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Orientation-Dependent19F Dipolar Couplings within a Trifluoromethyl Group Are Revealed by Static Multipulse NMR in the Solid State

Journal of Magnetic Resonance
Publication Date
DOI: 10.1006/jmre.2000.2127
  • Three-Spin System
  • Oriented Membranes
  • Flufenamic Acid
  • Homonuclear Dipolar Interaction
  • Fluorine Cpmg


Abstract The homonuclear dipolar coupling between the three equivalent 19F-spins of a trifluoromethyl group, rotating about its threefold symmetry axis, was studied by multipulse solid-state NMR. A modified CPMG sequence was used first to resolve the dipolar splitting of a powder sample, and then to follow its orientation-dependence in uniaxially aligned samples. Our aim is to employ the CF 3-group as a highly sensitive reporter to describe the mobility and spacial alignment of 19F-labeled molecules in biomembranes. As an example, the fluorinated anti-inflammatory drug, flufenamic acid, was embedded as a guest compound in lipid bilayers. Undistorted 19F dipolar spectra of its CF 3-group were obtained without 1H-decoupling, revealing a sharp triplet lineshape. When an oriented membrane sample was tilted in the magnetic field, the change in dipolar splittings confirmed that the guest molecule is motionally averaged about the membrane normal, as expected. A different behavior of flufenamic acid, however, was observed under conditions of low bilayer hydration. From this set of orientation-dependent lineshapes we conclude that the axis of motional averaging becomes aligned perpendicular to the sample normal. It thus appears that flufenamic acid induces a hexagonal phase in the membrane at low hydration. Finally, the dipolar 19F NMR experiments were extended to frozen samples, where no molecular diffusion occurs besides the fast rotation about the CF 3-axis. Also under these conditions, the CPMG experiment with composite pulses could successfully resolve the dipolar coupling between the three 19F-nuclei.

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