Role of molecular chirality and solvents in directing the self-assembly of peptide into an ultra-pH-sensitive hydrogel.

We report the self-assembly of an ultra-pH-sensitive hydrogel from a simple dipeptide derivative, ferrocene-diphenylalanine (noted as Fc-FF). The pure Fc--Phe--Phe-OH (noted as (L)-Fc-FF) or Fc--Phe--Phe-OH (noted as (D)-Fc-FF) peptide could self-assemble into right- or left-handed nanohelices in a mixture of water and organic solvent leading to formation of stable chiral hydrogels at a very narrow pH range of 5.7-5.9. Intriguingly, if we used a racemic mixture (noted as (L,D)-Fc-FF) of (L)-Fc-FF and (D)-Fc-FF, stable hydrogels could only be formed at much lower pH values with a weak mechanical property. The results indicated that the molecular chirality has great influence on the gelation of the peptides. Moreover, we investigated the gelation behaviour of the peptides in various solvents. Here the water molecules are essential in directing the chiral self-assembly of Fc-FF into entangled chiral nanostructures, leading to the formation of stimuli-responsive hydrogels. And in the presence of water molecules, a small amount of organic co-solvents also has significant effect on the gelation of Fc-FF peptides. This work demonstrated that the gelation of such a bioorganometallic molecule is very sensitive to the molecular conformation of itself and the self-assembly environment (pH, solvents, etc.), which has potential applications in chemical sensing and biomedicine.