title: Optimized protocol for synthesis of cyclic gramicidin S: starting amino acid is key to high yield.
abstract: [structures: see text] 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 (D)Phe. 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 19F-labeled phenylglycine derivatives and/or 15N-labeled amino acids were synthesized for solid-state NMR structure analysis.
authors.affiliation: Institute of Biological Interfaces, Forschungszentrum Karlsruhe, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. parvesh.wadhwani@ibg.fzk.de
authors.forename: Parvesh
authors.initials: P
authors.lastname: Wadhwani
authors.forename: Sergii
authors.initials: S
authors.lastname: Afonin
authors.forename: Marco
authors.initials: M
authors.lastname: Ieronimo
authors.forename: Jochen
authors.initials: J
authors.lastname: Buerck
authors.forename: Anne S
authors.initials: AS
authors.lastname: Ulrich
mesh_terms: D000596:Amino Acids / Q000737:chemistry*; D002942:Circular Dichroism; D003500:Cyclization; D006096:Gramicidin / Q000138:chemical synthesis* / Q000737:chemistry; D006860:Hydrogen Bonding; D008958:Models, Molecular; D015394:Molecular Structure
chemical_list: D000596:Amino Acids; D006096:Gramicidin