Deuterium NMR study of the interaction of phytanic acid and phytol with the head group region of a phospholipid bilayer. Evidence of magnetic orientation.

Deuterium nuclear magnetic resonance (2H-NMR) spectra of multilamellar dispersions of 1,2-dipalmitoyl-sn-glycero-3-phospho-[1',2'-methylene-2H4]choline (DPPC-d4) containing 20 mol% of the isoprenoid compounds phytol or phytanic acid in excess deuterium-depleted water, or Tris buffer (pH 7.4), have been recorded over the temperature range 15-55 degrees C. Phytol (20 mol%) causes a decrease in the residual quadrupolar splitting, delta vQ, of the choline 1'-deuterons (i.e., those adjacent to the phosphate moiety) by 3%, while delta vQ, of the 2'-deuterons decrease by 22%. Phytanic acid (20 mol%) reduces delta vQ of the 2'-deuterons by 43% and increases delta vQ of the 1'-deuterons by approx. 16%. First (M1) and second (M2) moments were calculated from the spectra. Plots of M1 and M2 versus temperature suggest phytol and phytanic acid have a significant effect on the pretransition. Spin-lattice relaxation times (T1) and transverse relaxation times (T2e, the time constant for decay of the quadrupolar echo) have been determined over the temperature range 15-50 degrees C. The presence of 20 mol% phytanic acid results in orientation of the DPPC bilayers in the magnetic field with the phospholipid long axes oriented perpendicular to the magnetic field as shown by 2H- and 31P-NMR. The sample must be carried through several freeze-thaw cycles in the presence of Tris buffer in order that the phospholipid magnetic field ordering occur. Possible explanations for the effect of phytol and phytanic acid on the dynamic structure of the choline head group of phospholipids in the bilayer are suggested.