Figure 2.

Gramicidin A structures in different environments. In addition to the atomic ball-and-stick structures, a ribbon is added to accentuate the handedness and strandedness of the helix. The two monomers have different colored ribbons: one yellow and one orange. The backbone carbonyl oxygens that line the pore of the channel are highlighted in red, and the indole ¹⁵N sites, important in dictating the strandedness of the structure in a membrane environment, is shown in blue. For each structure the Ala³–Leu⁴ peptide-plane orientation is shown with respect to B. (A) The solid-state NMR-derived structure from a bilayer environment: single-stranded, right-handed, and 6.5 residues per turn (ref. 1; PDB accession no. 1MAG). (B) An x-ray crystallographic structure of crystals prepared from Cs⁺/MeOH solution: double-stranded, right-handed, and 7.2 residues per turn (ref. 5; PDB accession no. 1AV2). (C) A solution NMR structure from an SDS micellar environment: single-stranded, right-handed, and 6.3 residues per turn (ref. 4; PDB accession no. 1GRM). (D) An x-ray crystallographic structure of crystals prepared from benzene/methanol solution: double-stranded, left-handed, and 5.6 residues per turn (ref. 7; PDB accession no. 1ALZ).