Amphiphilic properties of synthetic glycolipids based on amide linkages. II. Crystal and molecular structure of N-(n-octyl)-D-gluconamide, an amphiphilic molecule in head-to-tail packing mode

The crystal structure of N-(n-octyl)-D-gluconamide (space group P2₁, a = 5.252(1), b = 32.426(9), c = 4.805(1) A ̊, β = 94.96(5)°) was determined by X-ray diffraction methods and refined to R = 0.046. The molecules are V-shaped and occur in an all-trans conformation for both the aliphatic and D-glucose moieties. The crystal structure shows extensive intra- and intermolecular hydrogen bonding and the molecules are packed parallel in sheets, adjacent sheets being arranged in a very unusual head-to-tail fashion. Several homologues of N-(n-octyl)-D-gluconamide were also crystallized. Compounds with an even number of carbon atoms in their alkyl chain crystallize in monoclinic space-group P2₁, whereas those with an odd number of alkyl carbons occur in triclinic P1. At temperatures between 70°C and 100°C, all compounds undergo a phase transition in the crystalline state as seen in the polarizing microscope. The structure of an octylgluconamide crystal that was heated above the transition temperature and then cooled to room temperature was determined independently and refined to R = 0.071. The structure turned out to be largely identical to the 'native' one. Phase transitions were also observed in solution by small angle scattering experiments. These are discussed in relation to the formation of gels which has been reported for these compounds in a previous publication (Pfannemüller and Welte, Chem. Phys. Lipids, 37 (1985) 227-240).