Solution Synthesis, Conformational Analysis, and Antimicrobial Activity of Three Alamethicin F50/5 Analogs Bearing a Trifluoroacetyl Label

We prepared, by solution-phase methods, and fully characterized three analogs of the membrane-active peptaibiotic alamethicin F50/5, bearing a single trifluoroacetyl (Tfa) label at the N-terminus, at position 9 (central region) or at position 19 (C-terminus), and with the three Gln at positions 7, 18, and 19 replaced by Glu(OMe) residues. To add the Tfa label at position 9 or 19, a gamma-trifluoroacetylated alpha,gamma-diaminobutyric acid (Dab) residue was incorporated as a replacement for the original Val9 or Glu(OMe)19 amino acid. We performed a detailed conformational analysis of the three analogs (using FT-IR absorption, CD, 2D-NMR, and X-ray diffraction), which clearly showed that Tfa labeling does not introduce any dramatic backbone modification in the predominantly α-helical structure of the parent peptaibiotic. The results of an initial solid-state 19F-NMR study on one of the analogs favor the conclusion that the Tfa group is a very promising reporter for the analysis of peptaibiotic-membrane interactions. Finally, we found that the antimicrobial activities of the three newly synthesized analogs depend on the position of the Tfa label in the peptide sequence.