NMR of gel and liquid crystalline phospholipids spinning at the 'magic angle'

Recently Chapman et al. have been applying both natural abundance [l-4] and isotopic enrichment [5,6] nuclear magnetic resonance spectroscopy (NMR) of ‘H, 2H and 13C nuclei to a study of molecular mobility in both model membrane [ l-3,5] and natural membrane [4,6] systems. It has been established that the line broadening in such (unsonicated) systems is predominently dipolar [2,7] in fields up to -2T. Andrew et al. [8] and Lowe [9] have shown that dipolar broadening in solids can be greatly reduced by rapid sample rotation about an axis inclined at an angle j = set-’ \13 (the ‘magic angle’), to the magnetic field H,, . Schneider et al. [lo141 and Cohn et al. [ 151 have extended these experiments to proton NMR in various polymers, e.g. solid polyethylene [ 121 and polybenzylglutamate in solution [ 14,151 and in heterogeneous solid-liquid systems [ 10,131. DoskoGlova’ and Schneider [ lo] have originally used Gutowsky and Pake’s [ 161 linewidth formula for systems with anisotropic internal motion to characterise the motional requirements for line narrowing experiments to be effective, and Haeberlen and Waugh [ 171 and Andrew and Jasinski [ 181 have treated the problem theoretically in greater depth. In the simple treatment [lo] linewidth Au (arbitrarily defined with respect to line shape) can be expressed as 2 (Au)’ = A2 ~arctg