Helical channels in crystals of gramicidin A and of a cesium-gramicidin A complex: an X-ray diffraction study

Abstract

X-ray crystallographic studies of gramicidin A crystallized from methanol ( P2 1) and ethanol ( P2 12 12 1), and of a Cs + gramicidin A complex crystallized from methanol ( P222 1, P2 12 12 or P2 12 12 1) are reported here. The asymmetric unit consists of two molecules of gramicidin A in the native crystals and four molecules in the cesium complex crystal. Patterson analyses show that gramicidin A in these crystals forms a cylindrical helical channel. In the two types of native gramicidin crystals, the diameter of this channel is about 5 å and its length is about 32 å. Cesium ions are bound inside this channel in crystals of the cesium-gramicidin A complex. The channel in this complex is considerably shorter (26 Å) and wider (6·8Å) than in the native forms. The Patterson maps of these three crystal forms are compatible with either the single-stranded β-helix (Urry, 1971) or the double-stranded parallel or anti-parallel, β-helix (Veatch et al., 1974).