Solid State NMR Studies of Lipid Bilayer Structure
Phospholipid bilayers are an important class of lyotropic liquid crystals since they are the primary components of biological membranes. Using 2H, 31P, and 13C NMR we have examined a number of aspects of the structure and dynamics of these liquid crystalline systems. In particular, with 2H and 31P NMR we have examined the phase behavior of the ternary mixture dipalmitoylphosphatidylcholine (DPPC) — Cholesterol (CHOL) — H2O and we detect “phase boundaries” at both ~20 and ~30 mole % CHOL, thus reconciling conflicting reports in the literature, and with 2H NMR we have studied the microscopic nature of the gel-liquid crystalline transition. Angular dependent 31P studies of highly oriented samples permit a determination of the head group conformation in DPPC and also an assessment of the order in the bilayer plane. Finally, we show that magic angle sample spinning experiments allow observation of high resolution 13C and 31P spectra of aqueous phospholipid dispersions, thus circumventing the decade old controversy of the effect of “sonication” on the NMR spectra of these systems.