Halogen Bond‐Tuning of Responsive Supramolecular Amino Acid Hydrogels

N ‐Fmoc‐pentafluoro‐ L ‐phenylalanine ( F 5 ) forms stable supramolecular hydrogels, mainly through π‐π stacking of its aromatic groups. Hydrogen bonding‐driven co‐assembly of F 5 with suitable partner molecules has been reported to affect the gel's properties, in accordance with what is observed in their corresponding co‐crystal structures. Herein, we extended this hydrogel modulation strategy to halogen bonding (XB) interactions by introducing an XB‐donating iodine atom in the para ‐position of the F 5 phenyl ring. Under conditions mimicking biological environments, both crystal packing and hydrogelation of the resulting N ‐Fmoc‐4‐iodo‐tetrafluoro‐phenylalanine ( IF 4 ), were significantly affected by iodine‐π interactions. Slower fibril formation kinetics and reduced strength of IF 4 hydrogels in phosphate buffer solution, compared to F 5 , mirrored iodine‐induced changes in Fmoc stacking in the solid state. Notably, the addition of strong XB‐acceptors – such as iodide anions or pyridine‐containing substrates, like vitamin B 3 – induced a significant increase in gel stiffness. These findings suggest the possibility of exploiting properly tailored Fmoc‐amino acids as “XB‐responsive” hydrogelators, useful for anion sensing applications or for trapping bioactive molecules.