Comments on the use of the order parameters obtained from 2H-NMR to describe the anisotropic motions of the methylene groups of the fatty acyl chains in lipid bilayer membranes

The informational content of a deuteron quadrupole splitting obtained from a methylene group undergoing anisotropic motion is inversely proportional to the degree of symmetry underlying this motion. To accurately assess the power and the limitation of ²H-NMR, the motional symmetry of the methylene groups was strictly examined. From the two orthogonal and geometrical planes of symmetry of a methylene group one was found to remain a plane of symmetry for the motion. The other plane was shown, by a quantitative evaluation, to keep its symmetry property only in first approximation. The direction defined by the intersection of the two orthogonal planes was not found to be a motional axis of axial symmetry (even as a poor assumption). However, to a good approximation the motion of this direction with respect to the bilayer normal can be specified by the deuteron quadrupole splitting arising from the particular geometry of a given methylene group.