Crystal structure of strychninium chloride dihydrate: Hidden helix in the water/anion tape

The crystal structure of strychninium chloride dihydrate SHCClK$2H2O was determined. The structure is composed of strychnine herringbone bilayer sheets with channels occupied by the hydrogen bonded anion/water tape between them. Considering different amount of water molecules in isomorphous crystal of SHCClK$2H2O and previously described crystal of strychninium chloride sesquihydrate, we found that water molecules and chloride anions form a left-handed helix. Similarly, anions and water molecules in the crystal of strychnine bromide dihydrate form a left-handed helix related by the two-fold screw axis symmetry. Contrary, in the crystals of strychninium chloride sesquihydrate and SHCClK$2H2O, the helices are related by only translation vector and are stabilized by one or two water bridges, respectively. q 2005 Elsevier B.V. All rights reserved.