31P NMR of Phospholipid Membranes: Effects of Chemical Shift Anisotropy at High Magnetic Field Strengths

The 31P NMR spectrum of sonicated dipalmitoyl lecithin vesicles consists of two chemically shifted resonances, separated by -0.15 ppm, which arise from phosphate groups in phospholipid molecules on the insideand the outside of the spherical bilayer vesicles. The widths of the resonances are remarkably sensitive to the crystallineliquid crystalline phase transition, the magnetic field-strength, and the viscosity of the surrounding aqueous medium. The results are interpreted in terms of two phosphorus relaxation mechanisms: modulation of the anisotropic phosphorus chemical shift and dipolar interaction with protons on adjacent methylene groups. At high magnetic fields (7.5 T) the modulation of the phosphorus chemical shift anisotropy by the Brownian rotation of the intact spherical vesicles dominates the linewidth. The chemical shift anisotropy in the sonicated vesicles is compared with that in the unsonicated dispersions. It is concluded that the structure around the phosphate group is not significantly disrupted by the sonication process. The order parameter for the internal motion of the phosphate group is estimated, and a tentative model for the motion of the phosphate group in the membrane is proposed.