Synthesis of - and ,-Heterocycle-Dipeptide Conjugates for Supramolecular Hydrogel Formation.

Small peptides with aromatic nuclei at the -terminus have been shown to form bioactive, biocompatible, and biodegradable supramolecular peptide hydrogels. Novel heterocycle-dipeptide conjugates with potential biological activity or application as drug carriers were synthesized by using (benzo[]thiophene) and ,(thieno [2,3-]pyridine and thieno[2,3-]quinoline) heterocycles as -protective groups for dipeptides l-Phe-l-Phe and l-Phe-l-Leu. The synthesis involved coupling heterocyclic carboxylic acids with trifluoroacetate salts of ethyl l-phenylalanyl-l-phenylalaninate and ethyl l-phenylalanyl- l-leucinate using HBTU and EtN, producing the corresponding six -heterocycle-dipeptide ester conjugates, which were then hydrolyzed to the carboxylic acids. These conjugates were subjected to gelation tests in water starting from 0.4 wt% concentration of the conjugates, using a pH-lowering method with GdL. Among them, only the conjugate of benzo[]thiophene with l-Phe-l-Phe-OH formed a hydrogel, with a gelation critical concentration of 0.15 wt% (GdL 0.6%) and a final pH of 6.8, which is important for biological applications. The hydrogel was characterized by STEM, revealing nanofibers with an average thickness of 17 nm that assemble into a 3D network capable of trapping water. Further rheological analysis demonstrated its viscoelastic behavior (G' = 3.03 × 10 Pa; G″ = 3.28 × 10 Pa), comparable to the extracellular matrix of certain human tissues, crucial for biomedical applications.