Optimized protocol for synthesis of cyclic gramicidin S: Starting amino acid is key to high yield

A simple and highly efficient Fmoc solid-phase protocol for synthesizing the antimicrobial decapeptide gramicidin S and various labeled analogues is presented. When preparing the linear precursor peptides (1a-e), a systematic permutation of the starting amino acid within the cyclic sequence gave different yields between 51% and 93%. Also the subsequent step of cyclization gave widely diverging yields between 26% and 74%, depending again on the starting amino acid. The ease of cyclization was found to correlate with the tendency of the respective linear precursor peptide to assume a preorganized conformation, as observed by circular dichroism. The overall yield is thus critically dependent on the starting amino acid and can be raised from 20% to 70% using DPhe. The choice of coupling agent and its counterion was found to play only a marginal role. Irrespective of being able to assume a preorganized conformation, none of the linear precursor peptides exhibited any antimicrobial or hemolytic activity. Using the optimized protocol, which involves only simple Fmoc-couplings and requires no intermittent purification steps, several gramicidin S analogues (3-8) containing F-19-labeled phenylglycine derivatives and/or N-15-labeled amino acids were synthesized for solid-state NMR structure analysis.