Line Shape Analysis of NMR Powder Spectra of2H2O in Lipid Bilayer Systems

Abstract

A NMR line shape/spin relaxation model is developed for 2H2O studies of structure and dynamics of the lipid−water interfaces of phosphatidylcholine bilayers. A line shape function describing the orientational dependence of 2H2O is derived. In addition, also expressions of the observed quadrupole splitting and the spin−lattice relaxation rate are derived within the same dynamic model. The model comprises two chemically interchanging fractions of water namely, “free”, and “bound”. There are four molecular parameters characterizing the “bound” water of the lipid water interface, namely, (1) the fraction water molecules bound to lipid molecules, (2) the local water order parameter , (3) the order parameter which describes the averaged “bound” water, and an effective correlation time (4) τc, characterizing water translational diffusion at the interface. This model allows for analyzing quadrupole splittings, spin−lattice relaxation rates, and water powder line shapes. Thus, dynamics as well as structural information about the water molecules residing in the water lipid interface may be extracted. In the reinterpretation of 2H2O powder spectra obtained for lamellar phases of dipalmitoylphosphatidylcholine (DPPC), the results clearly indicate that (Lα) > (Lβ‘) when comparing the liquid crystalline phase with 10−11 water molecules per lipid molecule and the gel phase with 3.5−4.2 water molecules per lipid molecule. Whereas the order of the perturbed water is similar in both phases, (Lα) ≈ (Lβ‘). The lateral diffusion is characterized by a correlation time τc > 60 ns but cannot be determined without measuring the spin−lattice relaxation measurements.