Unique backbone-water interaction detected in sphingomyelin bilayers with 1H/31P and 1H/13C HETCOR MAS NMR spectroscopy

Two-dimensional H-1/P-31 dipolar heteronuclear correlation (HETCOR) magic-angle spinning nuclear magnetic resonance (NMR) is used to investigate the correlation of the lipid headgroup with various intra- and intermolecular proton environments. Cross-polarization NMR techniques involving P-31 have not been previously pursued to a great extent in lipid bilayers due to the long H-1-P-31 distances and high degree of headgroup mobility that averages the dipolar coupling in the liquid crystalline phase. The results presented herein show that this approach is very promising and yields information not readily available with other experimental methods. Of particular interest is the detection of a unique lipid backbone-water intermolecular interaction in egg sphingomyelin (SM) that is not observed in lipids with glycerol backbones like phosphatidylcholines. This backbone-water interaction in SM is probed when a mixing period allowing magnetization exchange between different H-1 environments via the nuclear Overhauser effect (NOE) is included in the NMR pulse sequence. The molecular information provided by these H-1/P-31 dipolar HETCOR experiments with NOE mixing differ from those previously obtained by conventional NOE spectroscopy and heteronuclear NOE spectroscopy NMR experiments. In addition, two-dimensional H-1/C-13 INEPT HETCOR experiments with NOE mixing support the H-1/P-31 dipolar HETCOR results and confirm the presence of a H2O environment that has nonvanishing dipolar interactions with the SM backbone.