic residue balance regulates Cu(II)-induced supramolecular gel formation and drug release in histidine-containing tetrapeptides

In this study, the effect of the balance between aromatic and aliphatic residues in Cu(II)-responsive tetrapeptides on self-assembly and supramolecular metallogel formation was investigated systematically. Tetrapeptides with the general sequence His-X-X-X (X = Ile or Phe)-HIIF, HIFF, and HFFF-were designed to evaluate the cooperative contributions of metal coordination, hydrophobic interactions, and π-π stacking in the presence of Cu²⁺. Histidine-mediated Cu²⁺ coordination promoted the formation of three-dimensional supramolecular networks, whereas network density and mechanical strength were modulated by the relative proportions of isoleucine and phenylalanine residues. Rheological analysis revealed that the storage modulus followed the order HIFF > HIIF > HFFF, indicating that HIFF, with an optimal aromatic/aliphatic balance, exhibited the highest mechanical robustness. All gels displayed reversible gel-sol transitions and pronounced thixotropy, confirming the dynamic and reconstructable nature of the supramolecular networks. Doxorubicin (DOX) encapsulation further demonstrated pH-responsive release behavior. Minimal release was observed under physiological conditions (pH 7.2), whereas sustained release occurred under mildly acidic conditions (pH 5.6). The release efficiency followed the order HIIF > HFFF > HIFF, suggesting that higher aromatic content and a more compact network enhance drug retention. Collectively, these findings identify the isoleucine/phenylalanine ratio as a key design parameter for tuning metallogel mechanics and drug-delivery performance.