Orientational anisotropy of nuclear spin relaxation in phospholipid membranes

The observation that the spin-lattice relaxation (R_1Z) rates of pure phospholipid lamellar phases depend only weakly on their orientation in the liquid-crystalline state is explained. A relaxation model in which either segmental or molecular motions are described by anisotropic rotational diffusion in an ordering potential (M.F. Brown, J. Chem. Phys. 77 (1982) 1576) can account for the available ²H R_1Z data to within experimental error. One possibility is that rotational isomerization breaks the symmetry of the static electric field gradient, leading to an asymmetric residual tensor which is further modulated by molecular motions.