Lanthanide coordination in N-acyl diphenylalanine organohydrogels

Three different N-acylated diphenylalanine peptide conjugates with varying lengths of fatty acid tails were synthesized and characterized through ¹H NMR, FT-IR, MS, and elemental analysis. The organo-hydrogelation of these peptides was studied using two organic solvents, DMF and DMSO, as well as in the presence of Tb³⁺, which resulted in successful gelation for three previously non-gelling conditions. The viscoelastic properties of these gels were also investigated through rheological frequency sweeps. The morphologies of the organo-hydrogel hybrids were also examined through TEM and SEM. Through mass spectrometry, we demonstrate a variety of Tb³⁺–peptide–solvent coordinated species owing to the variable geometries characteristic of lanthanides and the self-assembling nature of physical gels, driven by noncovalent interactions. A 1 : 1 peptide : Tb³⁺ ratio was found to yield the highest fluorescence intensity at 546 nm, though even 0.2 eq. of peptide enhanced the innate fluorescent properties of Tb³⁺. Thus, through control of the peptide : Tb³⁺ ratio, a wide potential range of fluorescent gels with variable viscoelastic properties can be developed with tunable properties.