Exploring relationships between mimic configuration, peptide conformation and biological activity in indolizidin-2-one amino acid analogs of gramicidin S.

Indolizidin-2-one amino acids (I(2)aas, 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 I(2)aa residues in place of the D-Phe-Pro residues at the turn regions of GS: [(6S)-I(2)aa(4-5,4'-5')]GS (2), [Lys(2.2'), (6S)-I(2)aa(4-5,4'-5')]GS (3) and [(6R)-I(2)aa(4-5,4'-5')]GS (4). Although conformational analysis of [I(2)aa(4-5,4'-5')]GS analogs 2-4 indicated that both ring-fusion stereoisomers of I(2)aa gave peptides with CID and NMR spectral data characteristic of GS, the (6S)-I(2)aa 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)-I(2)aa 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)-I(2a)a analog 2 exhibited significantly better potency than the (6R)-I(2)aa diastereomer 4. Relative to GS, [(6S)-I(2)aa(4-5,4'-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)-I(2)aa 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.