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

Gramicidin S, a membrane-active antibiotic, was crystallized from solvent containing water, methanol, trifluoroacetic acid and hydrochloric acid. The structure was refined at 0.95 Å resolution and contains 1.5 molecules of gramicidin S, two trifluoracetic acid molecules and ten water molecules in the crystallographic asymmetric unit. In the crystals, the gramicidin S molecules line up into helical channels that differ from those observed previously.