Membrane curvature studied using two-dimensional NMR in fluid lipid bilayers.

Abstract

The method of two-dimensional exchange spectroscopy in deuterium ${(}^{2}$H) nuclear magnetic resonance (NMR), originally developed for the study of slow, reorientational molecular motions in solids [S. Wefing and H. W. Spiess, J. Chem. Phys. 89, 1219 (1988)], has been applied to the study of multilamellar vesicle samples of phospholipid bilayer systems in their fluid (${\mathit{L}}_{\mathrm{\ensuremath{\alpha}}}$) phase. As is well known, the $^{2}\mathrm{H}$ NMR quadrupolar splitting in such fluid membranes is proportional to (3 ${\mathrm{cos}}^{2}$\ensuremath{\theta}-1), where \ensuremath{\theta} is the angle between the local bilayer surface normal and the external magnetic field. Measurements of two-dimensional exchange $^{2}\mathrm{H}$ NMR spectra using a mixing time ${\mathit{t}}_{\mathit{m}}$ were analyzed to give the reorientational angle distribution function for the local surface normals of individual lipid molecules comprising the membranes. A quantitative analysis of this correlation function taking into account the lateral diffusion of the lipid molecules demonstrated the sensitivity of the technique to the local curvature and shapes of the multilayers, which were shown to exhibit surface roughness on the mesoscopic length scale between about 10 nm and 1 \ensuremath{\mu}m. The results were consistent with an average radius of curvature between 1 and 2 \ensuremath{\mu}m, but approximately 15% of the molecules were located on relatively flat membranes having larger radii of curvature.