Dynamical structure of phosphatidylcholine molecules in single bilayer vesicles observed by nitrogen-14 nuclear magnetic resonance.

A 14N nuclear magnetic resonance (NMR) study of lecithins, 24-65 degrees C for dipalmitoylphosphatidylcholine (DPPC) and 20-50 degrees C for egg yolk phosphatidylcholine (EPC), in single bilayer vesicles prepared by an ultrasonic method is reported. Choline 14N signals are found to be Lorentzian in these systems of definite molecular arrangement because of the smallness of the quadrupole coupling constant. This is due to the high symmetry around the trimethylammonium type nucleus and to the low ordering compared with thermotropic liquid crystals. An analysis of relaxation times T1 and T2 gives the following results. (1) The activation energies for the rapid local motion of C beta-N bonds are 36 and 31 kJ.mol-1 for DPPC and EPC, respectively. These are higher than that of a C alpha-D bond, indicating that the -N+(CH3)3 group is bound in the polar surface. (2) The vesicle radius seems to decrease toward higher temperatures, just below the start of the phase transition. (3) The lateral diffusion coefficient of the constituent lipid is obtained with the help of the Stokes radius. The 14N NMR techniques has the essential advantage of applicability without modification of the system.