Conformational studies of polymorphic N-octyl-D-gluconamide with N-15 (labeled) C-13 (natural abundance) REDOR spectroscopy

Magnetic dipolar couplings between the N-15 atom (labeled) and neighboring C-13 atoms (natural abundance) in three solid modifications of N-octyl-d-gl uconamide are measured with rotational echo double resonance (REDOR). A uni que spectral assignment of C-13 resonances is possible by means of their di polar dephasing. While in the monolayer crystal and in the fiber modificati on the assignment is amenable to the solution spectra, in the bilayer cryst allites a different assignment is found. The dipolar couplings in the range 45 to 1220 Hz are converted into CN distances. These distances are employe d in conjunction with the C-13 chemical shieldings of the CP-MAS spectra to determine sets of possible torsion angles, which define the molecular conf ormation in the neighborhood of the amide group. In contrast to the monolay er crystal, for the fiber and bilayer crystallite modifications a gauche be nd at the C2-C3 bond is found, giving the molecules the shape of a (2)G sic kle.