Composite hydrogels of phenylalanine dipeptides with trivalent metal cations

We studied the co-assembly of hydrogels from a short model peptide, Fmoc-protected diphenylalanine (Fmoc-FF), in the presence of trivalent cations of aluminium (Al) and iron (Fe). Additionally, we investigated the implicit effects of small molecules on the self-assembly of Fmoc-FF by buffering the model system with three commonly used buffers: HEPES, TRIS, and sodium phosphate. The formation and stability of the resulting hydrogels were analyzed through rheological characterization, and changes in the secondary structure of Fmoc-FF due to addition of metal cations were monitored using ATR-IR spectroscopy. Our results suggest that complexation occurs between the metal cations and the amide groups of the peptide. The main characterization method in this study used a spin-probing electron paramagnetic resonance (EPR) strategy, employing persistent nitroxyl radicals TEMPO, TEMPO-benzoate, and Fmoc-TOAC. Our EPR data reveal differences in the solvation shell of probes and peptides induced by the presence of metal cations. These differences were then correlated with the toughness of the hydrogels and the distinct types of interactions within them allowing to draw conclusions on the internal nanostructure of these composite materials.