Two-dimensional exchange 2H NMR experiments of phospholipid bilayers on a spherical solid support.

Abstract

Bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) on a spherical solid support consisting of silica beads are studied by two-dimensional exchange deuterium NMR (2D exchange $^{2}\mathrm{H}$ NMR) at two hydration states of the bilayer and at mixing times in the range 1--8 ms. The spectra obtained were analyzed in terms of a solution of the diffusion equation of molecules diffusing on the surface of a sphere with a radius corresponding to that of the solid support. This procedure gives a jump angle distribution function of the lipids for a fixed mixing time. The well defined geometry of the sample enables us to compare the experimental results with those obtained by a random walk simulation assuming that solely diffusional jumps of the POPC molecules contribute to the 2D exchange NMR spectra during the mixing time. Excellent agreement was obtained between the experimental results, computer simulations, and numerical calculations based on the diffusion equation. These results provide strong evidence that lateral diffusion of lipids is the dominant mechanism in determining the spectral evolution in 2D exchange NMR spectroscopy of spherical, solid supported lipid bilayers. From the experiments we obtained a lateral diffusion coefficient of the POPC molecules of D=(8.2\ifmmode\pm\else\textpm\fi{}3)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}12}$ ${\mathrm{m}}^{2}$/s (high hydration, T=30 \ifmmode^\circ\else\textdegree\fi{}C) and of D=(0.8\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}12}$ ${\mathrm{m}}^{2}$/s (low hydration, T=35 \ifmmode^\circ\else\textdegree\fi{}C) in good agreement with values obtained previously using other spectroscopic methods. From these results it is concluded that future applications of the 2D exchange NMR method may provide valuable insight into the shapes and the slow motion dynamics of biological membranes.