Double-stranded helical twisted β-sheet channels in crystals of gramicidin S grown in the presence of trifluoroacetic and hydrochloric acids

Gramicidin S is a nonribosomally synthesized cyclic decapeptide antibiotic with twofold symmetry (Val-Orn-Leu-D-Phe-Pro) 2; a natural source is Bacillus brevis. Gramicidin S is active against Gram-positive and some Gram-negative bacteria. However, its haemolytic toxicity in humans limits its use as an antibiotic to certain topical applications. Synthetically obtained gramicidin S was crystallized from a solution containing water, methanol, trifluoroacetic acid and hydrochloric acid. The structure was solved and refined at 0.95 angstrom resolution. The asymmetric unit contains 1.5 molecules of gramicidin S, two trifluoroacetic acid molecules and ten water molecules located and refined in 14 positions. One gramicidin S molecule has an exact twofold-symmetrical conformation; the other deviates from the molecular twofold symmetry. The cyclic peptide adopts an antiparallel beta-sheet secondary structure with two type II' beta-turns. These turns have the residues D-Phe and Pro at positions i + 1 and i + 2, respectively. In the crystals, the gramicidin S molecules line up into double-stranded helical channels that differ from those observed previously. The implications of the supramolecular structure for several models of gramicidin S conformation and assembly in the membrane are discussed.