title: Exploring relationships between mimic configuration, peptide conformation and biological activity in indolizidin-2-one amino acid analogs of gramicidin S.
abstract: Indolizidin-2-one amino acids (I2aas, 6S- and 6R-1) possessing 6S- and 6R-ring-fusion stereochemistry were introduced into the antimicrobial peptide gramicidin S (GS) to explore the relationships between configuration, peptide conformation and biological activity. Solution-phase and solid-phase techniques were used to synthesize three analogs with I2aa residues in place of the d-Phe-Pro residues at the turn regions of GS: [(6S)-I2aa4-5,4'-5']GS (2), [Lys2,2',(6S)-I2aa4-5,4'-5']GS (3) and [(6R)-I2aa4-5,4'-5']GS (4). Although conformational analysis of [I2aa4-5,4'-5']GS analogs 2-4 indicated that both ring-fusion stereoisomers of I2aa gave peptides with CD and NMR spectral data characteristic of GS, the (6S)-I2aa analogs 2 and 3 exhibited more intense CD curve shapes, as well as greater numbers of nonsequential NOE between opposing Val and Leu residues, relative to the (6R)-I2aa analog 4, suggesting a greater propensity for the (6S)-diastereomer to adopt the beta-turn/antiparallel beta-pleated sheet conformation. In measurements of antibacterial and antifungal activity, the (6S)-I2aa analog 2 exhibited significantly better potency than the (6R)-I2aa diastereomer 4. Relative to GS, [(6S)-I2aa4-5,4'-5']GS (2) exhibited usually 1/2 to 1/4 antimicrobial activity as well as 1/4 hemolytic activity. In certain cases, antimicrobial and hemolytic activities of GS were shown to be dissociated through modification at the peptide turn regions with the (6S)-I2aa diastereomer. The synthesis and evaluation of GS analogs 2-4 has furnished new insight into the importance of ring-fusion stereochemistry for turn mimicry by indolizidin-2-one amino acids as well as novel antimicrobial peptides.
mesh_terms: D000595:Amino Acid Sequence; D000596:Amino Acids / Q000138:chemical synthesis / Q000737:chemistry / Q000494:pharmacology; D000900:Anti-Bacterial Agents / Q000138:chemical synthesis / Q000737:chemistry* / Q000494:pharmacology*; D001419:Bacteria / Q000187:drug effects; D002942:Circular Dichroism; D006096:Gramicidin / Q000031:analogs & derivatives* / Q000138:chemical synthesis / Q000737:chemistry / Q000494:pharmacology*; D006461:Hemolysis / Q000187:drug effects; D009682:Magnetic Resonance Spectroscopy; D008826:Microbial Sensitivity Tests; D015394:Molecular Structure; D011487:Protein Conformation; D013329:Structure-Activity Relationship; D013696:Temperature
chemical_list: D000596:Amino Acids; D000900:Anti-Bacterial Agents; D006096:Gramicidin