Ion-Induced Morphological Plasticity in a Self-Assembling Peptide Hydrogel

A hydrogel formed by a short peptide is presented that exhibits remarkable stimuli-responsiveness and plasticity, undergoing a morphological transformation from nanofibers to nanospheres in the presence of monovalent (Li⁺, Na⁺, K⁺) or trivalent (Al³⁺, Fe³⁺) metal ions under physiological conditions. The nanofibrillar structure of the hydrogel was examined using transmission electron microscopy (TEM), atomic force microscopy (AFM), small-angle X-ray scattering (SAXS), and X-ray diffraction (XRD) studies and atomistic molecular dynamics simulations, in complement, to explain the nanostructural transitions at the microscopic level. Interestingly, exposure to divalent metal ions (Mg²⁺, Ca²⁺, Co²⁺, Ni²⁺) induces a unique shrinking (syneresis) behavior, accompanied by a morphological shift to nanoribbons. Both simulations and SAXS analysis confirm that these ions cause a contraction in the packing of gelator peptides, significantly reducing the interpeptide distance. This ion-specific adaptability confers tunable physicochemical properties and morphological plasticity. Hydrogels incorporating mono- or trivalent ions exhibit enhanced thermal stability and mechanical strength relative to ion-free counterparts, underscoring the reinforcing role of metal coordination. Strikingly, shrunken gels formed in the presence of divalent ions display even greater stiffness than freshly prepared gels in the absence of any metal ions, suggesting that syneresis acts as a postassembly strengthening mechanism. These findings highlight a versatile, stimuli-responsive soft material in which ion-peptide interactions orchestrate nanoscale morphology, mesoscale network architecture, and macroscopic mechanical performance-opening avenues for adaptive hydrogel systems in targeted biomedical, sensing, and controlled-release applications.