On the conformational and packing behaviour of acyclic sugar amphiphiles: The crystal structures of N-(1-octyl)-d-arabinonamide and N-(1-dodecyl)- d-ribonamide and the supramolecular assembly-forming properties of N-(1-octyl)-d-pentonamides

The crystal structure of N-(1-octyl)-d-arabinonamide [space group P21, a=4.855(3) Å, b=30.45(3) Å, c=10.59(1) Å, β=94.46(6)°] shows two independent molecules arranged in an antiparallel manner in the asymmetric unit. Their head groups and octyl chains are extended, but the overall shape of the molecules is different. The packing arrangement in the crystal, with interdigitating head groups and oligomethylene tails, has never been observed before with aldonamides, but is identical with that of the glucitol amphiphile MEGA-8. Comparison of the packing of the arabinonamide molecules in the crystal with that of other amphiphiles crystallizing in an antiparallel arrangement reveals a similar behaviour, suggesting a common mechanism of crystallization for these compounds. N-(1-Dodecyl)-d-ribonamide [space group P1, a=4.815(2) Å, b=5.464(2) Å, c=18.084(4) Å, a=81.77(2)°, β=87.78(3)°, γ=83.83(3)°] displays one molecule in the asymmetric unit. It exhibits two straight-chain segments from C-2 to C-5 and C-2 to C-14, respectively, and shows an unexpected 1,3-syndiaxial interaction between C-1, the carbonyl carbon atom, and O-4. The hydrogen-bond patterns involve a quadrilateral homodromic cycle between between four molecules related by translation along the two 5 Å axes. The molecular packing is head-to-tail as observed in the crystal structures of amphiphilic gluconamides displaying an analogous homodromic cycle. It has been shown earlier that N-octyl-d-gluconamide forms solid micellar fibers in water, whereas the analogous ribose derivative investigated in this paper forms planar bilayer sheets instead of micellar threads, although the molecular interactions in the crystal sheets are similar to those in gluconamide. N-Octyl-d-arabinonamide, on the other hand, forms ill-defined tubular bilayers.