Orientation-dependent 19F dipolar couplings within a trifluoromethyl group are revealed by static multipulse NMR in the solid state

The homonuclear dipolar coupling between the three equivalent F-19-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 CF3-group as a highly sensitive reporter to describe the mobility and spacial alignment of F-19-labeled molecules in biomembranes. As an example, the fluorinated anti-inflammatory drug, flufenamic acid, was embedded as a guest compound in lipid bilayers. Undistorted F-19 dipolar spectra of its CF3-group were obtained without H-1-decoupling, revealing a sharp triplet line-shape. 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 F-19 NMR experiments were extended to frozen samples, where no molecular diffusion occurs besides the fast rotation about the CF3-axis. Also under these conditions, the CPMG experiment with composite pulses could successfully resolve the dipolar coupling between the three F-19-nuclei. (C) 2000 Academic Press.