Frequency dependence of 31 P NMR linewidths in sonicated phospholipid vesicles: Effects of chemical shift anisotropy

Phosphorus nuclear magnetic resonance (3’P NMR) is an increasingly important physical technique for the elucidation of structural features of phospholipid bilayer membranes [l-S] . In sonicated vesicles, however, the chemical shift differences between different classes of phospholipids are of approximately the same magnitude as the widths of the resonances themselves [5,6]. For many applications it is desirable to find conditions which optimize the resolution of these signals. In general, the resolution of chemically-shifted resonances is improved by increasing the field strength, as the separation of the resonances increases linearly with the field strength, while the width of the resonances is usually field-independent. The 31 P NMR spectrum from unsonicated phospholipid dispersions is, however, dominated by the chemical shift anisotropy of the phosphate group [2,4]. It might therefore be expected that the 31P NMR linewidths in the sonicated phospholipid bilayer systems would contain a term which arises from the modulation of this chemical shift anisotropy by the isotropic tumbling of the vesicles. In this case, the 31P NMR linewidths of sonicated bilayer vesicles would broaden as the