IntroductionHelical structures have been of significant interest since the formulation of the α­helix in protein structure by Pauling1and the discovery of the double helical structure of DNA by Watson and Crick (with a little help from Franklin and Wilkins at King's College) in 1953.2–4A great deal of work in supramolecular chemistry has been devoted to the preparation and characterisation of single, double and triple helical structures based on coordination chemistry.5Elegant work by Sauvage and co-workers has transformed Cu(I)-based molecular double helices into intricate knots and doubly interpenetrated catenanes.6,7Organic compounds that form molecular helices are also known, such as an acyclic methylchlorophyll biomolecule, held together by structure­organising R2C&z.dbd;O⋯HNR′2hydrogen bonds.8Helices with much larger pitch (and perhaps significantly greater degrees of optical rotation) are obtained by non­molecular chemistry, however. Thus the complementary 2,6­diaminopyridine (P) and uracil (U) systems, linked by tartaric acid units (T) to give PTP and UTU self­assembling units, give enormous, triple helical fibres displaying liquid crystalline behaviour.9A remarkable sixfold single helix with a repeat (pitch) of some 43.769 Å is obtained by co­crystallisation of the hydrogen bond donor [La(NO3)3(H2O)2(1,10­phenanthroline)] with the acceptor 15­crown­5,10although solid state structures with a pitch as long as 116.50 Å (9,9­dimethyl­2,2,4,6,6­pentaphenyl­3,5,8,10­tetraoxa­4­phosphabicyclo[5.3.0]decane) are known.11The helicity in the former case apparently arises from the approximate egg­shape of the lanthanum complex, which possesses three bulky coordinated nitrato anions on one side, balanced by the flat phenanthroline ligand on the other. In the latter example, even though are no strong intermolecular interactions, the molecular shape again determines the packing motif. Hydrogen bonded helices have also arisen from work by Hamilton and co-workers12in which an isophthalate derivative forms a 1∶1 hydrogen bonded helix with a long chain dicarboxylic acid. Similarly hydrogen bonded helices have arisen from amine–alcohol co­crystallisation,13while 2­methyl­1­phenyl­3­pyridin­4′­ylprop-2-en­1­one oxime self­assembles to form helical hydrophobic channels.14Even weak interactions such as π–π stacking and CH–π interactions may produce helical structures15and supramolecular helices may even exhibit spontaneous resolution in the solid state.16