Deuterium NMR study of the interaction of alpha-tocopherol with a phospholipid model membrane

The effects of 5, 10, and 20 mol % incorporation of alpha-tocopherol (vitamin E) on 50 wt % aqueous multilamellar dispersions of sn-2-substituted [2H31]palmitoylphosphatidylcholine (PC-d31), a saturated, deuterated phospholipid prepared from egg lysophosphatidylcholine, have been studied by deuterium nuclear magnetic resonance (2H NMR) and differential scanning calorimetry (DSC). Moment analysis of the 2H NMR spectra as a function of temperature and DSC heating curves demonstrate that the main gel to liquid-crystalline phase transition is progressively broadened and its onset temperature lowered by increasing concentrations of alpha-tocopherol. Below the transition temperature (40 degrees C) for PC-d31 bilayers, the 2H NMR spectra indicate that acyl chain motion is increased by addition of alpha-tocopherol and that this effect extends to lower temperatures with higher alpha-tocopherol content. Above the transition, average carbon-deuterium bond order parameters calculated from the first spectral moment establish that alpha-tocopherol increases acyl chain ordering within the PC-d31 bilayer by as much as 17% at 20 mol % incorporation. Profiles of order parameter vs. chain position, constructed from 2H NMR spectra following application of the depaking technique, show that despite higher order the general form of the profile is not significantly altered by alpha-tocopherol.