Systematic Analysis of the Relationship among 3D Structure, Bioactivity, and Membrane Permeability of PF1171F, a Cyclic Hexapeptide with Paralyzing Effects on Silkworms.

PF1171 hexapeptides, a family of cyclic hexapeptides produced by fungi, exhibit paralyzing effects on the larvae of silkworms via oral administration. To elucidate the structural features of PF1171 hexapeptides that are crucial for bioactivity, the relationship among 3D structure, bioactivity, and membrane permeability of PF1171F (the peptide with the highest bioavailability) was systematically analyzed through the synthesis of 22 analogues. The PF1171F analogues were prepared by the solid-phase synthesis of a linear precursor and subsequent solution-phase macrolactamization. Analysis by NMR spectroscopy and molecular modeling indicated that the major 3D conformations of PF1171F in various solvents resemble its X-ray crystal structure. The analogues with this conformation tend to exhibit potent paralysis against silkworms, indicating the significance of the conformation in the paralysis. The biological activity was dependent on the mode of administration, varying between hemolymph injection and oral administration. Parallel artificial membrane permeability assay (PAMPA) of the analogues revealed a correlation between membrane permeabilities and paralytic activity by hemolymph injection, indicating that the target molecule of PF1171F is present inside the cell membrane.

Structure Revision of Similanamide to PF1171C by Total Synthesis.

The total synthesis of the proposed structure of similanamide, a cyclic hexapeptide recently isolated from the marine sponge-associated fungus Aspergillus similanensis KUFA 0013, was achieved by solid-phase synthesis of a linear precursor and solution-phase macrolactamization. The NMR spectra of our synthetic final product were not identical to those of the isolated material and led us to conclude that similanamide is identical to PF1171C, a previously reported diastereomeric hexapeptide.

Total synthesis and biological evaluation of PF1171A, C, F, and G, cyclic hexapeptides with insecticidal activity.

The total synthesis of the cyclic hexapeptides PF1171A, C, F, and G has been achieved by solid-phase synthesis of a linear precursor and solution-phase macrolactamization. The synthesis includes a solid-phase peptide coupling with the weakly nucleophilic amino group of an anthranilic acid residue. This was efficiently achieved by in situ generation of an Fmoc-amino acid chloride using triphosgene. The natural products exhibit potent paralytic activities against silkworm larvae, whereas epi-PF1171A and epi-PF1171C, bearing l-Ala instead of d-Ala, were relatively inactive. X-ray crystallographic analysis indicates that intramolecular hydrogen bonds in PF1171 peptides are critical for maintaining their active conformations.