Self-assembly of synthetic molecules has been drawing broad attention as a novel emerging approach in drug discovery. Here, we report selective cell death induced by a novel peptide amphiphile that self-assembles to form entangled nanofibers (hydrogel) based on intracellular pH (pH i ). We found that a palmitoylated hexapeptide (C 16 -VVAEEE) formed a hydrogel below pH 7. The formation of the nanofibrous self-assembly was responsive to a small pH change around pH 7. The cytotoxicity of C 16 -VVAEEE was correlated with pH i of cells. Microscope observation demonstrated the self-assembly of C 16 -VVAEEE inside HEK293 cells. In vivo experiments revealed that the transcutaneous administration of C 16 -VVAEEE showed remarkable anti-tumor activity. This study proposes that distinct microenvironment inside living cells can be used as a trigger for the intracellular self-assembly of a peptide amphiphile, which provide a new clue to drug discovery.

We investigated D-amino acids as potential inhibitors targeting L-peptide toxins. Among the L-and D-amino acids tested, we found that D-tryptophan (D-Trp) acted as an inhibitor of melittin-induced hemolysis. We then evaluated various Trp derivatives and found that 5-chlorotryptamine (5CT) had the largest inhibitory effect on melittin. The indole ring, amino group, and steric hindrance of an inhibitor played important roles in the inhibition of melittin activity. Despite the small size and simple molecular structure of 5CT, its IC 50 was approximately 13 μg/mL. Fluorescence quenching, circular dichroism measurements, and size-exclusion chromatography revealed that 5CT interacted with Trp19 in melittin and affected the formation of the melittin tetramer involved in hemolysis. Molecular dynamics simulation of melittin also indicated that the interaction of 5CT with Trp19 in melittin affected the formation of the tetramer.

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